The update notifications of all my antenna pages on this website are now listed here.

 

[KGD-40-FD]                 [KGD-40-FD construction]                [KGD-20-FD]                  [KGD-80-FD   Mk 1]               [KGD-80-FD Mk 2]


 

No, it is not a KGB antenna, even though it's overall length makes it a stealthy antenna: 1m8 for the 40 m band! This antenna project is my implementation of the German "KGD" antenna: the "Kurz Geratener Dipol" (i.e., "the dipole that turned out short"). No, it is not a miracle antenna. But I am very pleased with its performance, and it definitely is an option for those with space restrictions.

The antenna was developed by a team of the DL-QRP-AG (Arbeitsgemeinschaft für QRP und Selbstbau im Amateurfunk). The manual ("Baumappe", ref. 1) is available from QRPproject. Unfortunately, they no longer sell the antenna kits: center section with pre-wound coils, and pre-cut aluminum tubes. But: so what? Amateur radio is about building yourself! The QRPproject website has an extensive forum [in German], where this antenna has been discussed (ref. 1).



The design of the KGD antenna - a base-loaded dipole

The antenna is a short, base-loaded dipole. In the original design, the dipole-legs are made of aluminium pipe. There is a loading coil at the base of each leg. A 50 Ω coax connects directly to the coils. A capacitor across the coax adapts the antenna impedance  to the coax.



Do not get near the antenna when transmitting!
Do not let any people or animals get near the antenna when transmitting!

Do not use more than 50 W continuous power!

This antenna has a high "Q". So, at the resonance frequency, very high voltages will be generated across the coils - even when transmitting with only a couple of watts! A fluorescent light held near a coil will light up, even at low power.


Yours truly holding a small fluorescent light tube near de coils of the KGD antenna
 

Note that the high voltage is basically across the coils, not across the capacitor!

 

Power

into 50 Ω

Voltage across capacitor

5 W 16 Vrms
10 W 22 Vrms
20 W 32 Vrms
50 W 50 Vrms
100 W 71 Vrms

So the capacitor does not have to be a high voltage type. However, the capacitor should be a low-loss type (low Equivalent Series Resistance, ESR, low Dissipation Factor, DF). Otherwise, the capacitor may become too hot (and disintegrate - dangerous), and the apparent current may exceed the capacitor's current-rating

The table below shows the dimensions of the original design for several frequency bands. Each leg has a length of only 1.75% of the wavelength (2% for 80m). All coils are made of 0.8 mm diameter enameled copper wire (a.k.a. CuL, magwire, transformer wire). You can also use AWG #20 wire (0.8128 mm Ø). In the original kit, the center section is made of solid polyamide, and has machined grooves for the coil turns. It is slid into the aluminium tube.

 

The original dimensions from the manual
(note 1: use 2 inductive coupling turns per coil, i.e. a Beta-Match coil);
note 2: alternatively, use no capacitor but interconnect the coils and tap each at 1.5 wdg from center)

As noted above, the capacitor only serves to adapt the coax impedance to the (low) impedance of the feed-point of the antenna. For instance, for  a feed-point impedance of 25 Ω, the impedance of the capacitor should be approximately 50 Ω at the design frequency: 50 Ω parallel to 50 Ω (coax) is about 25 Ω. At 7 MHz, a 470 pF capacitor would provide 48 ohm - close enough.

 

Note that the capacitor is not part of a tuned circuit. You can actually change the capacitor value e.g. 200% and the resonance frequency of the antenna will not change. "Doorknob" capacitors such as the one I used, tend to have a poor thermal coefficient, which makes them unsuitable for use in tuned circuits. However, in our application, they are OK (though over-dimensioned).

 

The feed-point impedance is the sum of the radiation resistance and the loss resistance of the antenna. It can be determined by measuring SWR at the feed-point, or using a network/antenna-analyzer at the feedpoint. The loss resistance is driven by the coil and the tubes. If you change the coil dimensions (core diameter, wire gauge, inter-wire spacing) the coil losses will change, and a different capacitor value will be required. Likewise, if a different diameter tube is used. Obviously this also affects the bandwidth of the antenna. If the losses are increased (not a good idea!!!), e.g., by using thinner wire for the coils, actually no capacitor may be required. Conversely, the coil dimensions may be changed so as to still have the desired antenna resonance frequency, but a feed-point impedance that can be adapted to the coax with a 1:2 or 1:4 balun.

Using a 1:1 current choke is recommended, and is what I use (placed close to the feed point of the antenna).

Per the manual: if the dipole is installed near the ground (height < 1/8 λ), it should be used vertically. The coax should be perpendicular to the antenna for at least the first 1 m (3-4 ft). It appears that the antenna characteristics are very sensitive to the latter, especially the first 10-20 cm! Obviously, the antenna should not be installed near metal down spouts, just like horizontal installation should not be near metal rain gutters etc.

 

A description of my 20 m mini-dipoles (2x 3 m / 2x 10 ft fishing poles and 2x 2½m / 2x 8 ft) is here.

 

References:

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Ref. 1: "KGD - Kurz Geratener Dipol", instructions and associated forum entries from QRPproject [in German]

I have decided to take the liberty of appending my initials to designator of my version(s) of this antenna. I still had a section of PVC tube from another project, and used that instead. It is dark-grey hard-PVC tube: regular Schedule-40 "rigid nonmetallic conduit - above ground and underground". It is marked as "UV resistant", which is good for an antenna! The ID is 15.7 mm and the OD is 21.5 mm. 

I decided to use copper pipe from the local Do-It-Yourself store. They did not have any diameters that came close to the OD of my PVC tube, so I got a smaller diameter: "14/16 mm" (ID/OD). According to the sticker on it, it even comes with a 30 year guarantee! Contrary to the original design, my copper pipe will have to be fitted inside the PVC-tube.

