Hi Jim, Fausto, LF,
It seems i have misunderstood what Fausto wants to do. Of course, having
the complete coil into the shack does not allow using RG213 ;-) I
thought Fausto only wants to TUNE the system into the shack and wants to
place the major part of the coil (say 90% or above) at the feeding point
of the antenna.
Placing the whole coil into the shack is no good idea in my opinion. But
placing a small fraction in the shack (e.g. to adjust when QSY from
137.7 to 136.177) is no problem...
73,Stefan
Am 06.01.2011 15:42, schrieb James Moritz:
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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