Last update: 16-Aug-2009
I have built the 80 meters spiral loop antenna designed by Harry, SMŘVPO. My construction and analyzer measurements are described here. 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. These antennas are easy to make and are inexpensive, so I gave it a try.
I made a quick-and-dirty Excel spreadsheet to play around with the dimensions. 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.
As with the 80 meters version, I have not been able use the antenna with my transceiver (other than for receiving): I don't yet have the required high-voltage (several kV) trimmer capacitor. A "high voltage" type is required as very high voltages will be generated across the capacitor. I used the same "regular" 5-15 pF trimmer capacitor that I used for my 80 meter version of this antenna.
Components:
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14 meters of 0.75 mm2 insulated wire ("zip wire", AWG 20 or 21, not critical); as suggested by Harry, I split 7 meters of 2x0.75 mm2 household hook-up wire (zip wire), and made a 14 m wire out of it. |
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15 pF high-voltage variable capacitor (at least 2 mm separation between the plates). |
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2-hole household terminal strip. |
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BNC or SO-239 coax connector. |
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2 sections of (at least) 120 cm (6 ft) PVC tubing, at least 16 mm (5/8") diameter, from your local Do It Yourself or building supply store. |
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PVC T-piece, for 4 cm 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). |
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4 meters of thin mason's string (e.g., 1 mm diameter non-stretch/pre-stretched multi-strand dacron or similar; nylon stretches way to much). |
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A short piece (5 cm) of shrink tube for the antenna wire. |
The main components, other than the "arms" of the square loop
Construction is simple. It is the same as what I described here for the 80 meters version (with dimensions adapted per the diagram below).
The 80 and 40 meter versions side by side
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 6.8 and 8.5 MHz. See plots below. Again, a very nice resonance dip of the SWR, and an impedance at resonance very close to 50 Ω - no tuner required. The SWR was ≈ 1:1.3.
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).
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. Supposedly the voltage Vc across the capacitor is Q x Vo, where Q is the "Quality" of the LC-circuit (resonance frequency divided by the bandwidth), and Vo the oscillator (transmitter) voltage.
The analyzer plots show a Q of 202. I assume that the output transistors of my transceiver are powered by 12 Vdc (or 13.8). That would result in about 2800 Vdc at the antenna... More on this (including actual arcing) in the description of my 20 meter version of this antenna.
(ignore the bandwidth/Q indications, they are not valid as I did not correctly
select the marker frequencies: SWR=2 bandwidth is actually abt. 35 kHz)
Resonance with low SWR and 50
ohms impedance at 6.8 MHz (trimmer at max) and 8.5 MHz (trimmer at min)
(ignore the bandwidth/Q indications, they are not valid as I did not correctly
select the marker frequencies)
Sweep from
13 to 30 MHz - outdoors via
15 mtrs of coax
©2009 F. Dörenberg N4SPP
