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Old 04-05-2008, 23:16   #1
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Testing VHF Coax cable

Is there a way to test the functionality of a coax cable inside the boat at the radio? Hmmm.... hard for me to describe.... My boat is old. The coax that comes out at the base of the mast is old. It is also fairly hard with a color akin to ancient gray gym socks. And it is not as thick as the coax I have seen on ham units. That being said, the radio does work.

I am having the forestay roller furler redone -- hence the rigger is going to the top of the mast. Seems a good time to do housekeeping aloft.

How is a coax cable tested? Can it be done in the cabin or does it also have to be done where it attaches to the antenna? If the cable has to be replaced, is there a thread here that talk about the specifics of the task? (I cannot find one)

Thank-you

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Old 04-05-2008, 23:54   #2
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The coax will most likely be for VHF, not SSB(Ham). To really test, you need some special test gear that I doubt you will find. A simple continuity test will tell you nothing more than the radio is now, that it is working. If you doubt the cable and can pull a knew one, then because the stuff is so cheap, go ahead and do so. But I think you will find the pulling of a new one is a difficult job. Often they are tied or run in a trunking to protect the cable from the halyards. If you can move the cable, then tie on the new one to it at the top and pull the old one down feeding the new in after it. Hope like heck you don't get it caught on the way down and bingo, your new cable is done. But if the old cable won't easily move, don't try it. It is a mast removal job.
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Old 05-05-2008, 08:37   #3
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Thank-you Alan. Another piece of the puzzle solved.


Michael
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Old 05-05-2008, 08:47   #4
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Michael,

I agree. Replace the coax, even at the cost of some extra hours to do so. VHF is a very important piece of gear, and your signal is only as good as the power reaching the antenna thru the coax.

Use RG-8X coax, and be sure the connections at each end are well done and watertight.

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Old 05-05-2008, 09:55   #5
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Bill: I am not going to replace the cable. Believe me, I would love to get some good and new cable up there. But, it works as it is thus there is no real need for testing. Bottom line is that there is no way am I going to drop the mast to get another cable through. I would rather have an external whip off the stern pulpit.

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Old 05-05-2008, 10:02   #6
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Michael,

In most cases, it's not necessary to drop the mast to replace the coax. There are several ways to do it, depending on the configuration of your mast wiring.

The fact that it "works" says nothing about the need to replace it. Coax deteriorates over time in the marine environment, as do connections thereto. While a second VHF antenna is a good idea in any case, I'd look long and hard at a relatively painless way to replace the old coax.

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Old 05-05-2008, 11:06   #7
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Coax test

A radio technician should have an accurate wattmeter (like a Bird with appropriate "slug") designed for VHF and a 50 Ohm dummy load. The tech tests the radio output into the dummy load. He goes up the mast and places the dummy load on the end of the coax and tests the radio output both at the drive end and at the load end. This way he can determine the cable loss at the VHF frequency as well as the SWR into a dummy load. The antenna cannot be used to determine SWR caused by the coax impedance mismatch.

I always make these tests especially with new coax and connector installations even though it is a pain to do sometimes. I agree with others that you should seriously consider replacing or adding a new cable. There are better jacketing materials available in coax now that does not deteriorate as much as the older ones over time. Moisture does migrate through the old jacketing material over time and vibration can also affect the dielectric.
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Old 05-05-2008, 12:06   #8
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Is there any safe way to run the coax external to the mast along the side? I just cannot handle the expense of dropping the mast now. Just cannot. And if as Wheels says, you get halfway down in the mast she jams up, then I am really stuck. I will look and see if the rigger knows how to test the coax. But damn..... the boat is so old that I think mast routing could be a nightmare.
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Old 05-05-2008, 12:35   #9
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Running coax outside the mast isn't a good idea for several reasons.

Running new coax inside the mast might not be such a big problem. Typically, you'll want to use some sort of messenger line or snake....you don't just drop the coax in.

Sometimes, if the coax is relatively loose inside the mast (e.g., in a channel or tube, but not tied) all you need do is use the old coax to pull in the new. I'd let a rigger do it, though, since it sounds like you might have some doubts about how to do it.

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Old 05-05-2008, 16:57   #10
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MV,

Over the years I have pulled a lot of wire through mast wire channels, one thing that has work very well when things get tight is soapy water. It won't damage the wiring but will make it a lot easier to pull the new coax. Just pour a bunch of soapy water down the wire channel while pulling back and forth on the coax until it moves easily.

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Old 06-05-2008, 01:37   #11
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If the radio works, don't trouble yourself. That is the best test. A simple continuity test will most likely be what any local will and can only do. He can say yes the cable has conductivity and no short to shield. But he can not tell you if the cable impedance is still OK. The impedance is determined by a couple of factors, but the main one that can change is the internal insulation. The only way you can test that is to have a special device that sends a Radio Frequency signal down the cable and measures things like loss and impedance and so on. Not a device any normal person would have.
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Old 06-05-2008, 02:51   #12
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Michael, perhaps every else knows this already, but do you have internal or external halyards and what else is already inside the mast apart from the coax?

If the halyards are external and the mast is reasonably "empty" you could leave the old coax in place (for "insurance") and just drop a new coax in "hanging free" and well secured top and bottom. Certainly not the best option but maybe a possibily in the short term. Only cost is the coax and some connectors.
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Old 06-05-2008, 08:41   #13
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Quote:
Originally Posted by Wotname View Post
Michael, perhaps every else knows this already, but do you have internal or external halyards and what else is already inside the mast apart from the coax? .
All the halyards are external. I am pretty sure of that. There are no lines coming out of the mast. I think the only thing in the mast are the wires for the non working wind instruments and the coax. In fact, now that I think about it, the one wire I can see -- I am not sure what it is for. It might not even be the coax.

I hate when this happens to me. I ask one question and my world starts to unravel.
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Old 06-05-2008, 13:29   #14
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Michael, that doesn't look like coax to me but it is a bit hard to tell from a photo. It looks more like the old wind instruments wiring especially given your original description of it.
Some more dumb questions for you .
Are you sure the coax is in the mast?
You should be able to see it exiting somewhere; either above the deck if mast is deck stepped or below the deck if the mast is keel stepped.

I don't know the background here like is this a new boat for you or have you had it for years or who installed the radio etc but it is possible that there is another antenna somewhere that is actually being used instead of the one at the mast head. Another possibility (but highly unlikely) is there NO separate antenna, just the coax running somewhere inside the cabin with about 20 inches of the outer braid stripped away from the inner and folder away from the inner. A home made dipole in effect. Rough but will work in a pinch (very directional though).

Have a good look around or just follow the coax from the radio end to see where it goes, if possible.

Given that the mast is "empty" and you don't want to spend a lot of money, you could just let a new coax float free inside the mast. I have even seen coax with cable ties attached (and the ends not cut off) radiating out like long flexible fingers (say 6 inches long) that act as "flexible horizontal supports" for the coax to stop it banging around inside the mast as the boat rolls and pitches. Doesn't provide any vertical strain support and is far from ideal but better than nothing.

Good luck
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Old 10-05-2008, 07:11   #15
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What's that plate riveted on just above the cable exiting the mast? If that's some sort of opening to the base of the mast take it off and you probably will get a good view of what's floating around in there. If it is, it would sure make the job of dropping a weighted fish line down from the top and grabbing it there easy. If you don't have one already a good heavy 1/4" pop rivet gun with an assortment of aluminum and S/S rivets is a great addition to the tool box to re attach the plate. You can pick these up fairly cheap and it'll pay for itself the first time you use it.
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