[RFI] 80/160M RFI in neighbor's oven circuitry (PART 2)

Jim Brown jim at audiosystemsgroup.com
Mon Feb 9 00:48:30 EST 2009

On Sun, 8 Feb 2009 15:16:23 -0500, WA3GIN wrote:

> But I had to connect the case of the Corcom filter to those 
>useless ground rods ;-) or it didn't work worth a damn!


A filter is essentially a frequency-sensitive voltage divider, so 
the bottom of the voltage divider has to be connected to SOMETHING 
to "work worth a damn." The bottom of the voltage divider is tied 
to the filter enclosure, so until you make the proper input and 
output connections to the shield enclosure, you haven't finished 
hooking up the filter. The "normal" connection point for that 
enclosure is the shielding enclosure of the equipment that the 
filter is filtering. A connection to EARTH is not necessary. 

You connected that point to ground rods, and that worked, because 
it gave the voltage divider current somewhere to go. It would also 
have worked if you had connected it instead to a long piece of 
wire, especially if the wire was an open circuit quarter wave. And 
it would have worked if you had tied the shield of the filter to 
the common or the shielding enclosure of the telephone, again with 
no connection to the earth! 

The earth connection would probably NOT have helped if you had 
made it to the telephone instead of the filter, because you would 
probably have increased the RF current flowing into the phone 
(unless you also bonded the filter common to the telphone common). 

>Sometimes EEs can get a headache trying to figure out why things
>don't work the way the text book says it should -- must be the 
>magic of radio ;-)

I'm sorry, but I don't subscribe to the "know-nothing" school of 
radio or electronics. EE or no EE, you've got to draw the circuit 
and study where the current flows. Once you do that, you 
understand, and there's no magic, just plain ordinary science. 


Jim Brown K9YC

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