[TowerTalk] Anomolous ground conditions

Tom Rauch w8ji at contesting.com
Mon Nov 7 18:45:25 EST 2005

 >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.

What balun are you using? Are the antenna in the clear?

>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).

That's not much of a ground system. Also, you are just going
by a feeling. To be truthful, most people can't feel several
dB if it is biting them on the rear like a Pit Bull.

 >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 ?

Which is it? 25 or 100? That would be important to know.

 >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.

You hear Hams talk about all kinds of things, that's mostly
what we do. Talk. ;-)

  >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.

It doesn't. However, if you extract the copper and lay it on
the ground in a sheet.....

73 Tom

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