Hi Bryan and all, I agree with all that has been said so far about earth
losses. One difficulty is that with amateur sized plots we all have slightly
different circumstances to overcome. To some extent we have to find out own
particular "best solution"
My own findings are that there is a limit with necessarily short
counterpoise wires to the reduction in loss gained. ( A 100 foot "radial" on
136 is like putting out 4 foot radial wires on 80m and expecting an
improvement!!). To some extent this depends on the local soil conductivity.
It is no use doing measurements at 50Hz or even DC as the result will be
toally different at 136kHz. Plots on my web site show how the loss varies
with the measurement frequency. I have suggested a simple bridge for the
measurement on my web site. I have found that a proper measurement is the
best way to avoid unnecessary hard work.....I am a comfirmed lazy B(oy). I
find it best (and easiest) to measure the wire alone (not resonated) ...you
can quickly see the result of any change of configuration.
However, when you find extra rods or counterpoise wires are not giving any
further advantage (usually very quickly) there are other unexpected things
you can do. The best was popularised by Laurie G3AQC....put more wire in the
air. This can be by paralleling extra top wires or just extending the ends
to " cover new ground". This obviously increases the aerial capacity but
with an "L" configuration it reduces the loss resistance considerably. A
dipole of the sort you describe will probably measre out at about 200 to 250
pF. If you double the wires with a spacing of at least 2 feet you will see
the capcaity rise to about 450 to 500 pF. This has two effects, most
obviuosly it reduces the size of the inductor required for loading
(resonating), but secondly it will most probably half the ground loss. This
is not effective unless the top wire is over relatively open ground. As
Laurie has found extending a wire over lush Rhododendrons actually increases
the loss. My tests with Finbar suggested that doubling the capacity, halves
the loss.
Recent tests at Malin where Finbar has retuned his "umbrella" to 136 again,
showed very poor aerial current levels initially. He achieved a considerable
increase in aerial current by running out an insulated "counterpoise" wire
under the top load (5) wires. The aerial configuration is described in my
web site. Note also that he is almost on the sea shore.
Cheers de Alan G3NYK
[email protected]
http://www.alan.melia.btinternet.co.uk
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