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THE 80 MTR SPIRAL LOOP - INTRODUCTION

One of my 2008 antenna projects was a square spiral loop antenna for the 80 meter band. The original design is from Harry, SM0VPO (ref. 1). So far I had really not been active on 80 meters, due to antenna limitations and its perceived suitability for DX (I am not interested in local rag chewing). So I thought I'd give this one a try.

This antenna is very compact: ≈ 120x120 cm (4x4 ft). It is basically a transformer loop antenna: it consists of a tuned loop and a small feed loop. The tuned loop is spiral-wound and square, has about ¼ λ total wire length. It is terminated with a 25 pF variable capacitor. The small coupling-loop (feed loop) is triangular, with a wire length of about 0.03 λ (≈2m75, ≈ 9 ft). The latter is center-fed and connects directly to 50 Ω coax.

Spiral loop

The original design by Harry (SM0VPO)

(source: ref. 1)

A "high voltage" type tuning capacitor is required, as very high voltages will be generated across the capacitor at resonance! The original design calls for a 25 pF variable capacitor. I used what I had in my junk box: a decent 5-15 pF trimmer capacitor. This was fine for initial measurements with my antenna analyzer and low power tests (10 W). My trimmer has 1 mm distance between the plates, so it should be OK for about 1 kV. I later bought several 5-75 pF high voltage air variable capacitors (Voltronic model NTM70 6E). They are quite compact: 7½ cm long (3"), 1.6 mm diameter (5/8"), and it takes 120 turns of the shaft for end-to-end travel. I got two with the standard rating of 3 kV (6kV peak), and two tested to 15 kV. Mid-2009 pricing was a reasonable $29 and $39 each, respectively (plus $13 for shipping them to my QTH in France). Now I have to design and build a remote-control motor drive for them...

Spiral loop capacitors

My 5-15 pF trimmer and 5-75 pF high-voltage air variable capacitor


Spiral loop

Ski partol with portable transceiver (1924) and "Schutzpolizei" in Berlin (1932) with receiver EA985 of Radiofrequenz G.m.b.H.

(source: Bundesarchiv Bild 102-10989 and 102-13262)

Spiral loop

Mobile loop antenna

(source: unknown)

CONSTRUCTION

I used the following components to build my spiral loop:

  • 25 meters of 0.75 mm2 insulated wire ("zip wire", AWG 20 or 21, not critical); as suggested by Harry, I split 13 meters of 2x0.75 mm2 household hook-up wire (zip wire), and made a 26 m wire out of it.
  • 25 pF high-voltage variable capacitor (at least several mm separation between the plates).
  • 2-hole household terminal strip (UK: "lustre terminal", "screw terminal"; D: "Lüsterklemme", F: "domino".
  • BNC or SO-239 coax connector.
  • 2 sections of (at least) 180 cm (6 ft) PVC tubing, at least 16 mm (5/8") diameter, from your local Do It Yourself or building supply store.
  • PVC T-piece, for 5 cm (2") diameter PVC pipe.
  • 6 meters of thin mason's string (e.g., 1 mm diameter non-stretch/pre-stretched multi-strand dacron or similar; nylon stretches too much!).
  • A short piece (5 cm) of shrink tube for the antenna wire.
  • A couple of tie-wraps.

Spiral loop

The main components, other than the "arms" of the square loop


Construction is simple. Drill 3 mm holes through the 16 mm pipe, at the distances from the center as marked in the diagram below. At the very tips of the pipe, drill a small hole through the pipe, in a direction perpendicular to the other holes. This will be use to connect the tips with a mason's string, to help the pipe "cross" from changing its shape when we install the antenna wire (the tubes are flexible).

Spiral loop

Make a hub for the crossing arms of the antenna structure. I used a PVC T-piece for this, intended for PVC pipe with a 5 cm (2") diameter. All my home-brew antennas are low-weight, and my "mast" is a section of 5 cm pipe. Drill the holes such that it is a tight fit for the 16 mm Ø PVC pipe! I pre-drill with a 15 mm drill (start out with smaller diameter, then increase to 15 mm), and finish with a conical grinding bit.

Spiral loop

Spiral loop

The PVC T-piece, with holes pre-drilled to 15 mm and finished with the conical grinding bit


Then slide the PVC tubes through the hub and center them. Turn the pipes such that the drill holes in each pipe face the other pipe. Next, use the mason's string to connect the tips of the pipe - the "arms" of the cross must remain straight, and form a cross, such that the loop is square. This helps a lot when stringing the antenna wire through the arms!

Note that if you make the cross brace and hub out of bare wood, the antenna will de-tune when the wood gets moist (dew, rain,...).

