Hi, I am not sure that that is the cause. If it were, you should be able to
see signals on the scope match when you are not transmitting. Local RF can
severely corrupt the operation of wideband detectors in antenna bridges. But
this is mainly because a good bridge will have in excess of 70dB nulls which
brings the null down to the level of the received signal, and the null is
"flattened out" and indistinct. When using bridges on LF it is necessary to
have a tuned detector such as a receiver or use a high enough drive level to
overcome the received voltage.
You are not "nulling" on the scopematch and if you had serious harmonic
effects they would be plainly clear as a distorted trace on the scope. My
suggestion is that you are amiss somewhere with your appreciation of the
parameters for the tuning of your antenna and are in fact trying to resonate
it at a harmonic frequency. Be very careful with short lengths of coax in
the feed, If not correctly terminated (ie in "off-tune" conditions) these
can resonate with link coils....invariably if Murphy is around at a
harmonic. If you do use coax relpace it temporarily by a wire and see if the
situation changes.....you may be surprised.
I suggest a old-fashioned absorption wave meter to cover 136 and its
harmonics could be useful here. Its why we all had to have one in the
"old-days". You may find that when you think you are tuned you have more
harmonic output that the required freeqency so the antenna is acting as a
"selector"
I think you need to check for this effect first.
I dont believe that a local signal except a very strong one would affect a
passive SWR indicator. It may lead to starnge readings on something like an
MFJ or Autek antenna analyser as these are effectively bridges with very
high sensitivity.
Another reason I dont think the scopematch is affected is that the main
proposer (though there were many involved in the idea) is Jim M0BMU. Jim
lives "just over the fence" from the Brookmans Park BBC transmission site.
So he has several MF half kilowatt BC transmitters a matter of yards
away.....he reports getting "several volts" of it on his Marconi !! But they
dont affect his tuning.
Remind me of the parameters of your antenna and loading coil, and lets throw
some figures around again.
Cheers de Alan G3NYK
----- Original Message -----
From: J. Allen <[email protected]>
To: <[email protected]>; <[email protected]>; Steve McDonald
<[email protected]>; Scott Tilley <[email protected]>
Sent: 04 July 2006 13:54
Subject: LF: Antenna Tuning
> Steve, Scott, and All,
>
> Last night, I was following a thread of another person and their problems
> with SWR detection in the presence of a nearby strong LF transmitter. It
> did not dawn on me until this morning that the problems with the LF
antenna
> tuning here relate to what looks like an even order harmonic on a system
> based on the square wave which generates high odd order harmonics, not
even.
> The antenna tuning problems are much harder to get any change in the
voltage
> portion of the SWR, and are more pronounced at low power used for antenna
> tuning than at higher power.
>
> I believe that the very strong signal from the LF beacon less than a mile
> from here which operates on about 240 kHz. is effecting the voltage
portion
> of the ScopeMatch readings and that this may be why the problem has been
> elusive to the point of discouragement.
>
> Here are three ideas which I put out for feedback and suggestions.
>
> 1. During my testing, vary the frequency slightly and see if it shifts the
> voltage wave shape because of the way the two signals combine. If it
does,
> my antenna is acting as a voltage pickup.
>
> 2. Put the LPF which I built out at the antenna terminals so that it
blocks
> the voltage signal from getting to the scopematch pickup.
>
> 3. Add a second FET to Steve's bulletproof antenna tuning transmitter, so
> that it produces a stronger voltage signal and masks the voltage from the
> nearby antenna.
>
> What do you think about these ideas? Are there any other ideas you can
> think of which will make testing more successful? Is there a different
kind
> of SWR circuit which works better in the presence of a strong signal? I
> wonder if any of my friends in EU have had similar problems and have found
a
> way to beat it.
>
> Help anyone?
>
> JA
>
> VY1JA.
>
>
>
>
>
>
>
>
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