Off Center Fed (OCF) Dipole Baluns

Does the operation of a treadmill or your exercise equipment wipe out your radio receiver reception? We have heard cases where the treadmill from a neighbor’s house wiped out the ham radio receiver reception from 2-30 Mhz! The treadmill source was finally isolated to a variable speed motor power supply that was injecting RFI into the AC wiring of the house and the AC wiring was acting as a transmitting antenna broadcasting the signal to the entire neighborhood. The case was difficult to track down since the broadband RFI interference only occurred at 7-8 pm each nite when the neighbor used the treadmill. As with all RFI cases the sequence is similar: Find the Source, Choke the Path, Protect the Victim.

Once the source was identified, a simple solution was to choke off the transmitting antenna with a AC line choke shown below which eliminated the interference problem to the ham receiver:

Testimonials

F400-31 AC Power Cord Choke completely cured the RFI problem I had with my wife’s treadmill. Now I can hear the weak signals!

73,
Jim K4AXF
Icom IC-725
Grottoes, Shenandoah Valley, Virginia
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Testimonial

Hi Bob,

Radio: Flex-6700 running barefoot -10 to 100 Watts max.
Antenna: MFJ-1216 – 160/80 meter OCFD wire antenna with matching network.
Elevation: Over 60 feet AGL – fully extended.
Installed: Mid November 2013

I operate mainly on the digital modes running less than 50 Watts.  All of the sudden after a few days operating with this antenna, I start to have RFI and my radio was acting up; such as changing bands, SWR changing values, radio failures and turning off. However, when I used my Hy-Gain 18 HT vertical on the same band none of the abnormality occurred.
I contacted MFJ and confirm that what I was noticing was a failure of their “matching network” AKA balun.  They offered no solutions. Then I contacted Palomar Engineers and bought the 1:1 new toroid RF beads.  I called Palomar Engineers and I happened to talked to you directly; regarding the on going issue with the MFJ antenna. I found out that the 1:1 balun was not what I needed (I was miss-informed by MFJ that it was 1:1 matching network.)
You offered to make the required balun 4:1 and the rest is history.

Here are my findings:

  1. No more RFI displayed on the monitors.
  2. Radio not acting up or turning off.
  3. The SWR at 160/80 meter is 1.2:1
  4. No antenna tuner required.  (radio goes to Bypass automatically.)
  5. Worst SWR is on 10 meter with a 2:1 but still usable, with antenna tuner 1.5.
  6. One happy OCFD user!

Thanks to Palomar Engineers design and engineering for solving this issue.

73, Ernest – W4EG