Mobile engine noise

K8DO at K8DO at
Thu Feb 1 17:37:41 EST 1996

I had a single experience with mobile noise, by helping a local ham, in the
parking lot before a club meeting...It was an employer supplied, leased, Ford
vehicle and he was forbidden to make any visible holes, changes to the
vehicle wiring, etc... He had this new vehicle 2 weeks and had been unable to
keep his usual mobile skeds on 40 due to noise...   With only a few minutes
to spare before the meeting I found that the fuel pump noise is directly fed
onto the metal skin of the vehicle, but only weakly radiated, even within a
fraction of an inch... the noise was loudest near the fuel tank, which was
mere inches from the antenna mount... The noise was not entering the rig via
the DC power cable, which ran directly from the battery, because unhooking
the coax silenced the receiver...  The noise was being directly coupled into
the coax at its connection point of the braid to the bumper-hitch/car-body...
As an experiment, I  (we) quickly insulated the antenna mount from the
trailer hitch with a plastic bag and some tape, and ran a 2 , 1/4 wave
radials, with insulated wire, along the two sides of the car, threading them
thru the door handles and wrapping around the car... we then fired up with
this floating antenna system and the pump noise was barely audible ( from
S8-9 before) and inaudible as soon as any signal rose above the noise
level... A quick qso on 40 showed that he was getting out "like
I suggested that he use phenolic sheet (Bakelite, et. al.) to insulate the
antenna base permanently...  and to put 2 to 4 ground radials of small
diameter coax under the vehicle, threaded round and round, along the rocker
panels and under the bumpers, using home made hangers, etc... making the
overall length of each coax 1/4 wave and carefully insulating and water
proofing  the far ends...

The last I heard he is happily mobiling on HF....  


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