The PVC is thermoplastic: it softens when heated. So, to avoid having to ream the ends of the PVC pipe, I checked, and after holding the end of the PVC pipe in boiling hot water for about a minute, the copper pipe can be pressed into the PVC pipe by hand (16 vs. 15.7 mm is not a lot...). My PVC tubes state that they are rated for 90º  C (200º F). So, boiling water is hot enough to do this (unless you are on top of a very high mountain, as water will boil there at (much) lower temps). CPVC (hot water conduit, some sprinkler system pipes) has a higher temp rating. It will be (much) more resistant to heating of the coils, and will not become malleable in boiling water. If you very (!!) carefully heat up the (C)PVC about 30 cm (1 ft) above a small burner of your kitchen stove, while turning it constantly (like a spit), it will also soften enough to work with. But I do not take any responsibility if you do this and burn yourself, or burn your house down.

The original design has some sort of plastic piece that is pressed into the tip of the aluminium tube sections. A threaded rod of about 20 cm (8") long and several mm diameter is screwed into this. The rods form a capacitive load at the tips of the copper tubes, and are used to tune the resonance frequency of the antenna. See plots further below.

I decided to build the 40 meter version of this antenna. Total length is only about 160 cm (≈ 5 ft 4") without the tuning rods. I had to recalculate the number of coil turns from the design diameter of 18.5 mm, to 21.5 mm. When increasing the coil diameter, the required number of turns is decreased by a factor that is equal to the ratio of the square of the old and the square of the new diameter (basically the ratio of the cross-sectional areas):

So, when going from 18.5 to 21.5 mm diameter, the number of turns would have to be reduced from 114 to 85. As you will see further below, the actual number for the PVC tubing that I used, turned out to be 92. The required length of wire (per coil) is about 5.8 m (reduced from 6.6 m). Using an on-line coil calculator (e.g., here), the estimated inductance is ≈ 45 μH (in both cases, hi).

 

Note that the required number of turns will have to be reduced by about 5% if you weatherproof the coils with something like shrink tube.

 

Read the measurements & performance section below, to see how I arrived at the final dimensions.

 


The final dimensions of my 40 m KGD antenna
(same coil parameters for 0.8 mm enameled copper wire and AWG #20)

 

As described further below, I had to rebuild my antenna after I melted the PVC coil core by running at about 100 W for several minutes. This time, I used AWG #20 enameled copper wire, instead of 0.8 mm. After tuning, the frequency plot of my miniVNA antenna analyzer closely matches that of my first build. So construction of this antenna appears to be repeatable.

 

Note that, as always,  "Harry's Law of Coils" applies. As Harry (SMØVPO) says: 
    1. You cannot wind coils like I, and I cannot wind coils like you.
    2. Coil-winding data is a constant that varies from person to person.


CONSTRUCTION

Components for the 40 mtr KGD antenna:

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1 section of hard PVC tube (Schedule-40), 32.5 cm long, 21.5 mm OD, 15.7 mm ID.

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1 section of PVC tube,  40 cm long, with an OD that fits snugly into the above PVC tube.

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PVC glue.

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2 sections of copper tube, 70 cm long, 16 mm outer diameter.

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2 x 6 m enameled copper wire (CuL, magwire, transformer wire), 0.8 mm diameter (or AWG #20).

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1 capacitor, 470 pF. I used a 16 kV Russian-made "doorknob" capacitor. See the notes above.

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2 ring-tongue terminals (to fix the end of the coil wire and the coax to the capacitor).

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2 sections of brass rod, 20 cm long, M6 thread.

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4 brass nuts, M6.

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2 round plastic inserts, diameter to tightly fit the inside diameter of the copper tube. A hole must be drilled through the center (lengthwise), and a thread tapped for M6.

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2 plastic inserts, at least 8 mm diameter, length to loosely fit across the inside of the copper tube. This is a spacer for the inserted tip of the brass rods, and will keep the rods centered inside the copper tube. A hole must be drilled across the middle of the insert, and tapped with an M6 thread.

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1 meter coax, 50 Ω, e.g. RG58.

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BNC or PL connector for the coax.

 

 


Dimensions of the PVC-tube center section - final design for the 40 mtr version
 


The main components of the antenna - the Russian-made 470 pF capacitor handles 16 kV
(copper pipe not shown to scale; tuning rods not shown)

 

With my first KGD antenna, I terminated the outer ends of the coils with a ring-tongue or spade terminal and screwed the terminals into an M3 hole that I had tapped into the copper pipe. The wall of the copper pipe is only 1 mm thick. On top of that, copper is rather soft. This is really not enough for a robust and solid thread. So, for my second antenna, I decided to solder the coil ends to the copper pipe.

 

CONSTRUCTION :

 

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Read the entire list of instructions at least once before you start this project!

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Cut the PVC pipe to length, per the table above.

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Drill holes in PVC pipe, per the diagram above

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the 5.2 mm diam. hole for the coax is drilled straight through the PVC pipe, i.e., two holes. If you only have a 5 mm drill, just move it in and out of the holes a couple of time to slightly (!) increase the diameter of the holes. It must be a relatively tight fit for the coax.

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the two 8 mm holes at the tips of the PVC-tube are for connecting the coils to the Cu tube. Make the holes oblong or oval, by tilting the drill.

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the four 0.9 (or 1.0) mm "tunnel" holes are for fixing and guiding the coil wires at the ends of the coils.

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The antenna characteristics are rather sensitive to the coax being at a right angle with respect to the large PVC tube. In my second version of the KGD I have expanded the original design with a second PVC tube that is fixed perpendicularly to the PVC coil core. The coax is passed through this second tube.

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The exit hole of the coax is shown at the center of the dimensional diagrams above. The entry hole is on the opposite side.

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Widen the exit hole with increasing drill bit sizes until the smaller diameter PVC tube almost fits into the hole. Carefully ream this hole with a conical grinding bit until the latter tube section fits tightly into the hole.

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Round off one tip of the smaller PVC tube until it has the same diameter as the inside of the large PVC tube. See photo below. This tube will be inserted later, after the coils are wound.

 

Shaped tip of small-diameter PVC tube and its perpendicular insertion into the PVC coil core
 

 

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With a conical grinding bit, chamfer the inside of the tube ends. This will facilitate insertion of the copper tubes.

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Wind the first coil:

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Insert the coil wire through the tunnel hole near the hole for the coax, in the direction indicated in the diagram.

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Make sure that about 10 cm of the wire sticks out.

