Hi Dick the problem at LF with a long low wire is that the horizontal
component cancels out in the ground and you are left with an aerial that is
effectively a 1m vertical ( the height of the wire above ground).... I can
hear some of the pundits sharpening their pencils....it is actually not that
simple, as the ground level is not the level of the "Earth-plate" you must
take into effect the skin depth of the ground at this frequency. This is
dependent upon the frequency and nature of the soil. Thus at ELF (< 3kHz),
the type of antenna you think of will work quite well (compared with
anything else) the skin depth is proportional to the frequency but the
radiation resistance is proportional to the (frequency)^2. I think you said
you ground was poor, sandy. That would work to some extent if there were no
surrounding lossy "environment"....like trees shrubs and houses. I suspect
this is why the VK antenna across the valley that Bob describes works so
well, the ground is probably very dry and the skin depth is quite deep. From
experience on both sides of the pond, the solution is a vertical, if it can
be up clear of any trees and buildings (at least its own height away from
these "absorbing" items.) If you have trees surrounding you and you cant get
away from them, then a big loop can be the answer. Because of the lower
voltage in a loop it induces much lower currents in the lossy "environment",
so though the radiation resistance may be less that could be achieved from a
vertical taking up the same volume, the loop can be much more effective.
If your vertical is out in the clear you need to look at loss points. First
the insulators... they need to have a long leakage path because the voltage
at the ends will be 10s of kV and any moisture or dirt will cause corona.
Double insulators may be needed and each should be 75 to 100mm long
(receiving antenna "egg" insulators are useless). You may need "corona
rings" over about 200W. These are necessary not only at the ends but at
every "sharp" bend on the antenna run.....try to get a 5 to 10cm radius on
all bends. This is a difficult one to find as you cannot see it and it only
occurs at high power. You can somtimes hear a "sizzeling" or see a dull
bluish glow on a dark night. You can even have corona around the coil,
particularly if it gets damp.
Next you need to get the ground loss down. It sounds as though you have done
as much as you can eaily under the feed point. The next area is to increase
the top load capacity.....what inductance do you need to tune the antenna??
every metre of top wire over "open ground" will add about 6pF to the antenna
capacity. Parallel top wires can be a little as a metre apart....it does not
matter how they are connected, a parallel fan or a meander with have almost
the same effect. Over your 'poor' ground, doubling the top-load capacity
will half the ground loss, giving you four times the ERP. This would be
difficult if not impossible to achieve with ground stakes and "radials"
unless the latter were 200m or so long. As I said earlier the only easy way
to check the ground loss is with a simple RF bridge. You should be aiming
for something less than 70 ohms, it is achievable, and between 40 and 70ohms
is a good target to aim at.
I have not tried this but it is an idea that occurs to me. A good check that
you have tamed the corona losses might be to plot the antenna current
squared against the PA output power...... It should be a straight line, if
it drops off at high powers then you may have "leakage problems". The ground
loss should be resistive and fixed.
Best of Luck de Alan G3NYK
----- Original Message -----
From: "Dick" <[email protected]>
To: <[email protected]>
Sent: 29 March 2005 21:49
Subject: LF: low long wire as TX-antenna???
I wonder if anyone here has experience in using a low long wire as TX
antenna on 136kHz.
I'm thinking of around 400-500m long wire at 1m above ground.
I still have a lot of 'portable'beverage stuff lying around from my
previous activeties:
mediumwave and NDB-beacon DX.
Maybe it's worthwile to try.........
Dick, pa4vhf
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