[TowerTalk] RFI in the AC UNIT

[David Jordan]

Hi Folks,

Was camping with last weekend and met W3BEZ. He was using a screwdriver
antenna
mounted on the bumper of his 30ft. 5th wheel. Brand new...No problems on
75meters
but on 40 meters when he operates his TS-120 in SSB or CW his air
conditioner goes on.
He had an MFJ 259. We used it to look for resonant wiring. Found out the
thermostat wiring is resonant on 15Mhz. We had a radio shack ferrite
sandwich so we wrapped one turn of the thermostat wire around it by the
control unit in the AC unit and had some one inch long beads that we
placed over the thermostat cable (all six wires) at the thermostat unit
on the wall. Those precautions reduced the RFI to the point where the AC
unit would not go on all by itself in the OFF position and didn't go on
when the thermostat was set about 10 degrees above the trip temp. but
when the control was set just a few degrees above the
trip temp. the AC unit would activate.

So here is what we planned to do:

o    Move the screwdriver antenna up from the bumper till it is level
with the roof of the trailer.
o    Installed a low pass filter to help reduce harmonics that might
trigger the AC unit circuits (15Mhz resonant thermostat line).
o    Place a line filter on the AC line to the Air Conditioner and on
the 12volt line that feeds the thermostat circuits.

Got one question for the TT'ers...when we were looking at the power
disto box we noticed that the neutral line was
connected to the chassic ground of the trailer but the low side of the
single phase AC line was floating on a plastic
block were all the other low sides from the 115volts outlets /AC unit
were terminated. So,  the the low side  makes it all the way out to the
30AMP camp receptical (no connection to the trailer ground) and down to
the secondary of the in-ground power transformer. We had no way to
visually inspec whether the neutral and low side were connected at the
XMFR.
This is different from how our house fuse boxes are wired. In both of
our homes the neutral and low side are both
tied together in the fuse box. We're wondering why it was left floating
in the trailer and if connecting it to the chassic would
perhaps help with the RF floating around the rig. We're going to call
the manufacturer but wanted to have a second opinion
from the pros!

thoughts,
dave


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