[TowerTalk] Anomolous ground conditions

K4SAV 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?

Jerry, K4SAV

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.
>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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