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The end that sticks out should be bent at a right-angle, towards the hole for the coax.

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Wind 95 turns. During "tuning & pruning" of the antenna, this will be reduced.

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Wind the coil tightly. 

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Once the coil is wound, terminate the coil:

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keep tension on the wire, and thread it trough the tunnel hole near the oval hole at the tip of the PVC-tube, in the direction indicated in the diagram.

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Cut the wire such that about 10 cm of the wire sticks out.

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The end that sticks out should be bent at a right-angle, towards the oval hole.

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Repeat for second coil.

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Wind the second coil in the opposite direction: if the first coil was wound clockwise, then wind the second coil counter-clockwise, and vise-versa. Note that this is per the original design. I have wound the coils in opposite directions as well as in the same direction, and found no difference in antenna characteristics. But we might as well stick with the original design...

 


The PVC center-section with the coils wound and round-tongue crimp terminals
(photo taken before rewinding one of the coils in counter/clockwise direction)

 


Copper tubes inserted into the PVC center-section
 

Let's prepare the copper pipes:

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Cut the copper pipe to length: 70 cm (27 9/16"). With a file, chamfer the outer edge of the pipe. This will make it easier to insert it into the PVC tube.

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Apply solder to the last 4 cm of one end of each pipe section. This is the end that will be inserted into the PVC tube.

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I do this by heating the end of the pipe over the kitchen stove, applying rosin-core solder, and turning the pipe to evenly distribute the molten solder.

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I hold the pipe with self-locking pliers, about 30 cm (1 ft) from the hot end.

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When done, and the solder is still hot, tap the end of the pipe to remove excess solder.

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Use a kitchen scrubbing sponge (one of those green things) to clean off the solder flux.

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If the layer of solder is thick or uneven (with "gobs" of solder) then file of the high spots.

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Cool off the pipe in cold water.

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Mark the copper tubes at 4 cm from the end that will be inserted into the PVC tube - that's how far they will be inserted. I use a piece of painters tape for this.

 

We are now ready to join the copper pipes into the PVC coil core. The PVC is thermoplastic: it softens when heated. My PVC tubes state that they are rated for 90 deg C (200 deg F). So, boiling water is hot enough to do this (unless you are on top of a very high mountain, as water will boil there at (much) lower temps).

 

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Put a small cutting board up against the wall of your kitchen counter. Put a large cutting board against it, on your counter.

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Put enough water in a sauce pan such that it is a little over 4 cm deep, and bring the water to the boil.

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Have your copper pipes ready to go!

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Put one end of the PVC tube vertically into the boiling water for one minute. It is OK to let it rest on the bottom of the pan.

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Quickly insert the copper tube into the PVC tube:

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you only get one chance to get it right!

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Quickly put the PVC tube onto the cutting board, perpendicular to the wall, cold end against the wall.

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Quickly line up the tinned end of one of the copper pipes with the hot end of the PVC pipe, and  immediately press it  into the hot end of the PVC tube.

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Make sure that you you press the pipe in straight! If the centerlines of the pipe and the tube are not aligned, you will probably end up with a crooked antenna!

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Push in the cupper pipe up to the 4 cm marker.

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The PVC will cool off quickly, shrink back, and fix the copper tube in place - its position can no longer be adjusted! (though if you very (!!) carefully heat it up about 30 cm (1 ft) above a small burner of your kitchen stove, while turning it constantly (like a spit), the PVC will soften again and you can straighten the PVC and maybe adjust the position of the copper pipe...).

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Repeat the process with the second copper tube.

 

The antenna is now taking shape. We'll proceed with connecting the coils and the coax:

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The wire of the two outer coil ends is sticking out of the "tunnel holed" in the PVC tube, and is bent towards the nearest oval hole. Guide them over the center of the hole, cut off the wire 1 cm beyond that center. Bend that last 1 cm of the wire at a right angle, i.e., perpendicular to the centerline of the PVC tube.

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Remove the enamel of the last 1 cm of these wire ends. This is easiest done with a heavy-duty soldering iron (not a soldering gun!). Once the soldering iron is hot, apply enough solder tin to the tip of the iron to obtain a large drop of molten solder. Hold each wire ends into this gob until the enamel is burnt off.

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Solder each wire end to the (pre-soldered) copper pipe that is visible through the oval hole.

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The other coil ends will be connected to the capacitor and the coax. Hold the capacitor over the hole for the coax, about 1 cm away from the PVC pipe, with the "nuts" facing away from the PVC pipe (see photos below). Guide each coil-end wire perpendicularly away from the PVC tube, bend them up to the respective "nut", and cut them off at the "nut" (no, it won't hurt your nuts, hi!).

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Thread the coax through the designated holes, and leave about 10 cm sticking out of the hole where the capacitor will be mounted.

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Strip enough of the insulation off the coax such that the braid can be made into a pigtail that can be folded around the capacitor, almost up to one of the "nuts".  See photos below. For my doorknob capacitor (2½ cm (1") diameter, 2 cm between the screw-in contacts), I needed to strip just over 2 cm.

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Fold the center conductor of the coax around the capacitor, almost up to the opposite "nut". Strip ½ cm (≈¼") of the center conductor's insulation, and apply solder to the center conductor.

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Solder the pigtail and one coil-end wire into a ring-tongue terminal and screw it to the capacitor , between two flat washers. Fasten tightly.

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Solder the center conductor and the other coil-end wire also into a ring-tongue terminal, and mount it on the opposite nut.

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Pull the coax such that the capacitor is pulled against the PVC tube.

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Slide the small-diameter PVC tube over the coax, rounded end first, and slide it all the way to the coil-core tube. Apply PVC glue to the rounded tip and to the edge of the hole in the large PVC tube. Fully insert the small tube into the hole (make sure to turn it so that the roundness of the tip aligns with the inside diameter of the large tube), and let the glue cure.

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Install a coax connector (PL-259 or BNC, per your preference) on the opposite end of the coax.

 

 

Coils hooked up to the copper tubes, capacitor and coax installed
 

Close-up of the coils and capacitor

 

Before we can tune the antenna, we must install the tuning rods:

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Install two nuts at one end of each brass rod, and tighten the nuts against each other. These counter-tightened nuts will give use something to hold onto when turning the rods for tuning.

