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vhf/stereo interference

Old 05-11-2008, 07:41 PM
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Default vhf/stereo interference

Set up my electronics box like many others that have
posted their set-ups. Stereo on the left, VHF on the right. Both are wire to the fuse panel in the box. VHF antenna is on the port side, just inside the hand rail. Stereo antenna is on the port side too, but only about 6-8 inches from the mast light.

Stereo works, sounds good. VHF seems to be ok getting weather, etc. Stereo is a SONY, VHF is a uniden.

Was playing around tonight and discovered that when you key
the mike, the stereo goes to static. As soon as you release
the mike, stereo resumes as it was.

Any idea what I have going on here? Solutions?
Old 05-11-2008, 07:51 PM
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Default Re: vhf/stereo interference

Don't listen to the stereo while talking on the radio!

Just kidding (but it's true). Have you tested your antenna connection? Shakespear makes an inexpensive watt meter that will measure forward power, VSWR, and will test receiver sensitivity. A VSWR reading greater than 2 may indicate a bad connector. The lower the reading the better and 1.2 is very good if you can get it that low.
Old 05-11-2008, 08:16 PM
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Default Re: vhf/stereo interference

Interference from the VHF into the stereo. It is going to be hard if not impossible to get rid of. What I suspect is happening is that you are de-sensing the stereo. The signal strength from the VHF is overpowering the weaker signals from the distant FM towers. So the radio is getting blanked out. You can try low power. That may help. But even at low power the signal strength from the nearby transmitting antenna is going to be much stronger from the distant FM tower - even if the distant FM tower is cranking out something to the order of 100,000 watts. However, if it really bugs you - you can get a band pass filter that will make your stereo deaf to the marine VHF band but will allow the FM stereo frequencies through.
Old 05-12-2008, 08:48 AM
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Default Re: vhf/stereo interference

If the VHF signal is coming in from the FM antenna, than a low pass filter with a cutoff frequency around 110 MHz in the FM cable to the radio might be able to help. Remember the signal decays with the inverse square of the distance between the antennas, so the farther you can space the antennas apart the better off you are going to be. Also most FM stations are circular polarized now which means they transmit in both the horizontal and vertical planes, so try putting your FM antenna in a horizontal plane rather than a vertical plane, that will reduce the VHF signal into the FM antenna by some 20 DB. The strength of the received FM station will also be part of this equation, the stronger the FM station the better chance you have of not being interfered with.

Is your FM receiver a marine stereo or just a standard stereo? I wonder of marine stereos have a built in filter for VHF frequencies?


Jim

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