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Re: RF Ground Quality
** Reply to note from Tesla List <tesla-at-poodle.pupman-dot-com> 09/16/96 10:54pm -0600
> From: pierson-at-msd26.enet.dec-dot-com
> To: mail11: ;
> Cc: pierson-at-msd26.enet.dec-dot-com
> Subject: RF Ground Quality
>
>
> There are standard procedures for such, i suspect in an IEEE
> standard, somewhere. Anyone have acces to a good library? (I don't
> or i'd summarize.) Lightning Protection standards would be where I'd
> start....
>
> My dim recollection is that the usual methods involve, as reccomended by
> others, measuring to temporarily seperated multipoint grounds (I believe some
> ground systems are explicitly made serable, to allow for testing) AND/or to put
> in a temporary ground to do the measurements to.
>
> SOMEwhere I've got some info on that. I'd have more faith in meaurements MADE
> AT RF (signal gene, low power coil) than with a straight DC instrument...
>
> regards
> dwp
I've tested munition area and distribution systems grounds with an instrument called
a Vibraground. I believe that's the name used by Biddle instruments for their earth
resistance tester. They make insulation resistance testers and other equipment for
the high voltage industry. It's a battery powered tester that used uses a vibrator
to produce a 90 cycle pulse. It uses 90 cycles to keep stray 60 cycle power from
affecting the measurement. A lead is connected to the grounding system in question
and another lead is connected to a ground probe 50 to 75 feet away. There are three
calibrated dials that control a resistance network. You turn up the dials until the
needle on the meter balances in the middle. At this point you have balanced the
resistance network against a precision resistor inside the unit and you can read the
ground resistance right off of the calibrated dials.
As far as using something like this for checking an RF ground, the readings would
only be relative. You could experiment with different grounding sytems and see
which are better and which are worse, but to be able to stand back and say "That
ground rod has a resistance of exactly 7.6 ohms to earth" would be stretching it a
bit.
-Mike McCarty