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Re: [CQ-Contest] SO2R 2nd Harmonic Reduction Question

To: cq-contest@contesting.com
Subject: Re: [CQ-Contest] SO2R 2nd Harmonic Reduction Question
From: Jim Brown <k9yc@audiosystemsgroup.com>
Reply-to: k9yc@arrl.net
Date: Sun, 11 Sep 2016 20:47:55 -0700
List-post: <cq-contest@contesting.com">mailto:cq-contest@contesting.com>
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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