G'day All,
Having recently received permission for operating an experimental
transmitter from this location on LF, I am now faced with the problem of
erecting an LF transmitting antenna. Although in the first instance I was
tempted to use a loop, the body of evidence seems to point to a
capacitively-loaded vertical being the better choice. Trouble is, the loop
seems to be much easier to get going in the first place and as my antenna
experience is mostly confined to the VHF up region, I need some guidance.
The antenna configuration I have decided to attempt first is a
capacitively-loaded vertical mounted on the roof of the house. This is the
only tree-free area on the block. The loading coil I want to place as far
up as possible to maximise the current in the vertical part of the antenna.
Elevated radials will cover the roof as far as possible around the base of
the antenna and connect to surrounding metal roofs, sheds, chook runs and
an aviary and also to ground.
A picture of this can be found at:
http://www.zeta.org.au/~ollaneg/images/AXSO_ant.gif
The TX will be connected to the base of the antenna via 50 ohm cable.
The questions I have are:
1. How do you measure the capacitance to ground to be able to calculate
the inductance of the loading coil? I doubt you can just attach a
capacitance meter between the base of the antenna and ground. I am
guessing that you just make an estimate and find the required inductance by
trial-and-error.
2. Is it possible to connect a smaller *variable* inductance (about 10% of
the loading coil inductance) between the base of the antenna and the
grounded elevated radials to effect a convenient tuning arrangement?
3. Given the arrangement outlined in 2. above, is it possible to tap into
that smaller inductance at the base to effect matching to the 50 ohm line
or don't you get the transformer effect required because the major portion
of the inductance is elevated and not inductively coupled?
Any answers/suggestions would be most appreciated.
73s Steve Olney (VK2ZTO - QF56IK : Lat -33 34 07, Long +150 44 40)
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email: [email protected]
Web Page: http://www.zeta.org.au/~ollaneg
LowFer: http://www.zeta.org.au/~ollaneg/lowfer.htm
LF Receiving - FRG-100, 8-turn 10sqm Loop
or Ferrite Loop + K0LR Preamp
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