Hum problem with VHF radio on transmit

[BBC16385]

We have a 1997 Apelco VHF 5160 radio that has developed a loud hum when transmitting. Receives fine. Radio is mounted in cockpit. I have inspected the connections on back of unit and oiled them with the blue spray for electric connections. Have not inpected the connection at masttop between wire and antenna.I would appreciete any thoughts on what might be the problem and how to fix it.

 

[Jim Rushing]

Question?

When you are transmitting, the receiver should be squelched. Are you saying that the receiver is still receiving or is the hum being heard in another receiver when you transmit?

 

[Don]

Hum

Jim's question is a good one but assuming you are talking about one radio, it's likley a bad ground. Faulty ground connections at either the radio output or at the antenna will cause a ground loop for RF which can cause the hum you describe while not effecting receive quality. A bad ground includes faulty coax outer braid corrosion caused by water intrusion in the coax.Best fix is to remove connections and inspect.The only other possibility I can think of is a resonant length of coax causing RF to get back into the radio but that would make sense only if you have changed the coax length just before the hum began.Don

 

[BBC16385]

Hum is heard by those receiving transmission

Sorry, I could have been more clear.My transmissions have a hum so loud that those hearing it can hardly understand what I am saying.On 1 watt, the hum is not as bad and I can be understood. Transmission range at 25 watts is still good but the noise [hum] is the problem. Nothing has changed in hookup of system except age of installation and now I have an XM Sat radio. But I still have hum when the XM is off.From what Don says, it sounds like I need to go up the mast and inspect/service the antenna connection.

 

[Ernie]

Could be bad ground or...

it could also be noise from your battery charger. You didn't specify whether you were underway or at the dock when the hum occurs. I'm assuming it happens while underway, which would negate my suggestion of the charger, but I've seen it happen while at dock.I'd check the ground connections first.

 

[Don]

RF feedback

BradleyBeing it's on the transmit signal others can hear but at a reduced level with low power pretty much confirms the problem as RF feedback into the radio caused by a bad ground someplace either at a connection or water in the coax. Unfortunately a simple multimeter ground test won't work in this situation.

 

[patrick]

Try eliminating various sources

There are a lot of variables. First thing to try is unhook from the dock, get away from the marina, and turn everything else off. Transmit using the battery only. If that fixes the problem, turn on things one at a time.If the problem is still there while operating on battery power alone with everything else turned off, you might try a different and temporary antenna with a different coax to connect to it. A short length of coax with the antenna hung outside will do. You won't have the range but you can see how it sounds.My bet is on the XM setup and some kind of grounding problem with it. I would check that out very very carefully. You might need to completely remove it from the power source and all return and audio circuits. Mixing up power grounds with rf ground and or bonding systems is a common issue. You might be eating up your neighbors prop and not even know it.Your description of the sound is not exactly clear either. Is it really a hum or more of a buzz and what is the frequency?

 

[Don12364]

Shore Power?

It sounds as if you are still on shore power when this happens. If so it is your battery charger modulating the battery voltage which, in turn, transmits 120 Hz hum. Worse when drawing more power so, non existent on receive, a little on 1 watt transmit and more on 25 watts.

 

[BBC16385]

Not on shore power

This is happening when on battery, but with the inverter on. Never had the hum before in 7 years of use.I'm thinking loss of grounding or wear on connection/coax cable at top of mast. I need to plug another radio into my antenna and see if it transmits hum.Thanks to everyone for your ideas.