Mike Dennison wrote:
ON7YD wrote:
>A bit desperate I started my final attempt, again inspired by Mike,
G3XDV,
>who reported an improvement of his antenna by adding some inductance at
the
>top of the vertical section of his antenna. [big cut] DCF39 seemed
about a S-point stronger, but that could be
>just QSB. Also G3AQC who was CQ-ing sounded stronger, but the ear can
be
>very subjective. I called Laurie, being curious about my report. To my
>pleasure I got a 1 S-point better report than I got so far from him, so
it
>was not just my imagination that despite the lower antennecurrent
signals
>were stronger.
>The same evening I managed to work Finbar, EI0CF, (got 439 / 539) for
the
>first time after having called him over and over for the last year.
>The next days reports from several stations (G4GVC, G3YXM, DK8KW)
showed an
>increase of 1 S-point.
Very interested in this report from Rik. At the time I announced my
success, Rik spent some time running through the maths with me to show
that theoretically there should be no improvement - and I could not
argue with that. I recall replying that if practice and theory did not
agree, believe the practice. Recently, Dick, PA0SE, also argued
mathematically and by modelling that there was no improvement to be had.
Later, after many e-mails and discussions with others, I came to wonder
whether my experience was unique, and was probably due to the vertical
part of the antenna being very close to the house (therefore anything
that increased the current high up - above the house - gave me a better
signal that it would for anyone else). This, of course, could still be
true as Rik has considerable screening by trees.
I shall be shorting out the top coil later this month so I can use the
antenna in the CW WW 160 contest, and will carry out some before and
after tests at that time.
I believe that optimum results should occur when the antenna is made
self-resonant, but I suspect it will be affected badly by changes in the
environment (trees in summer/winter, rain, ice, etc).
It's probably not worth reopening the debate which has already been well
aired, unless someone can make the maths fit the experimental results.
--
Mike, G3XDV
IO91VT
Most of the modelling software available to amateurs does not model real
ground, and neither is there a way of accounting for trees and shrubs in
the antenna environment, but there are suggestions that "greenery"
losses occur (I think it was Finbar who obtained higher current after
avoiding clutter).
Commercial beacon stations do not employ loading coils at the top of the
"up wire", but they likely have very good ground systems along with a
clear site, so dielectric loss in the soil or in trees is avoided by
engineering their site conditions. Amateur operation from home stations
is where a difference occurs, with clutter and greenery losses waitng to
soak up LF power :( Thus there could be advantages in having much of
the loading coil further up the antenna system, so that losses near the
up wire are reduced (lower voltage means lower field strength gradients
around the up wire, thus lower losses in greenery, etc).
The practical disadvantages of an elevated loading coil are mechanical
support needed, and it surviving high voltages on transmit, during wet
or humid weather. I am not a supporter of using ferrite cores in
loading coils, as it could become nonlinear under transmit current.
High Q air cored coils for LF loading are rather large, and heavy, hence
the practicality of using them at the bottom of the up wire ...
I also agree that practical evidence is what counts for assessing the
possible improvement with having loading inductance at the top of the up
wire. The evidence so far is that it can improve the efficiency of an
amateur LF antenna system.
While I have not implemented this myself, I had considered how to obtain
MULTIBAND use of the LF antenna system, for the likes of 160 and 80
metres. My thinking is to instead of having only a loading inductor at
the top of the up wire, additionally use a parallel capacitor so the LC
"trap" is resonant at say 500 kHz, making it usefully inductive at LF,
but at higher ham bands the LC network has a low enough capacitive
reactance to make use of the top loading wires to carry useful currents
on 160 and 80 metres. I have not done the calculations as to what L/C
ratio is suitable, nor what voltage rating the capacitor would need to
be for not breaking down on LF transmit. The idea is floated for
consideration.
Bob ZL2CA
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