[TowerTalk] Anomolous ground conditions
RadioIR at charter.net
Mon Nov 7 11:38:29 EST 2005
It's difficult to get a 40M dipole to have an SWR of 2 to 1. If you
have perfect ground and the antenna is at 15.5 ft the SWR should be
about 2:1. Lower heights raise the SWR, and poorer ground lowers it.
But I think your antenna is not this low.
For higher antennas, if you have a perfect ground (or salt water), the
worse case for SWR will occur at 1/3 wavelength or about 47 ft height.
This will be about 96 ohms or 1.9 to 1. If you change to the best soil
available in the US (.03s/m, diel=20) the SWR goes to 1.8. Average
ground produce about 1.7:1, and extremely poor ground produces 1.6:1.
If you keep the ground perfect and lower the antenna to 40 ft you get
1.8:1, and at 30 ft you get 1.5:1.
So, if you are getting 2:1 SWR out of an antenna at 25-30 ft, you must
have something wrong. Maybe 75 ohm coax? Bad balun? Maybe other
resonant objects too close? SWR meter picking up local RF?
Steve London wrote:
>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
>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.
>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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