Spiral loop

The square cross-brace, spreaders fixed in place with mason's string


Tie a knot in the antenna wire, about 25 cm (≈10") from the end. Thread the other end through the outer-most 3 mm hole in one of the arms. From there, thread it through the outer-most hole in the adjacent arm. Put some tension on it (not a lot!). I clamp a hemostat on the wire, right at the "exit" hole. If you don't have one of these medical instruments, you can improvise with small pliers and a heavy rubber band on the handles. Now thread the wire the outer-most hole in the adjacent arm, hold the tension and move the hemostat or pliers to this "exit" hole. Repeat until you have made a 5-turn loop. When you put some tension on the wire, the arms of the cross will bow; this is OK and actually helps maintain tension.

Leave about 25 cm wire sticking out of the final hole, and cut off the rest of the wire. Slide the piece of shrink tube over the wire, all the way up against the final "exit" hole (see bottom of photo 5), and - while maintaining tension on the wire - shrink the tube with a hairdryer or soldering iron.

Attach your variable capacitor somewhere on the pipe section that has the start and end of the loop wire. I used a couple of tie-wraps for this. Trim the wire ends to length and connect them to the trimmer. I also fixed the wire ends to the pipe with some tie-wraps.

Now install the feed loop. This is a small triangular loop. A right-angled isosceles triangle, to be precise (two equal-length sides and one 90º angle). It has two sides of 80 cm (79 in my case), and a hypotenuse of about 114 cm. So we'll need about 3 meters (10 ft) of antenna wire. We need to make two tie-offs out of mason's string, such that the 45º angles of the feed loop are at a distance of 4 cm from the arms. The tie-offs are tied to the innermost 3 mm hole in the arm (i.e., the one closest to the hub). String the antenna wire through the innermost 3 mm hole of the arm that has the start and end of the spiral loop. Position it such equal lengths of wire stick out of the pipe. Thread each wire end through a 5 cm long tie-off, put some tension on them, and lead them to the hub. Mount the terminal block on that same pipe, close to the hub. Cut the wire ends to length, strip off ½ cm of insulation, solder the wire, and mount them in the terminal block. Mount a coax connector just above the terminal block, and wire it to the same terminal block.

Spiral loop

A trimmer capacitor closes the spiral loop


Spiral loop

Attachment of the feed loop


Spiral loop

Close-up of the "hub" of the antenna


Spiral loop

Close-up of the "hub" of the antenna


That's all! Now you can erect the antenna. My antenna weighs 800 grams. When the construction is complete and checked out, I may house the capacitor in a small waterproof box, and weatherize all electrical connections with a good coat of Dip It®/Plasti Dip®.

Spiral loop

The 80 mtr spiral loop antenna erected on my terrace

(one of my dipoles crossing in the background)

In 2011 I found the following mounting hardware in the furniture section of my local do-it-yourself store. They are intended for joining 4 or 5 tubes with a diameter of about 2.5 cm (1"). They can be used for making a hub for a spiral loop or some other antenna:

Spiral loop

MEASUREMENTS

For starters, I hooked up my miniVNA antenna analyzer directly to the coax connector at the antenna. I measured the SWR and R of the antenna, both for the trimmer at its minimum and maximum value. I could easily tune it between 3.64 and 4.3 MHz. See plots below. These plots show a very nice resonance dip of the SWR - no tuner required. What more could you ask (other than radiation efficiency)? SWR=2 bandwidth appears to be several 100 kHz.

As I am interested in the low end of the 80 meter band (3550 - 3650 kHz), I will need to get a capacitor with a larger max value than the trimmer I used for this experiment.

The second set of plots below show a full HF sweep from 1.5 to 30 MHz. With the antenna outdoors, there are additional dips at multiples of the base resonance frequency. As to be expected, they don't coincide with 50 Ω impedance frequencies.

Spiral loop

My miniVNA antenna analyzer hooked up directly at the antenna


Spiral loop

Resonance with low SWR at 3.65 MHz (trimmer at max) and 4.2 MHz (trimmer at min)

(ignore the bandwidth/Q indications, they are not valid as I did not correctly select the marker frequencies)

Spiral loop

Sweep from 1.5 to 30 MHz - indoors (left, directly at the antenna) and outdoors (right, via 15 mtrs of coax)


Spiral loop

Resonance with low SWR at 3.578 MHz


Note: I have not done experiments with the spiral installed horizontally instead of vertically, nor have I tried to see if there is any effect (polarizaion?) when rotating the vertical spiral.

Important: ideally, like dipoles, this type of antenna should be installed at least 0.1 λ above the ground.