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Screw the rods halfway into the round plastic inserts (the black ones in the photos below) that will end up at the tips of the copper pipe.

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Apply glue to the hole in each of the two small spacer inserts (the white ones in the photos below). Screw one onto the tip of each brass rod (to the end opposite of the nuts). I used Gorilla Glue® (it expands while curing, so don't use a lot).

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Apply glue all the way around the (black) tip-inserts, but don't get any glue near the brass rod! Insert the rods into the copper tube, and fully seat the tip-inserts. You may want to hold them in place with some sticky tape while the glue cures.

 

The tuning rods and associated plastic inserts

 


A tuning rod inserted halfway into the copper tube

 

Now we're basically ready to tune the antenna. But before we can do that, we'll have to install the antenna outside, away from metal objects if possible. As stated before, if the antenna can't be installed at least 1/8 λ (5 meters or 16 ft for the 40 mtr antenna), it should be installed vertically. The coax should be perpendicular to the antenna for at least the first 1 m (3-4 ft). As stated above, the antenna characteristics are sensitive to this, esp. for the first 20-30 cm - which is why we installed the guide-tube for the coax! 

 

  

Installation of the antenna on my terrace - mounted 2 m off the ground

 

Detail of the tip of the antenna boom

(the boom is 40 mm Ø hard PVC, with a vertical through-hole just under 16 mm Ø, for tight fit with copper tube)

 

This antenna has a very narrow bandwidth around the resonance frequency. Tuning without access to some sort of antenna analyzer or SWR analyzer is difficult. You'd probably have to slowly sweep the frequency of your transmitter while transmitting a carrier, and look at the SWR meter of your tuner to find exact resonance frequency and associated SWR. The tuning procedure below is based on having an antenna analyzer (in my case, a miniVNA hooked up to my PC. Options are to hook up the analyzer right at the antenna, or in the "shack" at the tuner/transmitter at the end of the coax. I do the latter (measurements will include the feedline (12 meters of coax in my case), so I will be seeing what the tuner/transmitter will see later on.

 

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Install the antenna where you will be operating it! Tuning results significantly depend on installation location.  

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Hook up the analyzer and sweep from, say, 6-8 MHz. You should find a nice, sharp resonance dip in the SWR curve.

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Determine where you want the resonance frequency to be, keeping in mind that the antenna has a nominal bandwidth of about 45 kHz between the SWR=2 frequencies. At the resonance frequency, you should be able to operate the antenna without a tuner. On 40 mtrs, I primarily operate with digi-modes (Hellschreiber, PSK31). I decided to try and tune the resonance frequency to the boundary between the narrow-band and wide-band segment of the 40 mtr band: 7040 kHz.

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As the coils have more turns than we expect to need (it is easier to cut wire off than to cut it "on"), the resonance frequency should be below the low end of the 40 mtr band frequency range. Reducing the number of coil turns will increase the resonance frequency.

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Turns are removed at the end of the coils nearest the copper pipe. The same number of turns must be removed from both coils.

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Assuming the initial resonance frequency is indeed too low (if not, something has gone wrong!), we'll remove one turn from both coils. This will also give us a feel for how much the resonance frequency shifts per removed turn. This isn't a constant delta-frequency! The increase-per-removed-turn goes up as more turns are removed. I have observed 35 to 55 kHz shift per removed turn.

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With my second 40 mtrs KGD, I started with a generous 100 turns, and the tuning rods sticking out about 85 mm. Resonance frequency was 6680 kHz, SWR 1:1.1, and Rs=49 ohms.

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Removing 2 turns raised the resonance frequency to 6749 kHz (about 35 kHz per turn). SWR and R hardly changed.

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Removing 3 more turns (now down to 95) raised the resonance frequency to 6871 kHz (about 41 kHz per turn). No change in SWR, R went down to 47.5 ohm.

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Removing 3 more turns (now down to 92) raised the resonance frequency to 7000 kHz (about 43 kHz per turn).

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I could have removed one more turn, but I did not want to overshoot my target frequency. So, I pulled up the resonance frequency by screwing in the tuning rods until only 68 mm was left sticking out. With that, the resonance frequency was right at 7040 kHz, though SWR ended up at 1:1.5 and R at 53 ohms.

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one of the two rods was a little hard to turn. I applied one (!!) drop of gun oil (high viscosity ( = "thick") penetrating oil for firearms and fishing reels) right where the rod enters the insert. After turning the rod back & forth a couple of turns, this did the trick.

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I have left it at this for the time being. Of course, I could have removed one more turn, and pull down the resonance frequency by screwing the tuning rods outward as necessary...

 

I have also measured the resonance frequency, SWR, impedance, and bandwidth as function of the insertion/extension of the tuning rods (0-165 mm, in 5 mm steps). See the plots below. Note: these plots are for vertical installation, and with a total of 13 mtrs (40 ft) of RG58A/U coax! The resonance frequency varies linearly with the rod position, until the (20 cm) rods are sticking out of the copper tubes by more than ≈12 cm. For more than ≈15 cm, the resonance frequency actually starts going up again! The SWR is constant at ≈1.5, up to ≈12 cm rod extension. Given the observed |Z| and Rs, it appears that the value of the capacitor is basically correct.

 

 

 

 

 

 

Now the antenna has been tuned, we are ready for action. Keep in mind that this is not an antenna for "QRO" high transmit power, transmitter with an "afterburner", and other forms of waste of energy. The photos below show you what will happen if you overheat the antenna by continuously applying more than 50-70 watts. But who needs that much power anyway? You can work digi-modes fine with 30-50 watts.


When operating several minutes with 80-100 W, the coils get hot enough to soften the PVC
(the PVC is rated for 90 ºC (200 ºF) max)

After having built the antenna on a rainy afternoon and cleaned up the tools etc., I hooked up my miniVNA antenna analyzer. The resulting plot for this original set-up is shown below. It was not good! Resonance frequency about 400 kHz too high, and bandwidth appeared too wide (low "Q") - this turned out to be an "operator failure". Below are my notes of how I went about fixing things. They are in reverse order (latest notes first, for those who are impatient).

