© 2010 F. Dörenberg N4SPP

 

One of the prime design parameters of loaded antennas, is the location of the loading coil between the feed point and the tip of the radiator. One option is to put the coil at the feed point of the antenna, i.e., at the base. I have built a number of base-loaded short vertical antennas for 80 mtrs. They are discussed here.

 

The advantage of base-loading is that the construction is easier and more robust: the higher the coil is installed, the more top-heavy the antenna becomes. The antenna's current-distribution is sinusoidal, with the maximum current at the base (feed point) and zero current at the tip (maximum voltage).

 

This means that the required antenna lengthening-inductance ("loading") is the smallest when the coil is placed at the base. I.e., the smallest required coil. However, maximum current also implies maximum coil losses (and heating). So especially at this location, it is important that the coil has a high "diameter vs. length" ratio, heavy coil wire gauge, possibly even silver plated.

 

Obviously, we can put a loading coil anywhere between the base and the tip of the antenna. As the coil is moved farther away from the base, the required loading inductance goes up. At the tip of the antenna, infinite inductance is required. This is the point where inductive loading is useless, but capacitive loading ("top hat") is most efficient. The most efficient inductively loaded antenna is obtained when the coil location is about half way between the base and the tip. However, when the antenna is much shorter than 1/4 wavelength, the taper of the current distribution becomes less pronounced, and hence the placement of the loading coil is less of an issue. Obviously, inductive and capacitive loading can be combined.

 

With material left over from my base-loaded designs, I have decided to make a center-loaded vertical for 80 mtrs. I have a section of 1.5 m copper tube (16 mm OD), and 2.5 m aluminium tubing (7½ mm OD). OK, so the coil will not be exactly at the half-way point of the 4 mtr "short" antenna. And: yes, I know, I will be "mixing metals", which is not optimal from a corrosion point of view. I  will use my standard PVC "mast".

Articles:

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Ref. 1: "Loading of short antennas" by  Doug/WB6BCN

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Ref. 2: "How Does an Inductor or Loading Coil Work?" by Tom, W8JI

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Ref. 3: "Vertical antennas", by Ulli Weiss, DJ2YA

I determined the required coil value via the K7MEM on-line calculator. Note that the calculator is for a straight line, flat top dipole, sufficiently placed in "free space". A vertical "mono-pole" is just half of a dipole, and uses the same loading inductance as the dipole.

 

Another on-line loading coil calculator is here.

 

Based on an on-line calculator for helical coils, the required 96 μH coil should take about 47 turns of 0.812 mm CuL (AWG #20) on a 55 mm core (0.12 mm inter-winding space, which is what my hand-wound coils tend to have with this wire gauge). Of course, I will start with more than 47 windings: it is easier to remove windings than to add!

 

Estimated coil parameters

 

As usual, I will use PVC part from the do-it-yourself store. I will use a 2 mtr tall PVC mast with 40 mm outer diameter (OD). The 16 mm OD copper tubing easily fits into a short section of 32 mm  PVC tubing (smaller diameters are too flexible). So the PVC base of the antenna begins with a 40-to-32 mm adapter piece. To prevent the antenna falling out of the bottom of the base, I glued in a PVC disk that I ground out of a 32 mm end-cap (eft-hand corner of photo below). It has drain holes and a center hole, for the bottom bolt of the Cu tubing. The top of the PVC base consists of a 32 mm female-to-female connector and a 32 mm end-cap. A made a 16 mm hole in the end-cap, so the Cu tubing can pass through it. It has to be slid onto the Cu tubing before soldering the brass end-caps onto the latter.

 

Components of the PVC base

 

The radiator comprises a 1.5 mtr section of Cu tubing, and a 2.5 mtr section of Alu tubing. Both ends of the Cu tubing will have a brass end-cap soldered onto them. The end-caps have a 6 mm hole drilled into their center. A 6 mm bolt is mounted into each end-cap, with flat washers and lock washers. Use hex head screws and a socket wrench, not the round heads shown in the photo below. Once soldered on, you can no longer get to the screw heads, so they have to be really tightened.

 

Components of the copper and aluminium radiator sections

 

Base of the copper tube radiator, end-cap soldered on, wire bolted on

 

The coil is wound onto a 50 mm female-to-female PVC connector. It has an OD of 55 mm. A 50 mm OD end-caps are installed into this connector. Both have a 6 mm hole in the center. The bottom cap will receive the bolt that is installed at the top of the Cu tubing. The end-caps have a screw-on cap. The bottom screw-on cap is PVC-glued onto its threaded ring. The ring is then PVC-glued into the 50 mm sleeve.

 

The top cap will get a 6 mm bolt that stick out the top. The Alu radiator will be screwed onto it. A deep 6mm thread has to be tapped into both ends of the 7.7 mm Alu tubing. Large washers are used on the inside and outside of the end-caps. This is for reinforcement purposes.

 

Components of the coil core

 

Three rows of small holes are drilled into the 50 mm PVC sleeve. The wire end at the start of the coil wire (on the right in the drawing below - this is the bottom end of the coil) is woven through the single set of three holes. The wire is pulled tight through these holes; this helps secure the windings.