MY 40 MTR SPIRAL LOOP

As described above, I built the 80 meters spiral loop antenna designed by Harry, SMØVPO. It is basically a mono-band antenna. As I am interested in adding a 40 meter antenna to my antenna collection, I decided to adapt the 80 meters spiral loop for operation on 40 meters. That's what I did in August of 2009. These antennas are easy to make and are inexpensive.

I made a quick-and-dirty Excel spreadsheet to play around with the dimensions (ref. 4). In the 80 meters version, the total wire length of the triangular feed-loop is about 0.03 λ. I decided to apply the same ratio for the 40 meter version. I also retained the distance of 5 cm between the innermost spiral winding and the feed-loop, as well as between the spiral loop windings themselves. The total wire length of the spiral loop is about 1/4 λ. Start and end of the spiral must be on the same spreader arm. The spreadsheet showed that this can only be done with a 4-turn spiral, instead of the 5-turn spiral of the 80 meters version.

The resulting antenna is not 50% but 33% smaller than the 80 meters version: it measures about 80 cm square, compared to 120 for the 80 meters version. Very compact indeed! It also connects directly to 50 Ω coax, and uses a 25 pF trimmer capacitor for tuning. With 450 grams, it weighs a tad over half of my 80 meters version.

The list of components for my 40 mtr version of the spiral loop is similar to the list for 80 mtr (differences in bold):

  • 14 meters of 0.75 mm2 insulated wire ("zip wire", AWG 20 or 21, not critical);  I split 7 meters of 2x0.75 mm2 household hook-up wire (zip wire), and made a 14 m wire out of it.
  • 15 pF high-voltage variable capacitor (at least several mm separation between the plates).
  • 2-hole household terminal strip (UK: "lustre terminal", "screw terminal"; D: "Lüsterklemme", F: "domino".
  • BNC or SO-239 coax connector.
  • 2 sections of (at least) 120 cm (4 ft) PVC tubing, at least 16 mm (5/8") diameter, from your local Do It Yourself or building supply store.
  • PVC T-piece, for 5 cm (2") diameter PVC pipe. (I used 5 cm diam. for my 80 meters version but decided to go down one standard size. Works just fine. A standard reduction piece adapts this to my 5 cm diam. PVC mast).
  • 4 meters of thin mason's string (e.g., 1 mm diameter non-stretch/pre-stretched multi-strand dacron or similar; nylon stretches too much!).
  • A short piece (5 cm) of shrink tube for the antenna wire.
  • A couple of tie-wraps.

Spiral loop


Spiral loop

The 80 and 40 meter versions side by side


In 2013, Mauro (IK1WVQ) built my 40 mtr design, and successfully used his antenna indoors with 15 Watt:

Spiral loop

The 40 mtr spiral loop of Mauro, IK1WVQ

(photos © Mauro, used with permission

The capacitor is home-made. It is a 1-rotor/1-stator butterfly capacitor made of two pieces of circuit board: it works!

Spiral loop

Butterfly capacitor made of circuit board

Spiral loop

The plot below shows the stations that Mauro communicated with in WSPR-mode over a 24 hr period:

Spiral loop

MEASUREMENTS

I hooked up my miniVNA antenna analyzer directly to the coax connector at the antenna. I measured the SWR and R of the antenna, both for the trimmer at its minimum and maximum value. I could easily tune it between 6.8 and 8.5 MHz. See plots below. Again, a nice resonance dip of the SWR, I did not need a tuner.

I did a very quick "sound check" with my receiver. Received signals were at least as strong as with my short Cobra dipole. I did not turn the antenna to get a feel for its directionality. The trimmer cap that I am using now has 1 mm distance between the plates, so it should be OK for about 1 kV. I have tuned the antenna with 70 watt and there were no signs of arcing. I'm not sure how high I can go without arcing and I don't want to destroy the trimmer.

Spiral loop

(ignore the bandwidth/Q indications, they are not valid as I did not correctly select the marker frequencies)

The second set of plots below show a full HF sweep from 3 to 30 MHz. There is an additional resonance dip around 23.5 MHz (3rd harmonic of the base resonance?), but I did not check where this one goes when changing the setting of the trimmer capacitor).

Spiral loop

Resonance with low SWR at 6.8 MHz (trimmer at max) and 8.5 MHz (trimmer at min)


Spiral loop

Sweep from 13 to 30 MHz - outdoors via 15 mtrs of coax


MY 20 MTR SPIRAL LOOP

As for my 40 mtr spiral loop, I used my spreadsheet calculator (ref. 4) to find the dimensions of the arms and the placement of the drill holes. Again, retaining the circumference of the triangular feed-loop at about 0.03 λ, I had to reduce the number of loops by one to three (from 5 for the 80 meters, and 4 for the 40 meters version). Again, going to half the wavelength reduced the size of the antenna by 1/3 to 60 cm square (down from 120 cm for the 80 meters, and 80 cm for the 40 meters version). Weight has gone down to 340 grams (from 800 grams for the 80 meters, and 450 grams (1 lb) for the 40 meters version).

