[RFI] Rcvr burn-out from HV field

dgsvetan@rockwellcollins.com dgsvetan@rockwellcollins.com
Tue, 4 Sep 2001 16:28:37 -0500


Just a brief note about a bad experience I had on August 19th, with the
hope of reducing the chances that someone else will have this happen:

I operate an Alinco DR-590 dual band (2m and 70cm) FM rig in my mobile,
which is a minivan.  The DR-590, being an early generation dual band rig,
has separate ports for each band, thus making it a real treasure in my
mind.  (I feel that separate antennas offer less compromise in peformance
than dual or multi-band designs.)  I have separate 5/8 wave whips on the
roof (yes, I drilled the metal), and the rig is permanently installed in
the vehicle.  (That means the cabling is all buried, the chassis is bonded
to the vehicle frame and bolted into it, and the power feed is direct from
the battery and under-hood chassis ground point - NOT the NEG (-) terminal
of the battery.)  This configuration has worked well - no RFI between car
and rig.

I was driving the van along I-380, about 20 miles north of Cedar Rapids,
IA.  The 2m side of the radio was actively receiving a Cedar Rapids
repeater.  Temperature was around 76 to 80 F (close to 25 C), and humidity
was moderate and comfortable, probably about 40 to 50 percent.   There are
two sets of HV lines from a nearby major power plant that cross the
highway. These are on the very large support towers and insulators, with
those lines operating at somewhere around 500 to 750 kV.  The belly of the
line sag clears the highway by at least 50 feet [16 m] (visual guess).

As I approached the area of where the power lines cross the highway, I
noticed some increase in 60 Hz "rasp" added to the signal from the
repeater.  The rasp got VERY loud just as I was passing beneath the wires,
and then the repeater signal was gone.  A quick check confirmed my fears:
blown receiver front end, on the 2m side only. The 70cm portion of the
radio still works, and the 2m transmitter output is not affected.  No, I
did not hear any arc.

I have passed beneath those same wires before, in the same vehicle with the
same large 2m antenna in use, but with a different rig.  No problem.  At
this time, I am preparing to send in the DR-590 for repair.  Assuming that
the input FET can be replaced, my plans for avoiding a recurrence of this
situation include fitting the 2m antenna connection with a coaxial "T" and
placing an RF choke off the leg of the "T".  The choke will have to have
minimal effect (high Z) at 2m, but will need to present a very low
impedance to 60 Hz.  I don't believe that gas discharge devices will help,
as they can not limit voltage across the antenna terminals to a level low
enough to save the FET.  After all, the rig delivers 50 watts of output on
2m, so any protective device must not clamp or limit normal voltage for
that power level, nor must it introduce significant VSWR.  (Note:  The
input gate of the Rcvr FET in the DR-590 is AT DC GND.  The circuit uses a
varactor in series with this coil to tune the FET across the band.  The
coil is therefore between the input gate and circuit ground.  There is a 27
pf cap between the input of the rcvr and the T-R switching.  There is a
Schottky diode [just one, not a back-to-back pair] between the varactor
side of the 27 pf cap and ground, with the cathode grounded.  It appears
that enough field was coupled thru the cap to damage the FET in spite of
the DC path to ground and one-way clamping action of the Schottky.)

I have no way of knowing whether my previous passes beneath those wires,
without incident, were non-eventful because of different atmosphere
conditions (temperature and relative humidity) or because my other rig has
a more resistant RX front end;  either way, I don't intend to test again!
(I travel that stretch of road a few times per year, but I never know when
I might drive elsewhere and encounter a similar HV line crossing.)  My 2m
antenna is a large Comet, NMO mount, standing more than 1m above the
minivan roof.  Obviously, some real "volts per meter" can be coupled onto
it from the fields which surround these lines.

I would be interested in knowing if anyone else out there has had this sort
of thing happen to them.  Two local hams have informed me of having their
rigs damaged by fields from the large type of HV lines. One lost the rcvr
of his HT, the other a mobile.  Hopefully, none of you on the reflector
have had, or will have, this sort of experience.

73, Dale