Measuring Common Mode at KY6R

The MFJ-854 RF Current Meter

Yes, I will have fun today sniffing around the shack and antennas looking for Common Mode. I’m lucky – besides finding and reading Jim’s Receiving Antenna Choke Cookbook:

http://k9yc.com/RXChokesTransformers.pdf

I ran my ideas by Andy Ikin at Wellbrook, and he reminded me that loop antennas are not as susceptible to Common Mode as verticals are – or other antennas that have radials or are otherwise ground:

“Common mode choke should have negligible effect on the feeder. The effectiveness depends of the ratio of the impedance to ground on each end. For example, if the choke is at the loop end and the loop to ground z is negligible, then the choke wont be effective. However, if the antenna is  a vertical with a ground connection the common mode choke will be effective.

Now, if you have common mode noise on the RX chassis via the power-line connection. This noise could radiate from the feeder screen to the antenna and contaminate the signal. Hence, a choke at the Rx equipment may help.

BTW, the loop antennas have a high rejection of common mode noise.”

This makes total sense – and its why so many try loop antennas of all types – large and small, horizontal and vertical, electric and magnetic. Isn’t this hobby wonderful? Think of all the things you can try and play with. Honestly – making contacts is the boring part, really.

I’ve been using the “Scientific Method” lately to great advantage:

  1. Come up with a hypothesis, in this case “Common Mode is more likely in my shack, not at the loop antennas”
  2. Use the MFJ-854 to measure all cables in the shack and at the antennas out back
  3. Mitigate any noise using Common Mode Chokes
  4. Re-measure and see if I was successful

One thing that I might do is take screen snapshots of the noise with Win4K3Suite’s audio scope feature:

Win4K3Suite Audio Scope – showing noise on 80M

I did just took a look at the audio scope on my 80M Diamond inverted vee vs. the UrbanBeam on both 80M and 40M, and sure enough, there is a LOT more noise on those antennas than the two loops – on all bands. Is it Common Mode? How would I know without the MFJ meter – I guess unhooking the antenna at the antenna and at the radio is one way – see if the noise is present or absent in each of these tests. Maybe do this test and also use the meter? I’ll keep thinking . . .

While I’m waiting for the copper to get here tomorrow, I’ll trench the pipes that the wires are in, and I also have some tree trimming to do just in general and for the second loop – which I will build today and get ready to replace the W6LVP Loop tomorrow – if I get lucky and get the copper early enough tomorrow.

I’ll also wait to add the receiver guard and preamp modules to the NCC-2 until I have the copper on both frames and have them in the rotators. All of this is quick and easy – the hard work has been done of the past few weeks, and my experiments have been a lot of fun. I’ve learned a lot and have been humbled after being so excited about that “KY6R Optimized 40M Delta Flag” – what a dud that ended up being – the EZNec model really was “Too Good to be True”.

Oh well – if learning by mistake makes us smarter, then I’m a Stable Genius.

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