The list of components for my 40 mtr version of the spiral loop is similar to the list for 80 mtr (differences in bold):

  • 7 meters of 0.75 mm2 insulated wire ("zip wire", AWG 20 or 21, not critical);  I split 3.5 meters of 2x0.75 mm2 household hook-up wire (zip wire), and made a 7 m wire out of it.
  • 15 pF high-voltage variable capacitor (at least several mm separation between the plates).
  • 2-hole household terminal strip (UK: "lustre terminal", "screw terminal"; D: "Lüsterklemme", F: "domino".
  • BNC or SO-239 coax connector.
  • 2 sections of (at least) 80 cm (2.6 ft) PVC tubing, at least 16 mm (5/8") diameter, from your local Do It Yourself or building supply store.
  • PVC T-piece, for 5 cm (2") diameter PVC pipe. (I used 5 cm diam. for my 80 meters version but decided to go down one standard size. Works just fine. A standard reduction piece adapts this to my 5 cm diam. PVC mast).
  • 4 meters of thin mason's string (e.g., 1 mm diameter non-stretch/pre-stretched multi-strand dacron or similar; nylon stretches too much!).
  • A short piece (5 cm) of shrink tube for the antenna wire.
  • A couple of tie-wraps.

Construction is simple. It is the same as what I described here for the 80 meters version (with dimensions adapted per the diagram below). As the PVC tubes are only 80 cm long, they are stiff enough to not need a guy wire strung between the tips of the tubes.

Spiral loop

Spiral loop

My 80, 40, and 20 meter spirals side by side


In 2010, Bill (W7ZT), built the 20 m spiral using my design, and is happy with it:

"Frank: Thanks for info on the Spiral Antenna. Built the 20 meter version today and works wonderfully !!

I found a 9-58 [pF capacitor] in the junk box that worked on the top end of 20 meters, but was very narrow banded. I'm looking for an air cap to allow the full band. Did you build a motor drive for the air capacitor ?? If so, can you share details.

See attached for my newest fun antenna !! Took the picture before I "cleaned" it up. The BNC to Binding post worked well for input.

I get great pleasure in building QRP antennas. Been a Ham for 52 years and having more fun now than ever.

Thanks again Frank.

Bill W7ZT

Spiral loop

The 20 m spiral built by Bill, W7ZT


MEASUREMENTS

Again, I hooked up my miniVNA antenna analyzer directly to the coax connector at the antenna. I measured the SWR and R of the antenna, both for the trimmer at its minimum and maximum value. I could easily tune it between 12.1 and 16 MHz. The SWR 1.3 and 1.6 at these extreme frequencies, well outside the 20 mtr band. See the plots below. Again, a very nice resonance dip of the SWR, and an impedance at resonance very close to 50 Ω - no tuner required.

Spiral loop

Resonance with low SWR and 50 ohms impedance at 12.1 MHz (trimmer at max) and 15.8 MHz (trimmer at min)

(ignore the bandwidth/Q indication above & below: they are not valid as I did not correctly select the marker frequencies)

Spiral loop

Sweep with antenna tuned to 14230 kHz (SSTV frequency): SWR 1.12


Tuned to 14230 kHz with my 15 kV variable capacitor, the analyzer claims a Q of 127. The transistors in the output amplifier of my transceiver are powered by 12 Vdc (or 13.8). My understanding was that this would result in a max of 13.8 x Q = 13.8 x 127 ≈ 1750 Vdc at the antenna at resonance. For Vac (= RF), you should double that rating - definitely exceeding the rating of my original trimmer caps. Tuning at low power (≈ 10 W), everything looked fine. As soon as I changed to high power (about 70 W in my case), the SWR meter pegged! After interrupting transmission, and then resuming with low power, everything was OK again.  I suspected some "ionization" effects at the trimmer capacitor at the antenna. I asked my girlfriend to have a look at the antenna while I keyed the transmitter (it was night time): "Yes, the little blue light bulb is glowing!". Say no more... The small trimmer cap was enveloped in a light blue luminous ionization discharge aura. No visible arcing damage, though it only takes tiny damage spots on the capacitor plates to significantly reduce the voltage rating! This is why in 2009, I bought variable air capacitors with 6 kV and 15 kV rating...


REFERENCES


External links last checked: October 2015


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©1999-2016 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.