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Go Back   SailNet Community > On Board > Gear & Maintenance > VHF Problem
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Thread: VHF Problem Reply to Thread
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Topic Review (Newest First)
06-14-2007 06:11 PM
SEMIJim
Quote:
Originally Posted by brak
Check that the coax jacket does not come into contact with any metal, especially anything connected to the boat ground system.
Other than the fact this would be indicative of a breached jacket (generally not considered a Good Thing): Why? The shield is already at or near ground potential, anyway. (Tho, admittedly, you don't really want random grounds .)

Nit-pick: By "jacket," I believe you meant "shield." The jacket on most all common flexible coaxial cable is non-conductive.

Quote:
Originally Posted by brak
You can verify that by testing the resistance between the coax jacket and something that is definitely grounded (engine, mast, whatever). If you get anything less than infinity - that may be not too great.
With the coax disconnected at both ends. Otherwise there'll be a gound connection through the chassis at the radio end, and possibly one through the antenna at its end. (I honestly don't know if marine VHF antennas are normally grounded on "one side," as are [most] land- and mobile-use VHF antennas. New area for me.)
06-14-2007 05:55 PM
SEMIJim
Quote:
Originally Posted by btrayfors
WHAT through-deck plug/socket??? There should not be such an animal in your VHF line. The coax should pass through the deck whole, using a waterproof fitting.

I'd suspect any such "splice".

Bill
Other than the fact it may not be common: Why? There's no reason a proper bulkhead connector couldn't be in the line. I'd tend to avoid it, because each connector in a VHF transmission line induces loss, and I'm a perfectionist about such things.

Jim
06-14-2007 03:46 PM
brak Check that the coax jacket does not come into contact with any metal, especially anything connected to the boat ground system.

You can verify that by testing the resistance between the coax jacket and something that is definitely grounded (engine, mast, whatever). If you get anything less than infinity - that may be not too great.
06-14-2007 03:28 PM
btrayfors
Quote:
Originally Posted by Terah
...and also the through deck plug/socket....
WHAT through-deck plug/socket??? There should not be such an animal in your VHF line. The coax should pass through the deck whole, using a waterproof fitting.

I'd suspect any such "splice".

Bill
06-14-2007 03:02 PM
SEMIJim
Quote:
Originally Posted by Terah
OK, infinite resistance with radio and aerial disconnected. Next to none with the far end shorted. Very high resistance (only shows on 20M scale) with aerial connected.
That would tend to indicate the antenna--unless marine VHF antennas are very unlike any other VHF/UHF antenna with which I've dealt. (And that's been a lot, over the years. Doesn't seem likely--but it's a strange world.)

Quote:
Originally Posted by Terah
Have swapped the aerial, no change.

With borrowed cable+aerial the radio still works fine.
Your cable and/or antenna are haunted .

Seriously: Unless it's something in the antenna-end connector that only shows itself when the connector is screwed on to the antenna, I'm at a loss.

Can you try the borrowed coax on your antenna?
06-14-2007 01:44 PM
Terah OK, infinite resistance with radio and aerial disconnected. Next to none with the far end shorted. Very high resistance (only shows on 20M scale) with aerial connected.

Have swapped the aerial, no change.

With borrowed cable+aerial the radio still works fine.
06-14-2007 12:09 PM
SEMIJim Oh yeah: Depending on the antenna design, it may or may not show appreciable DC resistance (up to a couple ohms or so) when the coax is connected to it. (Don't have direct experience with marine VHF radios, so I don't know what they've generally got mounted up there.)
06-14-2007 11:57 AM
SEMIJim
Quote:
Originally Posted by Terah
Hi,

I've recently installed a new aerial (old cable was rotten)
Must've been very old cable. It was all inside the mast or cabin, no? Should've lasted until about the heat death of the universe. (Unless you had a leak at the top of the mast, in which case water may run down the inside of the jacket--and that'll destroy coax for certain. And in relatively short order!)

Quote:
Originally Posted by Terah
and am having problems with signal strength, both transmitting and receiving. When receiving there is a hiss that disappears if I ease the plug out of the set so the conductor is still connected but the braid is not.
That doesn't make sense. FM VHF radios have a (almost always adjustable) "squelch" control. With that set just on "the edge" of squelching, you may well experience the radio going into and out of squelch as you mess with the antenna connection, but, other than that, the "air noise" shouldn't change appreciably with the antenna connection.

At least not in the direction it is for you. Now, if the antenna or coax is shorted, the "hiss" may disappear when you plug in all the way.

Quote:
Originally Posted by Terah
I've replaced the plug at the radio end of the cable, and also the through deck plug/socket, and tried another aerial and the hiss persists which seems to narrow the problem down to the cable but I'd appreciate any thoughts on the possible cause before I rip out the cable and purchase yet another one.
Disconnect the connector at the radio and deck. Put an ohmmeter across center conductor and shield. It should measure infinity (open). Short the far end (alligator clips work fine). You should measure nearly zero (0) ohm resistance. Unplug the cable from the antenna end. Do the same for that segment. Plug the cable into the thru-deck connector and test the entire length the same way.

If all that behaves as it should, oods are either the radio's defective or the antenna is, probably. Antennas of this type are pretty simple affairs, so I'd tend to suspect "radio."
06-14-2007 11:23 AM
Terah Just borrowed another cable and aerial which works fine so set seems alright. Have also checked the existing cable and there are no breaks.
06-14-2007 11:17 AM
camaraderie Buy a new radio at West to test eherlihy's theory. do not install but hook up and plug into 12V and antenna. If the problem goes away...complete the installation. If it does not...it is your wiring. Return the radio to the store and buy some new wire.
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