[RFI] Solar water heater

[gator@wolfenet.com]

I have had "line noise" in a particular direction for some time.  On 20m, it
was S5 on the stingy S-meter of my Drake TR7, which probably would be about
S9 on the S-meters of normal (JA) radios.  Because the noise was constantly
present, I thought it was something on an electric pole.

Recently, our outstanding municipal electric utility, Seattle City Light,
sent their RFI-locating crew to help.  They used a VHF AM receiver with
manual RF gain and a handheld yagi, and quickly determined that the RFI was
not emanating from a utility pole, but from inside a neighbor's house.  With
the cooperation of the neighbor, they found the source to be a component of
a solar-thermal hot water system.

At first take, the defective component was thought to be a plug-in
transformer.  Unplugging it knocked the RFI off the air.  This transformer
looks like the 9V DC modules that are used for calculators, radios, etc.,
but in this unit was simply a transformer with a 12 VAC secondary.  It is
listed by UL as a Class 2 (power-limited) transformer, used for signaling
circuits, furnace controls, burglar alarms, etc.  The
communications electricians and I thought that it was generating RFI due to
a faulty temperature-cutout device, similar to those on old doorbell
transformers.  But then they brought a Variac (variable auto-transformer)
from their shop and put it in place of the plug-in transformer and we still
had RFI.  :-(

The transformer powers a small plastic box that is similar in appearance to
a programmable electronic thermostat.  It has a digital readout that was not
functioning.  It is labeled with the name "Quadra-Temp."  We thought that
this was a temperature sensor, necessary for the proper operation of the
solar hot water system.  The homeowner likes his solar hot water and did not
want to unplug the transformer, figuring that it would disable his system.

Then my XYL Jean, KM7E, contacted a local solar system dealer who was
familiar with the brand of equipment that the neighbor has.  The dealer
explained that the suspect device is only a temperature *display*, and is
not necessary to the proper operation of the system.  The water temperature
is *sensed* from another box, the main control box ("Delta-T" by Heliotrope
General), which is powered directly from 120 VAC and is not generating RFI.

With this new information, since the display was not functioning anyway, the
homeowner agreed to disconnect the transformer.  In fact, Jean asked him to
give it to her, and he did--that'll keep it off the air!!  It could be that
when the digital display failed, its electronics went into some strange
failure mode that began generating the RFI.  Or it could be that this brand
of device just is a poorly-designed, RFI-generating device; I don't know.
But my RFI is now gone!

WOW--I can't remember the bands sounding this quiet!  On the high bands, now
it is S-zero in that direction instead of S5.  On 80m, now it is S1 (in the
daytime)  instead of S4.  Since I never work any DX on 80 or 160
that is stronger than S4-1/2, this will help a lot!  Too bad I'm already
deaf from listening to noise on 80m and 160m...

In about a day, the electric utility helped clear up a noise source that has
plagued me 24 hours a day since at least 1996!  They have some good tools
and some knowledgeable people.  Most importantly, this success story would
not have been possible if we had not had the cooperation of the neighbor.

73,
Charley, K7NW
Seattle
gator@wolfenet.com


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