 

Update 12 October '09: rebuild

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Have built a new coil core and wound the coils with AWG #20 enameled copper wire (0.8128 mm diam.)

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Started out with 100 turns to have some margin. After tuning, I had the same number of turns as my first antenna with 0.8 mm wire: 92. The brass rods also have about the same insertion depth as before: 68 mm sticking out, instead of 85.

 

Update 27 August '09: melt-down!

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Had another long QSO last night with Rolf, DF7XH. Again to try out antennas and transceivers. At one point we could hardly copy each other, so I decided to more than double my output power to about 80-100 W (100% duty cycle mode). After a couple of minutes I noticed that my SWR jumped off the scale! I went outside to inspect the antenna and found it had folded over into a quasi-inverted-V dipole configuration: the coils had gotten hot enough to soften de PVC tube! Rolf now disrespectfully refers to my KGD-antenna as a "Schmelzantenne" and "Kurz Gebratener Dipol".

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So "do as I say, don't do as I do" and keep continuous power down to about 50 W max.

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As they used to say at the beginning of each episode of the 1970s TV series "The 6 million dollar man": "Gentlemen, we can rebuild him. We have the technology." So, it's time once again to gather materials and make a new coil-core... Maybe I'll try aluminum tube this time...

 

Update 20 August '09: Success!

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Late this afternoon, I fired up the antenna (7040 kHz, PSK31 mode, 30 W, no tuner), and called "CQ antenna test". After just a couple of calls, it was my friend Rolf, DF7XH, who answered. He was playing around with the Drake gear that he is reconditioning, and had stumbled upon my signals. Conditions at that time of day were not great, but our signals were strong. I have had QSOs with Rolf on 40 and 20 mtr before, using my Cobra multi-band dipole. That was a struggle. Today he could not hear me with the Cobra, but with the KGD copy was solid! Distance: 760 km (≈475 mi). Zero sunspots.

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We also tried my KGD in horizontal position, but could not copy each other (note that the KGD-manual recommends vertical orientation for installation close to the ground). Rolf successfully uses an "Up & Outer" antenna. Descriptions of that antenna are here and here.

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I am rather pleased! Try-outs will continue...

 

Update 15 August '09:

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I played around with the tuning-rods. When inserted half way, the the resonance frequency changed about 15 kHz per cm of insertion.

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I concluded that the coils did not have enough turns. I cut and drilled a new section of PVC tube, and wound the coils (in opposite directions), with 95 turns instead of 85.

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Resonance frequency with half of the tuning-rods inserted: 6841 kHz. Too low. But SWR very close to 1, and an SWR=2 bandwidth of 40 kHz.

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I reduced the number of turns by 3; at 94 turns, fres was 6903 kHz, and at 92 turns 7017 kHz: about 60 kHz per turn. Reducing one more turn would overshoot my target frequency, so I left it at 92. SWR=2 bandwidth is 44 kHz.

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Played with the tuning rods to see their effect. Pulling them out from 10 to 11 cm reduced fres to 7004 kHz - the wrong direction! Inserting 11½ cm (8½ sticking out) increased fres to 7039, right where I wanted it to be. Now the SWR=2 bandwidth is 43.6 kHz and SWR = 1:1.06. See plot below. Pretty much as per the spec!

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I added my 1:1 current choke at the end of the 15 mtr coax. The fres went up by by 6 kHz, and SWR increased to 1:1.46, see plot below. This still needs to be explained...

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The antenna analyzer plot goes all over the place if the coax is moved near the antenna; as the manual says: guide the coax away from the antenna at a right-angle, for at least 1 m.

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Hooked up the antenna to my transceiver. The received signal strengths are really quite impressive (compared to all the other home-built compact antennas that I have tried).

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As to be expected, I did not need an antenna tuner for frequencies around fres. The SWR meter of my antenna tuner claims an SWR of 1.5, but tuner-SWR indications are not known for accuracy...

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Will try it out as a transmitting antenna next, and see if anyone can hear me! TX performance will be reported here shortly.

 

Update 14 August '09: I have rewound one of the two coils into the opposite direction, and inserted the tuning rods. Measurement results and observations:

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my miniVNA connected at after the 1 m coax shows a SWR=2 bandwidth of 42 kHz, i.e., a respectable Q of 166. I hate to admit it, but the the bandwidth and Q that I reported previously where completely wrong. I assumed that the BW and Q indication of my antenna analyzer GUI was automatic, once the "BW and Q enable" box was checked. Well, it is, but not driven by the automatically determined "lowest SWR" frequency, but purely based on the two marker frequencies that are manually selected by the operator - me... Looking at the original screen capture below, I find a BW of 45 kHz and a Q of 166 (after 15 mtrs of coax).

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So, my PVC tubes are suitable for HF applications.

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The old adage holds true: "a fool with a tool is still a fool!"

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Based on the measurements, my conclusion is that it doesn't make a difference whether the coils are wound in opposite directions or not...

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Now I have to put the thing to the test!

 

Update 10 August '09: I have consulted with the kind fellow-hams of the QRPproject forum. Suggestions and observations so far are that:

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the particular capacitor that I used may not be suitable for HF.

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the PVC that I used may not suitable for HF-coils (should use polyethylene, nylon, ...)

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due to capacitive charge presented by the metal tubes, the tube diameter is a critical dimension.

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I may have reduced the number of coil turns too much.

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should be able to reduce the resonance frequency by adding the tuning rods at the tip of the tubes.

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the original design calls for the two coils to be wound in opposite directions. I.e., one clockwise, the other counter-clockwise. In this respect, the dipole does not have two identical halves!

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the tuning rods must not make contact with the copper tubes.

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check the frequency-marker settings (I should have heeded this comment!)

I briefly repeated the analyzer sweep with a 1000 pF capacitor. The resonance frequency did not change (this confirms what is pointed out in the manual: the capacitor is there to adapt the antenna impedance to the coax, not for resonance). However, the bandwidth increased four-fold (really bad) and of course the Q reduced correspondingly. Also, the real portion of the impedance has gone down by a factor of 2 (from 32 to 16 ohm). Looks like I should try 330 or 390 pF to get to 50 ohm.


