RFI: A Practical Example Of The Effect

A Communicator Reprise: January 2013

At the suggestion of Jim Smith VE7FO, I recently joined an RFI reflector at http://lists.contesting.com/.  Why?  Because I hoped to find some answers to a frustrating Radio Frequency Interference (RFI) problem that I was experiencing.  

I had installed in my bathroom an electrically heated floor and thermostat c/w built-in ground fault interrupter (GFI).  However, whenever I transmitted, even briefly, on 15m or 10m the GFI would trip and the power would go off.  The first time this happened, the thermostat failed to work again even after resetting the GFI.  First call was to the supplier (NuHeat) of the “Solo” thermostat.  

Their customer support team knew nothing about RFI and I had to explain that it was likely a problem with the thermostat and not a problem with my radio transmissions.  Without argument, they replaced the thermostat with a new one (#2).  Not wanting to take further chances with ruining another thermostat, I decided to install some RF protective devices, as I assumed that the 220 v house wiring was picking up RF and passing it through to the thermostat. 

While RF interference is not a new problem, it is most commonly experienced with telephones and audio devices.  However, nowadays the large variety of RF susceptible electronic devices in our houses has greatly increased the likelihood of problems.  The number of devices emitting RF has also increased accordingly.  Plasma TVs, computer monitors, electric fences, touch turn-on lamps, halogen lights, and wall-wart switching power supplies are just some of the items that are reputed to send out wide-spectrum RF, but there are many others.  What to do in this case? 

First, I applied the standard remedy: I clamped split ferrite cores around the power cable near the thermostat.  That appeared to have no effect, however this time the thermostat could be reset and made functional, which was progress of a sort.  Then I ordered an RFI kit from Palomar and when it arrived I added ferrite beads to the individual power leads.  No improvement.  Next I connected .01 uF disc ceramic capacitors between the hot leads and ground.  Also NuHeat had provided me with a “snubber” – a capacitor with a resistor in series – which I connected across the power leads.   However, the problem persisted.  Finally after many emails and telephone back and forth, NuHeat put me in touch with Honeywell, the manufacturer of the thermostat.  

Honeywell responded very quickly first asking me some questions about my power level, frequencies, SWR etc that might suggest a transmitter problem.  After they were satisfied with the answers, Honeywell couriered another thermostat (#3) to me for trial.  No cigar.  When I reported this failure, they promptly sent me yet another thermostat (#4) called “Harmony”.  This is a more expensive device of (apparently) different design.  It worked!  

One by one, I removed the RF chokes and capacitors until they were all gone.  After several days of testing at different frequencies, there has been no effect on the thermostat/GFI, so I mounted it permanently and thanked Honeywell for their efficient service. 

Lesson: if you have an RFI issue, try the standard remedies but when they fail to solve the problem, get the manufacturer on board with device replacement in mind, as with modern radios it is typically not the radio transmissions that are at fault.  This approach may not work with some of the off-shore manufacturers, so bear this in mind when purchasing electronic devices – they may not care about their reputation and customer service the way Honeywell does. 

ARRL publishes several good RFI guides as well as the Radio Amateurs Handbook and more information can be found on the Internet.  For unusual problems that defy the conventional solutions, the RFI reflector website mentioned above is another excellent source of expert advice.

~ John VA7XB

   Now enjoying toasty toes in his remodeled bathroom.