Laurie G3AQC wrote:
I have just measured my TX ant impedance(R term) and it is 120 ohm!
much higher than I originally thought. It is of course all ground
loss . . . .
With the simple verticals which I use on 136 kHz, I have always tried to
minimise the proximity between the radiator itself and earthy objects
such as our house; the garden fence; and nearby trees. From earlier
contributions to this Reflector (thanks are due to Peter Martinez G3PLX,
and others), I learned that the parallel loss caused by the proximity
between the radiator and lossy ground translates into a
series-equivalent resistance - which, in turn, contributes to the [120
ohm] "ant impedance". [I hope I have learned my lessons well, Peter!]
So improvements to your ground system may help, but it might also be
worth taking another look at your antenna to see whether there are
opportunities for increasing the distance between your antenna system
and nearby (earthy) structures.
. . . . I wonder whether anyone can offer me comparitive
figs for impedance and details of ground systems?
Before pruning trees in my neighbour's garden:
**********************************************
ANTENNA CONFIGURATION WET DAY DRY DAY
12 m vertical 160 ohm 130 ohm
20 m vertical 130 ohm 100 ohm
After pruning trees in my neighbour's garden:
**********************************************
ANTENNA CONFIGURATION WET DAY DRY DAY
12 m vertical 130 ohm 100 ohm
20 m vertical 100 ohm 75 ohm
Ground system
*************
Shack earth is isolated from the antenna ground connection using a
matching transformer. 'Cold' end of transformer secondary winding is
connected to outside water tap.
For more information, see:
http://www.alg.demon.co.uk/radio/136/experi_vert.htm
The trees (prior to pruning) can be seen just behind (apple tree), and
to the right (flowering cherry tree), of my vertical antenna at:
http://www.alg.demon.co.uk/radio/136/pictures/expvert1.jpg
Regards to all,
Steve GW4ALG
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