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I made my first multi band antenna this weekend for 20 and 15 meters for an EFHW wire with a 49:1 unun, to be used with 5 W rig. I used a toroid I had on hand, T80-2.

I hang this antenna in my living room as an inverted L. It's far from an ideal environment.

I have what I think is a 18 pF 1kV ceramic capacitor(marked 18J, NP0). I also have 100 pF 2kV caps.

I first thought I'd use the 100 pF cap since I'm sure of it's value and type. But then the inductance value becomes rather low with trap impedance ~75 Ohms at 21 MHz.

I figured that isn't going to look like an open circuit. I put in the questionable 18 pF cap and wrapped more turns (~20) on the T80-2.

I then added about four more feet of wire to the end and trimmed to get 20 meters in resonance. The lowest SWR I could get is around 2:1.

I probably did not build this correctly to start with. However, my question is: Should I aim for highest impedance possible at the trapped freq? Meaning the lowest capacitance value with highest number of turns on the inductor ?

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  • $\begingroup$ What are your target resonances? I'd be skeptical that this capacitor is 18pF. Well, maybe it is. Cap markings are always such an adventure. $\endgroup$
    – user21417
    Feb 28, 2022 at 21:12
  • $\begingroup$ @jdv I'd like to have 14.078 -14.350 MHz and 21.078 - 21.475 MHz. My transmatch allows tuning which is a start. The cap is quite small. Probably not 1 kV though. $\endgroup$
    – wbg
    Feb 28, 2022 at 21:27
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    $\begingroup$ Be very careful of relying on measurements made while hanging the antenna in your living room. SWR will change a lot when you move this up to an appropriate height in free space. In fact, everything will change a lot when you don't have household wiring, nails, plumbing, and everything else surrounding it, in addition to altitude. $\endgroup$
    – David Hoelzer ♦
    Mar 7, 2022 at 17:35
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    $\begingroup$ The tricky part is that your coils with have their own capacitance. $\endgroup$
    – user21417
    Mar 8, 2022 at 15:45
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    $\begingroup$ @jdv Yeah I'm aware and it makes it hard to predict. There's a web page toriods.info that also computes the base amount of capacitance for n turns but it hasn't helped ! I end up doing a lot of trial and error. I wish I could sit with an engineer at his bench and learning hands on with a teacher. I miss my college labs they were the best part. $\endgroup$
    – wbg
    Mar 8, 2022 at 19:04

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There are 3 different ways for a trap - all will result in the same resonance frequency. The best solution is to have the braid on both sides as the coil and on one side the inner conductor as a 'C'. An excellent reading that explains that: https://www.m0lxq.com/g4hfq/g2hcg/traps.doc

73's de OE1MWW Wolfgang

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  • $\begingroup$ Nice document on traps. $\endgroup$
    – wbg
    Aug 9 at 3:39
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Yep, antenna traps serve as a high-impedance wall at their designed frequency, while appearing mostly transparent to frequencies below this point.

Grossly speaking, a coil with fewer turns and larger diameter will generally have a higher Q and less self-capacitance... but there can be practical construction considerations that require compromise too... it's not like you can expand or contract the diameter of your toriod! So don't sweat it too much.

Anyway, you'll want to have something on hand that will let you measure reactance of your components so that you can adjust the windings on your inductor to appropriately match what you need based on the trap freq f and capacitance C in the equation above.

f = 1 / (2π √(LC)) is the equation you want, but you're really solving for L so:

L = 1 / (4π²f²C)

Happy building!!

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