Hi Dick, we have been having just such a discussion on the LWCA Lowfer group
reflector. There are lots who will tell you that 160m was unbeatable after
they did this or that....the problem is LF is not generally scalable as
there are many unknown parameters involved besides frequency.
My "research" suggests it might......but then again it might not !! I am
afriad there is no cookbook formula for this (at least I have not found a
working one) You really need to evaluate each location
separately.......something that is not easy to do on a DXpedition site,
except in preparation for next year !! I am afraid the answer is set up an
antenna and a ground spike. Measure it for C and Rloss, then quickly rollout
a wire and see if you get any reduction. First roll it out under any top
wire, then ground the remote end and try again. this way you can quickly get
a feeling for what works and avoid a lot of effort. You do need a simple and
reliable LF antenna bridge...... note that noise bridges do not work well at
LF. Many noise bridges do not work that well at all, because the designers
or makers do my appreciate the problems of balance over the frequency range.
You only have to measure a few resistors to determine this. Also you will
need about 3v rms feed to the bridge at LF to avoid false nulls from strong
nearby BC stations and general noise. See
http://www.btinternet.com/~alan.melia/aelossbr.htm#losses for some ideas.
Made properly a simple bridge will give sharp deep nulls and repeatable
results.
Again you do not have the luxury to try it on a Dxpedition but multiple long
spaced top wires, strung as high as possibble is the easiest way of reducing
loss. Wolf's reference to the "footprint" refers here.
One alternative is a very large loop. By that I mean with a circumference of
maybe at least 120m and a high as possible. These are low voltage antennas
so are not subject to loss by capacitive coupling nearby foliage or
buildings. The Yanks feed them via a ferrite transformer and mount them
right up in the trees. The ERP of the loop is not as high as a "T" of
similar dimensions but it can give a better ERP in some situations.
One comment we have agreed on is that for an inverted L or T antenna there
is not much point in running "radial" wires much beyond the span of the top
wire. If you were to manage 300 to 500m length ground wire you are on your
own, we have no data on the effect of a wire beyond the influence of the top
load, but a near resonant wire might work well.....but you will have to test
it by measurement. Dont be seduced by the 160m ideas, ground skin depth is
so much greater than effects can be totally different, but do depend on the
parameters of the ground (soil).
Good Luck with your LF signals.....though receiving might be the biggest
hurdle, I am sure we will hear you.
Cheers de Alan G3NYK
----- Original Message -----
From: Dick <[email protected]>
To: <[email protected]>
Sent: 13 September 2006 21:11
Subject: LF: the use of long radials
Anyone knows if it makes sence to use some long radials 100-300m long
in combination with a spiral-coil toploaded vertical with 4x20m toploading
wires.
Or is this just wasting time and wire......
Dick, pa4vhf
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