Elper wrote:
Hello,

I have a bizarre problem with my VHF and could do with a little advice...
When transmitting (any channel) the VHF 'overlays' a sound on the
transmission...
This varies from a buzz (only slightly annoying) to commercial FM stations
???
Apart from causing listening stations to say some VERY unpleasant things,
this
renders the radio useless. I am about to send it back to the manufacturers,
but
wondered if there is anything wrong with my instalation that could cause
this.

I would guess: shielding, intermodulation, or grounding.

At your breaker panel, turn off every circuit on the boat except for the
VHF and try the radio. If the problem clears up start turning circuits
back on one at a time until it comes back.

It is possible that the IF from an FM radio on your boat is getting into
the mixer in your VHF. If the case of the radio is plastic this could be
the problem. Note that some radios have a metal case for shielding
inside a plastic case that is intended to keep water out. You can buy
shielded cases to mount electronics in, but if your radio isn't well
shielded I'd return it and get a better one.

Intermodulation is the result of signals mixing and interfering with
something else. The mixing could be external to the radio. Rusty chain
link fences are famous for this: the chain makes a dandy antenna and the
rusty connections can function like a diode (like the old crystal
radios); fences won't follow you around (grin) so if the problem
exists everywhere and not just at your marina intermodulation (if it
exists) has to stem from something on your boat. Unlikely to be passive,
so the circuit test above should help pin it down. I'd suspect powered
TV antennas, FM radios, networked navigation equipment, and autopilots.

If your VHF and/or FM radio are not grounded correctly (see manuals and
a general reference like Nigel Calder's book) all kinds of signals can
floating around. The VHF should have a grounding stud on the back
(inside the console if surface-mounted) that should be wired to the boat
ground (either an electrical bus somewhere, a stud on the engine(s), or
a grounding plate connection somewhere -- check boat owner's manual).
The antenna should not be seperately grounded ... most marine VHF
antennas are balanced radiators that only connect to ground through the
shield of the antenna cable. The FM radio case should also be grounded
seperately from the negative power line.

These kinds of problems can be a real challenge to track down sometimes.
Good luck,

dave



Many thanks in advance for any replies, other than 'turn your radio off!'
;-)

Elper.