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  #1  
Old 10-18-2012
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VHF Receive but not Transmit?

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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Re: VHF Receive but not Transmit?

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.

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Re: VHF Receive but not Transmit?

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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Re: VHF Receive but not Transmit?

Quote:
Originally Posted by arf145 View Post
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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Re: VHF Receive but not Transmit?

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Originally Posted by KBuckley View Post
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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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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Re: VHF Receive but not Transmit?

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

Last edited by Capt Len; 10-18-2012 at 11:36 PM.
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Re: VHF Receive but not Transmit?

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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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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Re: VHF Receive but not Transmit?

Quote:
Originally Posted by KBuckley View Post
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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Last edited by asdf38; 10-19-2012 at 11:07 AM.
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