Posted by
Stuart Rohre on
Jan 25, 2007; 1:22am
URL: http://elecraft.85.s1.nabble.com/Rf-in-the-shack-distorting-audio-tp443216p443224.html
Bob,
The length of conductor from the rig chassis to the eventual ground rod,
including the length of the rod to the physical dirt, all makes a resonant
system. If such a "ground" lead is 1/4 wave or a substantial length at a
given band, you will have a poor RF ground.
As someone said, you need to find out if your antenna is causing the RF to
appear in your shack.
First is the antenna a dipole, and therefore balanced?
Next, is the antenna remote from the shack, not just over the roof just over
the shack?
Are both legs of the dipole, if it is, equally spaced from nearby metal and
conductors, including foliage, trees, buildings, etc.?
What are you feeding the antenna with? Are you feeding with coax to a
balanced antenna?
These are all thought questions to get you thinking in terms of what is
resonating to cause this RF to appear? How is it getting from the antenna,
feeder, or rig to the undesired places? The fact that adding quarter wave
elements did not change it, says the point at which you added quarter waves
was not the point where the RF was strongest.
Don't forget the effect of your AC wiring, it can function as an antenna if
its legs make quarter wave resonant lengths on ham bands.
Simplify your set up to just the rig, tuner and antenna, and work backwards
to add in any computer, jumper cables, other cabling, like phone, or
lighting, and see if there are complex paths the RF is taking.
The ARRL publishes a book on tracking RFI problems and eliminating them.
Make sure any grounding screws holding chassis boxes and cases onto the
radio are really conductors by loosening them slightly, then retightening
them, to break any buildup of oxides.
Make sure you have proper connections of low impedance on the mike cable and
its plug. Is the mike cable shielded? Sometimes there is a press to talk
pair in a mike, and separately, the audio line shield and those should not
be bonded together in some cases. Have you tried another mike and its
cable?
Well, that is a start at some diagnostics.
GL and 73,
Stuart
K5KVH
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