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Rethinking loop tuning capacitors

A project log for HF Short loop antennas: managing expectations

Make a portable loop antenna that works well on 40 meters, then make it autotune.

Andrew MitzAndrew Mitz 03/11/2022 at 15:220 Comments

I have been rethinking loop antenna tuning. The evolution of this new direction started with the design of a "fine tuning" capacitor. Rather than microstepping a high voltage 100 pF variable capacitor, it could be easier to adjust for low SWR with a fine tuning ("bandspread") capacitor. I did some spreadsheets to see what value capacitor was needed. The results surprised me. Specifically, what size (capacitance range) variable capacitor is needed to tune each band with a 1 meter loop and a 5 foot diameter loop. One meter is a very common loop size. 5 foot diameter is the size I use on most bands. The answer is: under 15 pF for 40 - 10 meter bands. You need substantially more on 80 m unless you go to a 7 ft loop.

No wonder tuning can be a challenge given the small capacitance ranges. Here is a table of capacitance range for each band.

   Dia  3.34     5.00    7.00 foot 
Band    
10m     1.14     0.69     0.47  pF
12m     0.10     0.06     0.04
15m     0.75     0.46     0.31
17m     0.27     0.16     0.11
20m     1.97     1.19     0.82
30m     0.77     0.47     0.32
40m    13.17     7.97     5.48
80m   153.37    92.84     6.38

Note that the WARC bands (12, 17, 30 m) have narrow assignments, so their capacitance range values are smaller. 

If we omit 80 m for now (not a very efficient band without a 7 foot loop), the largest variable capacitor we need is under 15 pF. 

New thought: If you want just a few bands, think about fixed capacitors for band changing and a small variable capacitor for tuning. More on this later.

For testing this capacitance range, I am going to keep my large variable capacitor to change the band, but use a small capacitor for tuning. Next, I will design and build a small home-brew capacitor.

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