In my 35+ years as a ham, I have probably built 100 dipoles, both for my
own use, and for others. From strictly an SWR perspective, they have
all behaved as expected, obeying the 468/f formula, more or less. Some
have required a little tweaking, but once they are up 20 or more feet,
they have the expected SWR curve, with a minimum SWR below 1.5 at some
frequency. Life is good !
I moved to my mountaintop QTH in New Mexico, and before I had any towers
erected I put up 20 and 40 meter dipoles. They weren't very high - in
the 20-30 foot range. To my surprise, I couldn't get the SWR below 2:1
at any frequency. Did the usual things - changed coax, tweaked wire
lengths, changed SWR meters, etc. Nothing would get them close to 50 ohms.
The towers and beams went up, working as expected, and I forgot about my
little dipole enigma until yesterday. For Sweepstakes, I put up a low
40 meter dipole to compliment the high 40 meter beam. Sure enough,
another dipole with a lousy SWR. Now I have to believe that it is
related to my ground conditions, and interaction with my ground.
Basically, I don't have soil up here - it's a mixture of limestone, with
quartz and other igneous intrusions. There is also a small percentage
of metals in the rocks - mainly copper and silver (see note in later
paragraph).
The real reason I care is to try to improve my 160 meter signal. I have
a full-size vertical, with 10 ground radials. To Europe, it feels like
it's down 6-10 dB from my 160 antenna at my Colorado QTH (1/4 wave
sloper, good ground conditions).
Anyone care to speculate on what my ground looks like to RF, and why it
would appear that a low dipole would have a radiation resistance of ~25
ohms (or ~100 ohms). If the ground was lossy, shouldn't the radiation
resistance be closer to 50 or 75 ohms ?
Unrelated side note: I occassionally hear hams on the air talk about
how wonderful it would be to have an antenna farm on a copper mine. The
USA's 2nd largest open-pit copper mine is 2 miles from here. My wife is
an engineer there. She says that at best, this mine has 2% copper in
the ore. I can't believe that rock with 2% copper would have any
discernable effect on RF. 2 miles in the other direction from here is
the Georgetown mining district, where many millions of dollars of silver
was mined in the early 1900's. I'm sure the silver content there wasn't
much higher than 2%, either.
73,
Steve, N2IC/5
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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