 

Holes in the coil core, for fixing first & last coil windings in place

(the bottom of the coil core is on the right)

 

The coil is tightly wound by hand. The wire at the opposite end of the coil is woven through the nearest set of three holes. The first winding of the coil (on the right in the drawing) is glued in place. I use waterproof super glue for this. The last winding (on the left) will not be glued in place until the coil is tuned - by removing windings.

 

The first coil winding is fixed in place with waterproof super glue

(the final winding is not glued until the coil is tuned!!!)

 

A brass 6 mm flat washer is soldered to each wire end:


The loading coil, before removing windings during tuning process

 

At this point, the bottom end-cap is glued into the bottom end of the coil core. The bolt at the top of the copper tubing is passed through this end-cap, the brass washer at the bottom wire end of the coil is slid over the bolt, then a lock washer, and then an M6 nut. The nut is tightened solidly with a socket wrench. Note that there is a large flat washer at the inside and outside of the PVC end-cap.

 

An M6 bolt is passed through a lock washer and the brass washer at the top end of the coil, then through the end-cap. There is a large flat washer at the inside and outside of the PVC end-cap. A lock nut is fully tightened onto the outside of the end cap. The end-cap is inserted into the PVC coil core. Do not turn the end-cap - or you will break the coil wire!!). As the end-cap will be removed several times during the coil tuning process, this end-cap is not yet glued in. it is held in place with 3 or 4 small self-tapping screws (small pre-drilled holes). See photo below.

 

Loading coil installed between Alu and Cu tubing parts of the radiator

 

A hole into the side of the 32-to-40 mm PVC adapter at bottom of the PVC base is for passing the wire at the bottom of the copper tube.

 

Base of the antenna 

 

At this point, the coil has 59 windings, quite a bit more than the calculated 47. The antenna is installed on top of a 2 mtrs section of 40 mm PVC tubing, stuck into a heavy umbrella stand. I use my standard single-radial of 7 mtrs, installed horizontally at 2 mtrs above ground level.

 

With my miniVNA analyzer I measured the following:

 

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with 12 mtr 300 ohm window ladder line, no balun:
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resonance frequency: 2710 kHz, SWR = 1.9, Rs=70 ohm, Xl=30 ohm

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same, with 4:1 balun at transceiver end of the feed line:
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resonance frequency: 2773 kHz, SWR = 1.9, Rs=40 ohm, Xl=30 ohm

 

After reducing the number of windings by five to 54:

 

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with 12 mtr 300 ohm window ladder line, no balun:
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resonance frequency: 2858 kHz, SWR = 1.93, Rs=85 ohm, Xl=23 ohm

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same, with 4:1 balun at transceiver end of the feed line:
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resonance frequency: 2925 kHz, SWR = 1.4, Rs=41 ohm, Xl=20 ohm

bullet with 12 mtr RG58 C/U coax, plus choke:
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resonance frequency: 2953 kHz, SWR = 1.22, Rs=51 ohm, Xl=10 ohm

 

Reducing the number of windings by five, increased the resonance frequency by 140 kHz, about 28 kHz per winding. I have plenty of margin, so I decided to reduce the number of windings by ten, to 44:

 

bullet with 12 mtr RG58 C/U coax, plus choke:
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resonance frequency: 3340 kHz, SWR = 1.2, Rs=51 ohm, Xl=8 ohm

 

Reducing the number of windings by ten, increased the resonance frequency by 387 kHz, about 39 kHz per winding. Note: the inductance varies fairly linearly with the number of windings, but the resonance frequency varies with the square of the inductance!

 

My personal "target" frequencies in the 80 mtrs band are 3578 and 3604 kHz. I gambled and removed another five windings. Now at 39 windings:

 

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with 12 mtr RG58 C/U coax, plus choke:
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resonance frequency: 3610 kHz, SWR = 1.25, Rs=53 ohm, Xl=10 ohm

 

Reducing the number of windings by five, increased the resonance frequency by 270 kHz, about 54 kHz per winding. The resonance frequency now is just above my upper target frequency! Darn, did I remove one winding too many? Well, no! Yes, I did get lucky, as it is only slightly above the upper target. With an integer number of windings, it is impossible to get right on target. Furthermore, I have two target frequencies, spaced 26 kHz. Also, the resonance frequency will vary with temperature, etc.

 

A slightly high resonance frequency is equivalent to a slightly short radiator. This is easy to fix! Both ends of the aluminium radiator section have an M6 thread in them (I have done this before, hi). The top end is open. An M6x80 screw will serve as tuning rod. This works very well!

 

Tuning bolt, screwed into the tip of the radiator 

 

By adjusting the amount of screw sticking out the tip of the antenna, the resonance frequency can be reduced as desired. The plot below shows an SWR=2 bandwidth of about 100 kHz. The residual SWR is easily reduced with an ATU.

 

SWR + impedance plot , made with my miniVNA analyzer

(measured with 1 radial of 7 mtrs, 12 m RG58 C/U coax, choke balun)

 

The antenna, held by its creator  

 

The antenna, installed atop a 2m tall PVC mast
(radial + coax also visible)
  

 



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