Sweep of the antenna with its final dimensions and 15 mtr coax


Sweep of the antenna with its final dimensions, 15 mtr coax and a choke-balun

 
Sweep of the antenna response for the prototype (too few coil turns) set-up and 470 pF
(I originally made the big mistake to misinterpret the indicated BW and Q values)


Sweep of the antenna response for the original set-up
- with a 1000 pF capacitor

 

I have also constructed the 20 meter version, just for fun, because 1) it is easy, 2) I was curious to see how well it performs, and 3) because I still had the coil core and coils of my first prototype 40 meter KGD. Using the formula above, I would need only 35 turns. Based on the final number of turns of my 40 meter version, I would end up with 38 turns. I started with 40, but this turned to be still too few (or the copper tube to short). With the tuning rods sticking out 60 mm, the resonance frequency was close to 14900 kHz, SWR about 1.6, R about 33 ohms (the latter could also be caused by tolerance of the 200 pF capacitor). Pulling out the rods to 150 mm lowered the resonance frequency to about 14400 kHz, and SWR to below 1.2. So I made a new coil core and used longer copper tubes.

 


The final dimensions of my 20 m KGD antenna
(same coil parameters for 0.8 mm copper wire and AWG #20)

 

The list of components for the 20 mtr KGD is basically the same as for the 40 mtr KGD, except that:

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the PVC tube is now 22 (≈ 9") cm long

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the sections of copper pipe are only 50 cm (20") long

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the capacitor is 220 pF (at least 5 kV rating)

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a total of about 7 mtr of 0.8 mm wire is used for the coils

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tuning rods are 25 cm (10") of 4 mm brass rod (not threaded)

 

Again, I added a 75 cm section of 15 mm diam. PVC tube to guide the coax perpendicularly away from the coil core. See KGD-40-FD construction above.

 

Dimensions of the PVC-tube center section - final design for my 20 mtr version

 

 


The components of my 20 mtr KGD prototype

(coil core and coils salvaged from my prototype 40 mtr KGD (35 cm Cu tubes; ruler scale in cm)
 

 

Update 6 Feb 2010: successful test!

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Of course, once tuned, I had to put the antenna to the test. I positioned the tuning rods for 14230 kHz, and used my antenna tuner to tune the residual SWR from 1.5:1 to 1:1. Very easy! I turned the antenna such that the legs are pointing east-west. Assuming a standard dipole radiation pattern, the lobes would be north-south.

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Now, how do you a reasonably objective test the antenna all by yourself? Simply by transmitting SSTV images and checking one of the standard SSTV webcam sites. I.e., use a remote receiver. I used the GØHWC SSTV-cam  (others that I use are here (AE6JN), here (WD7F) and here).

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I was very (!!) pleasantly surprised to find that the receiver of OZ5AGQ had actually received me, see image below. His station is located about 25 km northwest of Copenhagen, some1560 km (≈ 1000 miles) north of my QTH (25º E).  So the antenna works quite nicely for its size! Note that this test was done at noon time, with the antenna horizontal, and placed close to (and on the south side of) my apartment.

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One more observation: as pointed out before, these KGD antennas are for low-power! I transmitted with about 50 watts in Scottie 2 mode; after a single transmission (70 sec), the antenna coils do get warm to the touch!

 

 

 

Update 30 Jan 2010: final

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 I have built a new coil core with plenty of turns (2 x 44) and 2 x 50 cm copper tubes. I want to be able to use the antenna for narrow-band digi modes (14060-14080 kHz) and SSTV (14230 kHz). So I increased the resonance frequency until it was slightly high. See table below. The resonance frequency increased170 kHz (at 44 wdgs) to 250 kHz (at 33 wdgs) per reduction by one coil turn.

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Tuning down to the desired frequency is then done with the brass tuning rods. Last month I figured out an easier way to fix the tuning rods in place. It can be used with both threaded rods and smooth rods. It is a simple plastic strain-relief fitting for cables (UK: "cable gland", D: "Kabelverschraubung"). See photo immediately below. The ones that I use fit snugly into the copper pipe, and hold a non-threaded M4 brass rod firmly in place when tightened. This allows for very simple and quick adjustment, a lot easier than tediously screwing in/out a threaded M6 rod (the "official" design). I have since added small rings of shrink tube to the smooth rods, to mark preferred rod positions.

 

Update 28 Mar 2010: had a very solid Hell QSO with Jan, SP3AMZ (1465 km, 60 deg NE)). Last night, I nearly had a QSO with a station near Seattle (8600 km, 325 deg)

 


A tuning rod and strain-relief fitting

 


The two tuning rod constructions side by side

 

 

Turns

resonance freq (kHz)

SWR

Rs

Polarization

Tuning rod insertion

44

11660

1.09

54

H

4 mm   125 mm out

44

11708

1.17

57

H

M6    100 mm out

44

11680

1.56

72

V

M6    100 mm out

44

11840

1.29

59

V

4 mm   6cm out

44

11850

1.3

60

H

4 mm   6cm out

44

12130

1.4

60

H

none

44

12130

1.46

63

V

none

44

12130

1.16

43

V

none

41

12632

1.27

47

H

none

41

12160

1.23

42

H

4 mm   125 mm out

41

12074

1.27

40

H

4 mm   170 mm out

41

12188

1.25

42

H

M6    100 mm out

41

12634

1.29

44

V

none

36

13600

1.56

53

V

none

36

13624

1.5

55

H

none

34

14112

1.6

67

H

none

34

14118

1.75

69

V

none

33

14333

1.21

60

V

none

33

14342

1.4

67

H

none

33

14066

1.22

58

V

4 mm    54 mm out

33

14208

1.22

58

V

4 mm   22 mm out

Tuning data for the final KGD-20-FD
(via 40 ft coax, current choke)
 

 

Update 19 November 2009: experiment

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As stated before, the KGDs are rather narrow-bandwidth antennas. On 20 mtrs, I'd like to be able to work on two frequencies: 14230 kHz (SSTV) and around 14070 kHz (Hellschreiber). So I decided to add a switch to each of the coils of my prototype KGD-20-FD. The idea being to short out several turns in each coil, thereby shifting the resonance frequency. The photo below shows the switches installed across the last four turns of each coil. No significant voltage is induced across these turns, so no special switches required.

