Hi Alex,
I live in a subdivision that does not allow towers, much less elevated
antennas. So I am pretty familiar with your situation.
If the antenna is cut to resonance, meaning no measurable capacitive or
inductive reactance at the Frequency of choice, and the impedance is
something other than your desired impedance required to match the coax
transmission line, then I would use a Balun at the feed point. I have a
loop for 75M that is not even close to being a 1/2w off the ground. It's
impedance at resonance, due to height and surrounding area, is roughly 120
ohms. So I built a 2.5:1 Balun using a ferrite toroid and some formvar
coated wiring. This is all enclosed into a weather proof box. My SWR is
now ~1.1:1 or lower at resonance, and the antenna/Balun can take what ever I
can throw at it, given my meek 1KW amp.
If you would like more info on this, feel free to email me off list.
73,
Mike
W5CUL
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Alex Aimette
Sent: Friday, March 23, 2007 8:25 AM
To: amps@contesting.com; Joseph Brue
Subject: [Amps] 160 m antenna swr
Here is a problem that perhaps some of you have faced or haqve had
experience with.
On v4 we put up a dipole for 160 but the best we can do is about35-40
feet above "ground."
Gound here is a plateau about 75' wide by 250' long on top of a mountain
1200' above sea level.
We feed this antenna with RG8x 52 ohm coax which comes straight down
from antnna to the ground, thence to the shack about 60 feet away. The
best we have been able to do is get a swr of about 1.7-2.0 which
limits how much power we can feed to about net 800 watts (forward
1050-ref 250 w).
Reviewing the lit I "think" the feedpoint impedance is about 30-35
ohms--no measurements.
I was thinking maybe we could put some lumped matching device right on
the ground under the feedpoint to lower the swr.
Any ideas along these lines--configuration et al would be appreciated.
alex w2ox/v47kp
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