VHF Radio hum

[vanderd]

We’re running the ICW and my Standard Horizon radio has developed a background hum when another VHF’s mic has been keyed. It doesn’t seem to happen when the caller is close by. Not only is it annoying it makes communication difficult because it is loud enough to make hearing the message difficult.

[requiem]

I assume you see similar effects with many different callers and not just one? (I.e. to ensure the problem is on your end.) How is your own signal when you reply?

Since you mention it changes with distance my first thought would be to check the antenna and feedline (I'm thinking shorts, corrosion, water intrusion, etc). Do you get the same problem when you swap in the spare antenna?

I'm also new to this radio stuff, so I'm just tossing in the basics. It also appears that if you just grab a multimeter to test continuity you can get different results depending on the antenna design, so read up before trying that.

[billknny]

It is not likely the radio, but rather interference from some other piece of electric equipment on your boat. LED lights would be the first guess, but if you have NO lights on, then start by turning EVERYTHING except the radio off, including the engine. Then turn one circuit at a time on until the hum is heard...

You might need to send a crew member ashore, or out on the dinghy, with a handheld on low power to be sure you have a transmission "on demand" to test with

[vanderd]

Most of the traffic on Ch 16 includes the hum. It doesn’t have to be a communication between me and another boat. Unfortunately I don’t have a spare antenna but I did check the antenna connection and it is tight and clean.

[transmitterdan]

On the radio should be a knob labeled Squelch. Turn this knob all the way left until you hear static from the radio. Do you hear the hum when no one else is transmitting? If you hear the hum turn off all other electronic gear including fridge/freezer, lights, shore power etc. Does the hum go away? If so turn on things one at a time to see which might be causing the hum. The most common item to cause hum is a battery charger but it can be any number of things such as a freezer.

Once you know the source of the hum then we might suggest possible solutions.

[vanderd]

Adjusting the squelch as you suggested did not reveal any hum. It comes from other people keying their mic and broadcasting. Close range broadcasts do not create any hum.

[a64pilot]

Mine does it too, my source is the alternator, picked up I believe in the amplified speakers audio line, the amp of course does what its supposed to and amplifies the noise along with the radio call.
I'm too lazy to fix it, on aircraft the usual fix is a capacitor on the alternators output.

You don't want to buy this one as it comes with aircraft pricing, but it explains what I'm talking about
https://www.aircraftspruce.com/catal...atorfilter.php

I've not tried it, but I believe my hum would be present with the squelch off, if I'm correct as to the source of mine.

alternator as the source can usually be proven by changing engine RPM, if its the alternator the whine will change with RPM

[requiem]

I was kicking myself for not mentioning the basic checks of powering everything else off, if only because it's far easier than messing with the mast. But, if you are still hearing it with everything else powered off (and not just in "sleep" mode), and far away from anything else that might have electricity running through it....

A very short cable with the appropriate connectors or adapters should let you use a handheld both before and after the feedline to help isolate the problem.

Regarding handhelds, I'd assume they'd also pick up any local interference in such cases, no?

[vanderd]

Thanks everyone for all of the thoughts and recommendations. I’ll check them out as time permits on our trip