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As shown in the table below: a frequency shift of 1.45 MHz (!) for shorting the last four turns of each coil. The switches work, but the antenna has to be tuned for the lower resonance frequency of interest, with the switches installed! I am only looking for a shift of 170 kHz, so I have to see if this can be achieved with shorting fewer turns or even a partial turn. Tricky!

 


Influence of the coil-shortening switches on the characteristics of the KGD-20-FD prototype

KGD-20-FD prototype nr.1 with coil-shortening switches across four turns
 

 
        No switches                  Both switches open            One switch closed             Both switches closed
(click on image to get full-size)
 

Fall '09:I have also tried to tune my prototype KGD-20-FD. The initial resonance frequency was already too high (14910 kHz, SWR 1:1.56, Rs of only 35 ohms). So reducing the number of turns wasn't going to help me. I could only play with the tuning rods.  Changing the rod position from 58 mm sticking out to 168 mm sticking out (the max), the resonance frequency went down to 14426 kHz, and SWR went down to 1:1.24. That is, the resonance frequency goes down when the rods are inserted. Note that this opposite to what I have observed for my 40 mtr KGD!. Oddly, lowest resonance frequency (14384 kHz) and lowest SWR (1:1.13) was obtained with the rods sticking out 150 mm. It looks like I need to change one or more parameters: increase number of turns even more, or use copper pipes that are longer than the original spec. I am using copper pipes that have a smaller diameter than the aluminium pipes of the original design. The difference may have a more pronounced effect as the pipes become shorter... The low impedance may at least partially be caused by the actual value of the capacitor being high: the cap has ±20% tolerance... I have several more of these capacitors on hand, so I tried several: no significant change. Removing the 12 mtr coax cable and attaching the analyzer directly at the short coax of the antenna did change resonance frequency, SWR and impedance significantly! See table below. The table also shows that the capacitor value does not change the resonance frequency. Per design, it is only there for adapting the antenna impedance to the coax impedance.
 

 
Influence of coax and capacitor on characteristics of the 20 mtr KGD prototype


I am using my ZS6BWK multiband dipole as my primary antenna for the 80 mtr band. At my location, I can only use it in an inverted-L configuration, and cannot leave it installed permanently. So, why not give the 80 mtr version of the KGD a try? I am using standard 32 mm diameter PVC pipe, sleeves and end-caps from the Do-It-Yourself store. As the antenna "legs" are at least 1m6 long (≈ 5ft), I have opted for 2½ meter long aluminium tubing with an OD of 7.5 mm (5.5 mm ID), rather than copper pipe. Copper would be to heavy. I will tap an M6 thread into the pipe.

Update 27 December '09: as the photo below shows, with nearly 5½ meter (18 ft) span, the dipole is a little wide for my terrace (no, this was not a surprise!). Also, my "mast" is only 2 meters high, so vertical installation is also not (yet) an option. Note that the original KGD-spec prescribes vertical installation if the height is less than 1/8 λ. Obviously my umbrella stand is not 10 m tall...  Anyway, I hooked up the analyzer to get a quick impression: resonance (with an impedance quite close to 50 ohms "real") at about 1.6 MHz, below the160 meter band! Oh well...

While I was at it, I did some more experiments. I removed one "leg" of the dipole, removed the coax and capacitor, connected the coils in series, connected the center of the coax to the end of the bottom coil, and the braid of the coax to the down spout of the rain gutter (total length over 70+ meters (230 ft)). Now I can install it on top of the mast.

Got resonance at 2.058 MHz with an SWR of 1.14. Resonance frequency too low (coil has too many turns and/or tube too long). I clipped a wire across the bottom coil and measured again. Resonance had gone up to 2.922 MHz with an SWR of 1.1. Also noticed a second resonance dip at 10.666 MHz but with an SWR of 1.8. The table below shows the measurement results of other configurations. In case you wonder why I used a length of 7 meters for the radial wire: I just happened to have a 7 m section of household hook-up wire laying around.

It (finally!!) occurred to me that in the configuration with the rain gutter, the actual radiator is the gutter and the "antenna" is the counterpoise. Other than the low resonance frequency, the configuration on the first line of the table is actually pretty good (besides efficiency). None of the other configurations appealed to me, so I have decided to abandon this antenna for the time being. I did re-use the coil core and aluminium tubing in my 80 m short vertical - with success!

Coil turns

radiator length (cm)

resonance freq (kHz)

SWR

Rs

Counterpoise

Feedline

112

250

2922

1.06

49

gutter

coax

100

250

3116

1.17

56

gutter

coax

100

240

3165

1.33

60

gutter

coax

70

240

3776

1.5

74

gutter

coax

70

250

3721

1.6

77

gutter

coax

70

250

3548

2.7

20

radial 7 m

coax

70

250

3924

1.2

59

radial 7m

450 ohm + 4:1 balun

70

250

2972

2.3

42

gutter

450 ohm + 4:1 balun

70

250

2972 / 7840

2.3 / 1.12

42 / 51

gutter

450 ohm

70

250

3558

2.8

18

radial 7m

coax + choke

70

250

3832

1.3

64

radial 7m

coax + choke

70

250

3828

1.4

69

radial 7+2 m

coax + choke

65

250

3974

1.26

62

radial 7m

coax + choke

83

250

3802

1.1

46

radial 7m

coax + choke

111

250

3240

1.13

45

radial 7m

coax + choke

106

250

3328

1.12

46

radial 7m

coax + choke

98

250

3476

1.11

46

radial 7m

coax + choke

92

250

3610

1.2

41

radial 7m

coax + choke

94

250

3568

1.1

46

radial 7m

coax + choke

 

 

APRIL 2014: I have decided to give the KGD-80 another chance - the short hatted 80m dipole: the KGD-80-FD Mk 2

 

Original KGD-80 (kit):

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dipole legs: 160 cm each (5 ft 3"), 20 mm (≈3/4") diameter aluminium tubing

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loading coils: 258 turns on 20 mm diam. delrin rod; 120 μH according to coil calculators (ref. 2-4)

