[Elecraft] Elecraft: RFI Problems

[Ron D'Eau Claire]

Bob, G3VVT wrote:

...Opted like Vic for a balanced antenna with balanced feeder. Using 100W
have  
not seen any problems unless I try to operate on a band that the antenna is

not resonant on. Using 75 ohm balanced twin as the feeder as the use of 300
ohm 
 ribbon or higher would be impractical from my shack. The 75 ohm twin seems 
quite  docile in use and can be installed in close proximity to metallic 
objects which  is not recommended for 300 ohm or higher impedance twin
feeder.
-------------------------

All of the 75 ohm stuff that I've seen wasn't rated for high voltages
anyway, so it's likely to melt at the voltage loop when hit with high power.
AT the very least, it'll be rather lossy. 

The "rule of thumb" for any balanced line is to keep it at least 3 or 4
times the wire spacing from objects. With the narrow-spaced 75 ohm line
that's much easier than using twin lead or open wire lines as you noted. I'm
lucky to have my operating desk right at a window directly below the center
of my doublet, so my 2.5-inch spaced open wire feeders can come right down
to a header mounted in the window and into my balanced ATU. That's about the
only way one can deal with wide-spaced lines effectively <G>.

Even with the balanced line, if you happen to have a voltage loop - high RF
voltage point on the feeder - at the ATU output, you may see the whole rig
tend to "float" at the high RF voltage, producing some undesirable effects:
RF in the shack and RF feedback in nearby equipment in the house. One
approach is to cut the feedline to a length that makes the feed
low-impedance. Adding or subtracting up to a 1/4 wavelength on a problematic
band will do that. Often less will do fine. Another approach is the
time-tested 1/4 wave long wire attached to the rig chassis and insulated at
the far end. The far end will be very hot with RF and must be well
insulated, but it will show a low impedance and low RF voltage at the rig.

If by "resonant" you mean your antenna is 1/2 wave long, then feeding it in
the center will produce a decent match for 75 ohm line and the RF voltage
should be low all along it. 

Ron AC7AC