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I built a vertical out of a 5M aluminium pipe. It tunes fine (and wide!) on 20M. I decided to add a loading coil to operate it on 40M, and it also worked fine, albeit not so wide. While I was at it, I extended the coil for operation on 80M as well. It became super narrow! But it worked.

To test this, I used crocodile wires to bite into the coil. I decided make a remote switch out of relays. But it's not working as expected when operating 80M. The SWR jumps around like crazy on my meter, and at some point it just stays inifinite.

This is what I built:

enter image description here

What I suspect is that when both relays are off, there is a very high voltage across the relays and an arc forms, due to the high Q of the coil. This does not happen when the 40 or 20M relays are activated. The antenna works more or less as expected when the 40 or 20M relays are active.

Could this be the problem? Could it be solved somehow?

I thought of using traps instead of relays to make the antenna "auto switching", but I'm not sure if traps would add more loss than the relays, or if they would whitstand the high voltages appearing on the coil.

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  • $\begingroup$ What relays are you using? $\endgroup$
    – Mike Waters
    Commented Jun 9, 2019 at 17:37
  • $\begingroup$ What do you have for radials. If its just a ground rod you should add 1/4 wave radials equal to the lowest band of operation. This can effect SWR readings and directivity. $\endgroup$
    – KB1CHU
    Commented Jun 11, 2019 at 16:42
  • $\begingroup$ @KB1CHU I'm using the metal roof of my house as a ground plane. I added a random piece (3m) of 9mm aluminium tubing and that seemed to help a bit. The antenna is mounted to one side of the roof, not in the center. The roof is about 6x10m and it's connected to the coax braid. $\endgroup$
    – hjf
    Commented Jun 12, 2019 at 12:59

2 Answers 2

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It may very well be that the open contacts are arcing, and I'm betting that's what is happening.

Here are some possible solutions. How about using different relays that have less tendency to arc:

  1. Open-frame relays with much wider contact spacing
  2. Vacuum relays
  3. Multiple relays in series

Another possibility might be that RF on the control lines may be activating the relays. Suitable RF chokes and bypass capacitors might help that.

It's a myth that all traps are lossy. If they don't get hot to the touch at sustained 100W or higher, then their loss is insignificant. Scroll to the bottom of https://www.w8ji.com/traps.htm:

3.) Do traps create noticeable loss, perhaps one dB per trap typically?

NO! Even the worst traps (coaxial traps) in the worse possible condition of operation are only 1.6dB loss for BOTH traps!

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  • $\begingroup$ What about these? tme.com/ar/es/details/li05-1a85/reles-electromagn-de-lengueta/… they are inexpensive reed relays, rated for 4KV $\endgroup$
    – hjf
    Commented Jun 9, 2019 at 17:47
  • $\begingroup$ @hjf What voltage are your present relay contacts rated at? $\endgroup$
    – Mike Waters
    Commented Jun 9, 2019 at 17:50
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    $\begingroup$ Just regular "ice cube" no-name chinese relays with generic specs. probably 250V. $\endgroup$
    – hjf
    Commented Jun 9, 2019 at 18:03
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    $\begingroup$ No, it's a physical fact that all traps are lossy — it's a myth that you should worry about that loss in ordinary circumstances :) $\endgroup$
    – hobbs - KC2G
    Commented Oct 22, 2019 at 1:14
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Your suspicion is justified. Voltage increases (and current decreases) as you move toward the open-circuited end of the antenna. The most common solution is to situate the loading coil - and, perhaps, a resonating capacitor - at the base of the antenna. A base-loaded vertical only 5m tall will not be very efficient on 80m, but a center-loaded antenna will probably not be great either, as it's only about 1/4 the length of a resonant monopole.

The voltage along the 5m radiator can be reduced and efficiency can be improved by effectively lengthening the radiator with "top hat" conductors. This will increase the current at the top of your 5m aluminum pipe and, depending on the size of the top hat, reduce the amount of inductive loading you need to add.

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    $\begingroup$ But a hatted 5M vertical won't tune in 20M, correct? $\endgroup$
    – hjf
    Commented Jun 9, 2019 at 16:30
  • $\begingroup$ @hjf If it won't, disconnect it with a HV relay as suggested in my answer here. $\endgroup$
    – Mike Waters
    Commented Jun 9, 2019 at 17:31
  • $\begingroup$ @hjf Since you have to add compensating reactance, you might decide to make the system resonant on 40m and "tune" it on 20m and 80m. The permutations of lengths and loading are endless! $\endgroup$
    – Brian K1LI
    Commented Jun 9, 2019 at 18:37
  • $\begingroup$ @BrianK1LI Added that link you your comment. ;-) $\endgroup$
    – Mike Waters
    Commented Jun 9, 2019 at 21:18

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