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coupling: beta-match coil, 2+2 turns

 

KGD-80-FD Mk2 - VERTICAL:

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dipole legs: 200cm each (6 ft 7"), 20 mm   (≈3/4") diameter aluminium tubing = standard DIY store in this area

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loading coils - V1: 1.5 mm diam CuL on 50 mm diam (2") PVC core

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loading coils - V2: 1.5 mm or 0.8 mm (#20 AWG) enameled copper wire on T200-2 iron powder core

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tuning rods: 50 cm (20") each, 4mm diam. brass tubing (rod would be OK, but heavier)

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center insulator: 20 mm delrin rod

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end-hat loading: 6 spokes/radials of 50 cm (20") length,  3mm diam. brass rod, with perimeter wire ("skirt") = 1.25 mm diam installation wire, stretched with bench vise (to straighten it)

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installation: horizontal 50mm PVC tube x 2m, 50 --> 32mm redux, 32 mm T, clips

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horizontal 300 ohm ladder line/window line to automatic antenna tuner

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coupling: beta-match shunt-coil, 4 turns; to be optimized empirically and/or via 4NEC2 simulation; simple, wide-band, low loss, reduces static charge ( = noise)  between dipole legs

 

Yes., I know: I am mixing metals (aluminimum, brass, stainless steel) and there will be bi-metallic corrosion effects at their junctions. But this antenna is experimental, and not a commercial product...

 

Adapter for mounting the dipole's center insulator to a 2m long horizontal section of PVC tubing

 

 

Insert plot: 4NEC2: vertical dipole, h = 2m4, 2 x 2m Alu tube, no coils, no hats

Insert plot: 4NEC2: vertical dipole, h = 2m4, 2x 2 m alu tube, no coils, hats

Insert plot: 4NEC2: vertical dipole, h = 2m4, 2 x 2 m alu tube, coils, hats

Insert plot 4NEc2: horizontal dipole, h = 2m4, 2 x 2m alu tube, same as above --> NVIS: high take-off angle

 

As stated above, the original design calls for for loading coils of 120 μH at the feed point. They are wound onto the 20 mm diam. insulator between the dipole tubes. I decided to try something else. I had two iron-powder toroidal cores from an antenna tuner project. They are type T-200-2 (i.e., 2.00 inch outer diameter and material nr. 2). Iron-powder cores are suitable for high "Q" inductors, and are hard to drive into saturation - unlike ferrite cores. Cores of "type 2" material are used in automatic antenna tuners such as the LDG model AT-100 (1 kW) and the Elecraft model KAT2.

 

Material "type 2" has an AL of 120: 100 coil turns should result in 120 μH inductance. These cores fit about 5 meters of 0.8 mm enameled copper wire. With 95 turns (and my winding technique) I obtained an inductance of 117 μH. Close enough.

Experimental loading coils: 95 turns on T-200-2 iron powder core = 117 μH

With these coils, the resonant frequency of the antenna (installed vertically, feed point about 240 cm off the ground) was 2.7 MHz. See the analyzer plot below:

Note that the SWR is 2:1, as I have not installed an impedance adapter at the feedpoint. I will experiment with a "beta match" coil between the dipole legs.

As the dipole legs are 0.4 m longer than the 1.6 m in the original design, it was to be expected that the resonance frequency is well below 3.6 MHz.

I also have two coils of 77 μH from an other dipole experiment. I will repeat the test with these coils.

Experimental loading coils: 42 turns of 0.8 mm (#20 AWG) enameled copper wire on 50 mm diam. PVC tubing = 77 μH

(according to various calculators, these coils should have an inductance of 67-71 μH; ref. 2-5)

 

Close-up of end-hat of six spokes of 50 cm (20") and tuning rod barely sticking out of the tube

 

I made a 4NEC2 model of this antenna, both with and without end-hats (ref. 8). The difference in current-distribution clearly shows the advantage of the end-hats: with the hats, the current does not taper to zero at the tip of the dipole legs. So the area of the surface that is delimited by the green current-distribution line and the pink dipolel legs, is larger then without the hats. 

Current distribution - KGD-80-FD without hats (left) and with hats (right)

 

 

Measured resonance frequencies vs loading coil inductance:

Interpolation --> 46 μH for 3.59 MHz; e.g. T130-2 with 65 turns, or T-200-2 with 62 turns (per ref. 3). Actual: TBD.

 

Beta-match coil (placed between the two dipole halves (ref. 6):

Clearly, physically the antenna is a symmetrical dipole. But due to the vertical installation, one end is much closer to the ground than the opposite end. This means that the loading by the ground is asymmetrical. This can be compensated by off-center feeding and/or by using a smaller loading coil in the lower dipole leg than in the upper leg (ref. 10).

 

Next gen: braze Alu rods onto Alu tubing with Alu welding rod and blowtorch, or welding. Avoid mixing metals, corrosion.

 

References:

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Ref. 2: "Helical coil calculator"

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Ref. 3: "K1QW Inductor Calculators"

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Ref. 4: "ON4AA Single-layer Helical Round Wire Coil Inductor Calculator"

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Ref. 5: "Turns-length calculator for ferrite and iron powder core toroids"

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Ref. 6: "The Beta Match: 2 Views", L.B. Cebik (W4RNL, SK)

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Ref. 7: "A triangle for the Short Vertical Operator" [hatted short dipoles], L.B. Cebik (W4RNL, SK)

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Ref. 8: 4NEC2 model of KGD-80-FD without hats and with hats (with loading coils and installed 2.4 m above average ground), , Frank Dörenberg, N4SPP, 25-May-2014

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Ref. 9: "Beta-match Coil Calculator for Known Antenna Impedance", Frank Dörenberg, N4SPP, 25-May-2014

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Ref. 10: "Vertical dipole for 40m (and higher)", Ed Bosshard (HB9MTN)

One of these days I will go back to playing with KGD antennas. I have a  projects in mind: remote motorized tuning of the resonance frequency. motorized tuning: the tuning rods would be moved in & out of the tube ends. Alternatively, a tuning core (ferromagnetic material, or brass rod) could be moved in & out of each coil

©2009-2010   F. Dörenberg N4SPP

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