Dear Rik, LF Group,
I think the main effect of having the elevated loading coil is too reduce
the voltage on the downlead of the antenna - this would have two advantages:
-Reduced voltage on the downlead, resulting in lower dielectric losses due
to objects in the field near the downlead.
-Reduced displacement current from downlead to ground, resulting in higher
current at the top of the vertical section of the antenna. This would
increase the effective height of the antenna somewhat, especially if the top
loading part of the antenna is quite small compared to the vertical section.
On the antennas that I tested, the tops of the loading coils were raised
about 2.5m above ground. The reasons for this were mainly to reduce losses
in the loading coils, and to keep the high voltage parts of the antenna a
safer distance away from accidental contact with people. The problem in
trying a fully-elevated loading coil, especially at my home QTH, is finding
somewhere to mount the coil. Due to my flimsy antenna supports this would
require an extra mast to support a loading coil that had low enough loss. I
am not sure the effort would be worth it, as the horizontal part of the
antenna passes closer to the trees than the downlead does, so I think most
of the losses would be in the horizontal section. There is also the problem
of how to change bands between 136k and 500k - at the moment, I have
separate tuners for each band, and swap the antenna wire between them. This
would be a lot harder if the coil was at the top of the mast!
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Rik Strobbe" <[email protected]>
To: <[email protected]>
Sent: Tuesday, December 11, 2007 2:23 PM
Subject: Re: LF: Antenna tests on 136k and 503k - Results
> Dear Jim,
>
> thanks for sharing your experimental result with us, most interesting.
>
> Did you ever try to put (a part of) the loading coil up in the air ?
> What I experienced with my 136kHz antenna was that putting a part of
> the loading inductance at the top of the vertical section worked miracles.
> In 1999 I placed a +/- 1.5mH coil at the top of the vertical section
> of my inverted-L antenna. This represented about 50% of the required
> loading inductance, so another 1.5mH remained at the bottom (1m above
> ground). It resulted in a +/- 5 dB signal increase (measured by PA0SE).
> The "upper" coil was rather low Q (to reduce weight): I used a 5
> liter container (the ones filled with purified water for ironing) and
> 1mm isolated wire.
> After mentioning this on the reflector I received a number of
> responses from others who also tried it and it seemed that it gave
> only a significant benefit when the antenna was surrounded by lossy
> objects. In some way the elevated loading coil seemed to "decouple"
> the antenna from the lossy surrounding.
>
> 73, Rik ON7YD
>
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