At 15:11 6/5/02 +0200, you wrote:
Hello Jim,
keep in mind that an elevated counterpoise will reduce the effective
height of the antenna (by "pulling up'' the RF ground level.
So a part of the improvement could be lost again due to the lower
radiation resistance of the antenna.
It is interesting to notice that the counterpoise increased the total
antenna current by 7.5% (32 Ohm vs 37 Ohm) while it takes 20% of the
current. Maybe it is a bit simplified, but based on that 20% one could
assume that the RF ground level was raised by 40cm (20% of 2m).
In that case the effective height would decrease from 9.5m to 9.1m (-0.37dB).
The increased current (7.5%) gave an improvement of 0.63dB, so the overall
gain could be as low as 0.26dB.
It seems that an isolated counterpoise is only worth the effort with :
- high antennas (so the loss in effective height is minimal)
- a lot of space (and $$ to buy the wire)
- very tolerant XYL and neighbours
73, Rik ON7YD
I'll have to respectfully disagree with Rik on this one.
There are lots of advantages to the use of counterpoises on LF. The biggest
one being
that the antenna is a lot more resistant to de-tuning from weather, dew,
etc. Using
high-Q coils, losses from even small amounts of de-tuning can be very high
indeed.
Also, I'm currently using a "stealth" counterpoise. Insulated #14 AWG house
wiring has
been stapled to the underside of my outdoor deck (which is very large).
I've literally
covered the bottom of the deck with wire. It is completely hidden from
view, was
much easier than attempting to bury wire, and the cost was minimal. Less
than $50
dollars total by buying the wire on sale at home improvement stores.
My tuning coil is mounted to a wooden post on the end of the deck, and a
flattop three-wire vertical is positioned on two masts on opposite ends of
my yard. All this on an average 1/4 acre lot.
Performance is unchanged vs. my former extensive ground radial system which
could only be used during the winter months.
I'm a huge fan of counter-poises and think that more amateurs should
give them a try. Since most of your losses occur at the base of the antenna
and it's virtually impossible to lay down resonant radials at this frequency
anyway, even small counterpoises can perform better than most
ground radial systems within amateur limits.
73,
Les Rayburn, N1LF
Les Rayburn, N1LF
Navy MARS NNNØHSI
"Proudly Serving Those Who Serve"
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4919 Cox Cove
Helena, AL 35080
Try the 1750 Meter Band:
http://www.highnoonfilm.com/xmgr
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