Hi Pete,
See my comments below.
To: <rfi@contesting.com>
>Date: Thu, 21 Oct 1999 09:10:57 -0400
>From: Pete Smith <n4zr@contesting.com>
>
>Greetings, all. I just recabled my shack to make the coax cable
>runs from TS-930 to SB-220 and SB-220 to entry bulkhead no
>longer than necessary, and to clean up a number of accessory
>cables. Now, to my chagrin, I find I'm getting RF into my
>transceiver.
>
>Symptoms -- happens only on 80m, and only when my 4-element
>array is pointed toward my chack (from 200 ft away). Happens
>only with QRO. The frequency display on the transceiver flashes
>in time with keying. Does not seem to affect frequency control
>or other aspects of the radio's performance
It sounds like you have a ground loop. This is just another name
for a parasitic antenna. If I had to guess, I would suspect that
the feedline into your shack and the power coming into the shack
for the rig takes a significantly different path. I realize that
this path usually _must_ be different for a large fraction of the
run. But you can probably control the last 5 to 10 feet of the
path enough to suppress the effect.
1. Do you have an established single point ground for the gear
in the shack? This should probably be on the rear edge of
the shelf the radio equipment is on or near this point. I
use surplus grey metal government bureaucrat desks to support
the radio equipment at my shack. Then I make the rear
vertical surface of the desk the single point ground (SPG).
2. Is the only ground connection between the various pieces of
radio equipment the feedline shields? Each piece of
equipment should be tied to the SPG by the lowest possible
inductance means (usually a short flat strap. These
connections should be relatively equal impedance
connections. In particular, these connections should have an
impedance that is a bit lower than the impedance of the
shield connections of feedlines that connect the equipment
together.
3. Power distribution to the gear should be from a receptacle
box or power strip which is attached to the SPG (safety
ground bonded to SPG). The AC power should be RF bypassed
to the SPG at this point. Spike suppression should also be
applied at this point. Spike (overvoltage) suppressors
permit the use of much cheaper bypass caps. And MOVs are
themselves pretty good disc caps for bypassing RF.
4. Any wires that leave the equipment table and go elsewhere
should be shielded and bonded to the SPG before leaving the
table. This includes control wires, feedlines, phone lines,
etc. Wires which for some reason cannot be shielded should
be RF bypassed to the SPG before leaving the area.
5. The wires leaving the SPG should follow the same route as far
away as possible. If the entry bulkhead is on a wall of the
shack, all the wires that leave the table SPG should go there
first before being routed elsewhere. In this case, all
lightning protection devices should be mounted to the entry
bulkhead.
This sounds a lot more complicated than it is. In fact, it is
very simple to do.
>
>I plan to go through the station this weekend, checking the
>integrity of grounds, etc. (it's on the second floor, so RF
>grounding, in particular, is always suspect, but I didn't change
>the ground connection's length from before, when there was no
>inward RFI.
>
>My suspicion is that there are common mode currents on the
>jacket of my coax, and that changing the length of the
>interconnecting cables may have caused a voltage or current peak
>to occur at the transceiver. Is that plausible? If so, would a
>coax common mode choke, either between transceiver and amp or at
>the entry bulkhead, make sense? Any other ideas about things to
>focus on?
>
>73, Pete N4ZR
>Sometimes a tower is just a tower
I hope these thoughts are useful to you.
73, Eric N7CL
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