|Subject:||Re: [RFI] Earth isolation|
|From:||"Ian White, G3SEK" <G3SEK@ifwtech.co.uk>|
|Reply-to:||"Ian White, G3SEK" <email@example.com>|
|Date:||Wed, 8 Sep 2004 22:39:22 +0100|
Jim Brown wrote:
On Wed, 8 Sep 2004 19:29:12 +0100, George Shaw wrote:
What you say is true, Jim... but having been picked up by accidental antennas, that RF current also needs somewhere to flow *to*. Whatever mother nature hasn't enlisted as an "antenna", she'll try to enlist as a "ground" or "counterpoise" instead.
There are some situations, particularly involving end-fed wires and inverted-L antennas, where large ground return currents are unavoidable. These return currents will divide among all possible ground-connected conductors, according to the series impedance that they see along each pathway. And if even a small percentage gets into the wrong places - especially the mains wiring - it can cause significant RFI.
I had that problem with the low-band antennas here. There was no possibility to change either the antennas or the feedline runs; both were in the only places they could be. Enough RF current was flowing back through the shack, and out again into the mains, to cause RFI to the neighbors. The cure was to place an RF block in the shack mains feed, using a three-wire filter which includes a ground-line choke. The unwanted RF current coming into the shack is greatly reduced, because it no longer has a low-impedance exit path into the mains.
These results have also been verified by measurements with a clip-on RF current meter.
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