Hello James,
I haven't another mast but is possible to put an horizontal wire (or more
parallel wire) from top of the tower
to TV antenna mast that is in the center of the roof at about 30 meter from
the tower. Top end of the mast
is 18 meter above ground. There is also 18-20m tall oak (not visible in the
right of the photo)
at about 25 m from the tower.
A total of 55 m of wire (about 300 pF) or 500-600 pF if i put three widely
separate wires.
It is also a good idea descend with one wire from top hat to the shack in an
intermediate position
between the tower and the house trying to keep as much as possible vertical
the wire.
Of course i can put a certain amount of inductance outdoor close to tophat.
73 Fausto IK4NMF
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Thursday, January 06, 2011 3:42 PM
Subject: LF: Re: Vertical antenna and connection line
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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