Dear Fausto, LF Group,
Beware of sweeping generalisations - I think the answer is that "it depends
on the antenna". You need to think about your particular antenna carefully.
At 136k, 20m of transmission line feeding a high impedance load (such as an
electrically short vertical antenna) will behave essentially as a shunt
capacitor. The capacitance of a high Zo open wire line is of the order of
several pF per metre, which may be increased by being close to the ground.
So your feeder would have capacitance of a few 100s of pF. This will be in
parallel with the antenna impedance, which will be equivalent to a capacitor
in series with a resistor. If the antenna capacitance is large compared to
the feeder capacitance, the feeder will make little difference, while if the
antenna capacitance is small compared to the feeder, the feeder current at
the TX end will be much higher than the antenna current, leading to
increased losses in the antenna tuner and feeder. So for small antennas,
having the tuner close to the antenna is very desireable.
So what is the capacitance of the antenna? According to your info on
QRZ.com, the mast is 25m high, and you will be using it to suspend an
inverted L for 137k - unless you have another 25m mast, I guess the inverted
L top loading wire will actually be sloping... Your QTH seems to have plenty
of space, so I guess it would be no problem to have at least 100m of wire in
the top load. As a rough estimate, an isolated wire has about 6pF/m, so with
a downlead, the capacitance might be 700pF. This would be about right for
the variometer in the picture. So this would be substantially more than the
capacitance of the feeder, which is good.
But as Stefan and Rik say, another problem wil be the high voltage on the
feeder. The voltage depends on the required antenna current. At your good
QTH, with a sloping wire 25m high at one end, it should not be difficult to
get an effective height >10m. This would have radiation resistance of about
30milliohms, and to achieve 1W ERP, an antenna current less than 4A would be
needed. The reactance of 700pF at 137k is about 1700ohms, so 4A would result
in a voltage of about 6.6kV. This is quite high, but should not be a problem
for the antenna - many amateurs have managed to operate LF antennas at 20kV
or so with reasonable reliability. However, it would require wide spacing,
and careful design of insulators, for the transmission line.
But why do you need the ladder line? If you want to have the tuner in the
shack, from your photograph it looks like you could simply run a downlead
from the top of the mast directly to the shack... This would have the
minimum of loss and insulation problems.
Hope this is helpful,
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Fausto Coletti" <[email protected]>
To: <[email protected]>
Sent: Thursday, January 06, 2011 11:28 AM
Subject: LF: Vertical antenna and connection line
Hello,
A question for RF expert:
you think that i can connect a Marconi antenna and relative gound system to
the variometer with a high impedance ladder line?
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