# VHF Receive but not Transmit?



## arf145 (Jul 25, 2007)

We've got a 25-yr-old Uniden VHF in our cabin with an antenna at the top of the mast. We receive transmissions fine--loud and clear from fairly far away. We almost never transmit on that unit--we use a handheld to signal drawbridges.

But I tried the Seatow automatic radio check recently with the Uniden and got no response, and then tried to hail a nearby marina and got no response. The mic is connected and the light on the unit sees when the talk button is held. To me this all suggests that my Uniden is no longer transmitting--time for a new one. But would an antenna ever be involved in receiving but not sending?


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## Gene T (May 23, 2006)

I recently had a mic go bad. The radio is at the nav station with a mic in the cockpit. It simply quit, but would receive just fine. Not an uncommon problem as I found out. Found a used mic on ebay and all works fine now. If you have a detachable mic see if you can borrow another to test it with. Otherwise - I would doubt the antenna is the problem if receive is ok so would suspect the radio. Radios are cheap and have more functions so maybe it is an excuse to upgrade anyway.

Gene


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## KBuckley (Sep 17, 2008)

Probably not the antenna so much but rather the connections between the radio and the antenna. 

I would do two things. First pull off the connectors at the radio and at the antenna (if you can get to that one) and clean any corrosion. Second, check for a voltage drop on the 12VDC going into the radio. The voltage should drop a bit when you transmit - this at least tells you that the radio is trying to transmit. It still does not tell you the signal is getting out of the radio or to the antenna. If you have a voltmeter on your power panel that should show the drop when you key the mike.


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## SlowButSteady (Feb 17, 2010)

arf145 said:


> We've got a 25-yr-old Uniden VHF ...


For $100 or $200 you can get a brand new VHF with DSC and all sorts of other fancy bells and whistles. I would start there. Even if you wind up needing a new antenna/coax you're due for an upgrade anyway, if only for the added safety features of DSC.


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## asdf38 (Jul 7, 2010)

KBuckley said:


> Probably not the antenna so much but rather the connections between the radio and the antenna.
> 
> I would do two things. First pull off the connectors at the radio and at the antenna (if you can get to that one) and clean any corrosion. Second, check for a voltage drop on the 12VDC going into the radio. The voltage should drop a bit when you transmit - this at least tells you that the radio is trying to transmit. It still does not tell you the signal is getting out of the radio or to the antenna. If you have a voltmeter on your power panel that should show the drop when you key the mike.


Good debug advice checking for the voltage drop. But it doesn't sound like an antenna problem if you can receive from far away. The coax/antenna are just two wires (shield and signal), the same two are used for transmitting and receiving. So it's probably time for a new radio which as SlowButSteady says isn't that expensive and it's worth it.


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## KBuckley (Sep 17, 2008)

I should have explained my thinking. The signal levels when receiving are very very low current so the resistance of a poor contact has little effect. When you try to put 25W thru the poor connection by transmitting you will lose lots of power since the power loss is I^2R. If your problem is a corroded connection the new radio will do the same. Also, the power is reflected at the poor connection so you can potentially damage the radio. Hence the manufacturer's warning not to try transmitting without an antenna connected. A repair tech can put a meter between the radio and antenna and tell you the condition of the whole system.


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## Capt Len (Oct 9, 2011)

You can buy old radios by the pound, Go for a new DSC. Remember your mast head ant doesn't talk down worth a dam.(wave propagation) so the hand held is not a good tester. Isee Buckley beat me to the rest of my thought


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## arf145 (Jul 25, 2007)

I certainly get the "why not just get a new radio" thought--I'm thinking the same thing. Just wanted to know if the antenna could be a player. It sounds like the antenna connections could be a factor (thanks KBuckley) and could damage a new unit if they are bad. I'll check the radio connection right away and get up the mast when I can. The voltmeters are no longer hooked up, but I guess I can hook the multimeter up to watch for voltage drop.

And I'll probably get a new unit in the off season no matter what--I'd like to have a remote unit in the cockpit!


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## KBuckley (Sep 17, 2008)

I agree that a new radio is a great idea. Tying in your GPS to a DSC radio lets you send a mayday with your location at the push of a button. An associated MMSI number gives the coast guard all your info. I hope never to take advantage of the feature!


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## asdf38 (Jul 7, 2010)

KBuckley said:


> I should have explained my thinking. The signal levels when receiving are very very low current so the resistance of a poor contact has little effect. When you try to put 25W thru the poor connection by transmitting you will lose lots of power since the power loss is I^2R. If your problem is a corroded connection the new radio will do the same. Also, the power is reflected at the poor connection so you can potentially damage the radio. Hence the manufacturer's warning not to try transmitting without an antenna connected. A repair tech can put a meter between the radio and antenna and tell you the condition of the whole system.


First, RF is a bit of a black art - even experts in the field resort to trial and error. But in general you're over thinking it a bit. The power levels are different but the frequencies are the same and therefore, more or less, you'd expect the losses to be the same.

If you're receiving a far away signal then it's weak when it gets to your antenna - you can't afford to lose much more. If it's getting through then your antenna losses must be limited. If your antenna losses are limited then you'd at least be able to transmit short distances.

Take my example. Up until this season my coax was busted at the base of the mast (completely severed). Before realizing this I installed and tested a new radio. It could both transmit and receive short distances but not long ones.

Sure, spend some time debugging it if you want. But also keep an eye out for sales on new radios.

EDIT: Oh whoops, for some reason I mistook your post as being from the OP. Doesn't change what I said too much but explains why I say to look out for new radios..


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## night0wl (Mar 20, 2006)

long shot, but check the transmit mode. I once had the same experience on channel 78...turns out my radio was set to international rather than usa. I guess international, 78a is reserved, but usa its open for all.


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## arf145 (Jul 25, 2007)

Back from the boat, and while I didn't have time to try everything, I did _apparently_ fix it. And it does look like corrosion on the antenna connection to the radio was the problem, so thanks KBuckley! The coax connector outside looked fine, but unscrewing and pulling the connector off revealed a layer of corrosion inside, especially up in the pin. A replacement would have been good, but as a spot fix, I hit the area in question with a very small rasp attachment on my cordless Dremel. Screwed the connector back on, connected cable to the radio and was heard loud and clear at a marina down the bay. I'll try some other tests next time, including the auto Seatow radio check, but so far so good.

A new radio might still be in my near future, but I'd rather have it be an option instead of mandatory!

Thanks again everyone.


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## KBuckley (Sep 17, 2008)

That is great news. Thanks for the update.


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## btrayfors (Aug 25, 2006)

Tom,

If the connector is/was the problem, you don't need a new radio. Just cut the old one off, and put on a new PL-259 connector.

Bill


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## arf145 (Jul 25, 2007)

Oh, absolutely Bill. And I might replace any others I find in the antenna cable. I assume there's a connector up at the antenna. The talk of the new radio, was, well, just because it is old, and lacks features such as AIS, RAM mike, etc. But really, a new radio wasn't high on my constantly edited upgrade list. I really want cruise control!


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