Good work locating the culprit, Ken! Several thoughts and questions.
1) Is the conduit steel or PVC? If PVC, I suggest trying to re-pull the
power as twisted pair.
2) If the conduit is steel, it will provide shielding IF it is
continuous for its entire length, and if it is terminated to the
electrical boxes and panels that feed it, and to the shielding enclosure
of the A/C unit.
3) The RF can be coupling to the unit from its internal wiring. Current
flows in loops, the larger the loop, the greater the magnetic field
coupling and the greater the antenna action. If you have access to
internal wiring, it's worth rewiring the unit with twisted pair for as
much of every path as possible.
Twisted pair helps two ways. First, it turns the path into a
transmission line, so the magnetic field is quite small because the loop
area is small, and the field is confined to a very small region between
the conductors. Second, twisting causes the voltage and current induced
into the two conductors to be more precisely equal, so the differential
voltage is smaller.
4) Make certain that neutral is not bonded to the green wire (or any
other ground) at the motor. Neutral MUST be bonded at the service
entrance, but must NOT be bonded anywhere else. This is a common error,
especially with legacy installations. Some electricians don't know about
this.
73, Jim K9YC
On Sun,9/11/2016 5:22 PM, Ken Widelitz wrote:
After having my XYL flip circuit breakers while I was watching a bandscope,
I found that an A/C unit on the roof was causing an increase in 2nd harmonic
interference. Any suggestions for how to deal with this besides flipping the
breaker off during contests? I can't wind the power cable around ferrites as
they are fixed in conduit.
73, Ken, K6LA / VY2TT
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