Last update: 17 August 2009
After having built the 80 meters spiral loop antenna designed by Harry, SMØVPO, and successfully scaled it down to my 40 meters version. For fun, I decided to try the "limbo approach", in other words: "how low can you go?"
I updated the Excel spreadsheet that I made to arrive at the 40 meters version, with the possible solutions for a 20 meter version. 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).
Components:
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7 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) 80 cm (2 ft 6") 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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A short piece (5 cm) of shrink tube for the antenna wire. |
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.
The 80, 40, and 20 meter
spirals side by side
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. 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 - 1.6. 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 - definitely exceeding the rating of my original trimmer caps. I read somewhere that for Vac, you need to double that rating... 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! While continuing to transmit a test tone, I had to completely de-tune the two variable caps of my tuner and then re-tune back to the previous settings. Suspecting 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 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! Time to search for a variable capacitor with a (much) higher voltage rating. Several trimmer caps actually, as I am using the same type in my other spiral loops.
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 indications, they are not valid as I did not correctly
select the marker frequencies)
Tuned to 14230 kHz (SSTV frequency): SWR 1.12
(ignore the bandwidth/Q indications, they are not valid as I did not correctly
select the marker frequencies)
©2009 F. Dörenberg N4SPP
