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Old 21-06-2008, 07:25   #1
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Lightning strike and VHF antenna and cable

I was struck by lightning and it zapped the VHF radio and some of the electronics. Don't think it was a "direct" strike because steaming and anchor lights still work, as do some of the instruments (depth/speed, etc.)

I am replacing the wind instruments with Tacktick wireless, so no need to unstep/step the mast for that.

At some point in the future, probably in the winter, I'll step/unstep the mast and remove the now unnecessary wiring, and replace anything that needs replacing, but I'd rather avoid that right now, in the middle of the sailing season.

However, the VHF radio is a concern. The antenna is damaged but the coax cable seems to look OK on both ends. What I would like to do is replace the base and antenna at the top of the mast, and replace the radio down below, but not (right now) replace the cable.

Are VHF coax cables that have conducted a lightning strike usually unusable? Is what I am suggesting feasable? Advisable?

And if this might work, any good way to test the cable?

Thanks in advance.
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Old 21-06-2008, 08:29   #2
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To test the cable, you need a helper at the top of the mast.

Step A--with the cable disconnected on both ends, check the resistance between the shield and the inside wire (the shell and the pin of the PL259 connector). It should be at least 1 megohm.

Step B have the helper short between the pin and the shell of the connector on the top of the mast, and again measure the resistance between the pin and the shell at the radio end. I should read less than 5 ohm.

If it passes the test, it is probably good, and you can do the usual with the new radio, like listening for the weather broadcast and calling for a radio check...
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Old 21-06-2008, 09:35   #3
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While you have him up the mast you might as well put an SWR/power meter in the line between the coax and antenna and check the SWR and power there.

I had complaints about my signal strength some time ago and I put the meter in line at the radio. The SWR was under 2:1 and the radio was putting out 25 watts. I thought well that's good. My more savvy electronics friend said I must test it at the antenna. Well I knew there was a union down low in the bilge by the mast, so I put the meter there first before going to the trouble of going up the mast. 0 watts, and of course with no power, no SWR. Salt water had gotten into the cable between the radio and mast and corroded it. I forget whether the shield or the center conductor was open, but one of them was. The part that had surprised me was how the impedence (basically resistance at frequency of interest) of the cable had remained close enough to 50 ohms so that the SWR was still good enough that the radio continued to put out full power. Of course that means I was trying to use the coax laying in the bilge as my antenna. Probably explains the poor transmission and reception, ya think?

John
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Old 21-06-2008, 13:48   #4
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Quote:
Originally Posted by StanleyCup View Post
Are VHF coax cables that have conducted a lightning strike usually unusable?
Usually, it's the mast and rigging that conduct the brunt of current from a lightning strike. We've had several hits over the years at the Naval Academy on our sloops and only once has the VHF radio been damaged, while the antenna has been completely destroyed. Have never had a damaged coax cable either. Iv'e had lightning hit my HF tuner on the roof of the building where I work and found the cover of the tuner about 50' away and the inside of the tuner blackened and several components blown apart, but no damage to cables or radio. I suspect your cable is ok but as already stated, you can do a resistance check on it with a multimeter, but even better would be to check it with a megger with a voltage a bit under what the cable is rated for.

Eric
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Old 21-06-2008, 15:21   #5
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Quote:
Are VHF coax cables that have conducted a lightning strike usually unusable? Is what I am suggesting feasable? Advisable?
My neighbor was struck twice and lost radios in both cases. In his two cases everything but the radio was fine. After the second strike he added a protector for the coax. It added about 1/2 db of signal loss but it is hoped it will save the radio the next time. In his two cases he was indicted and actually struck. Induction is most common and you can get it easier if your boat is plugged in to shore power.

I had the Tacktick wind instrument on the last boat and it was a pretty amazing device. If I lost the Raymarine on this one I would get another one. If you pull the mast you can then pull out all the wires and review them and see what needs to be replaced. It's a good idea to do that anytime you pull the mast.
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Old 22-06-2008, 05:01   #6
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Originally Posted by StanleyCup View Post
I was struck by lightning and it zapped the VHF radio and some of the electronics. Don't think it was a "direct" strike because steaming and anchor lights still work, as do some of the instruments (depth/speed, etc.) ...
.... Are VHF coax cables that have conducted a lightning strike usually unusable? Is what I am suggesting feasable? Advisable? ...
... And if this might work, any good way to test the cable? ...
Given the many possible variables, the word “usually” is meaningless, when discussing lightning damage. Even similarly described “lightning events” can vary greatly in their etiology & consequences.

Utilizing an Ohmmeter (continuity tester), you can perform a basic DC Continuity (Open) & Short Circuit test on your “suspect” coax cable. These tests will reveal "gross" damage only .

If the antenna line has been damaged, it is possible that:
1. Either the inner conductor or the shield are “open”.
2. The shield could be shorted to the inner conductor.

1. OPEN (Continuity) Test):
Test both conductor & shield. With the antenna disconnected, and the inner coax conductor shorted to the outer shield, there should be zero resistance. You could determine which comnponent is open (the inner conductor is more likely to be open than is the shield); but it doesn’t really matter - the cable is “toast”.

2. SHORT Circuit Test:
With the antenna disconnected (open coax cable ends), there should be NO CONTINUITY between the inner conductor and the outer shield. A higher resistance will indicate degraded insulation. A lower resistance will indicate a short.

As previously mentioned, an Output Power and VSWR meter (designed for Marine VHF: 156 - 160 mhz) is a useful tool for testing connected antennas.
See: Shakespeare ART-2 ($50), or ART-3 ($100), and others such as MFJ (259B?) ...

See the excellent short tutorial “VHF Marine Antenna Fundamentals” ~ by Al Corkins
VHF Marine Antenna Fundamentals

What are the Chances of Lightning Striking Your Boat?
The following statistics are based on all of the BoatUS Marine Insurance claims for lightning damage over a five-year period. The percentages suggest the chances of the various types of boats being struck in any given year.

Auxiliary Sail 0.6% Sixty out of 10,000
Multi-hull sail 0.5% Fifty out of 10,000
Trawlers 0.3% Thirty out of 10,000
Sail Only 0.2% Twenty out of 10,000
Cruisers 0.1% Ten out of 10,000
Runabouts 0.02% Two out of 10,000

BTW: Thunder is more than just a rich source of loudness (audible up to about 25 miles).
It can also tell you interesting stuff, such as how close you came to being hit by lightning.
Since light travels at 186,000 miles per second, you see the lightning the instant it flashes. But sound, including thunder, travels about a mile in five seconds (1000Ft/Sec, 340m/sec) near the ground, at normal temperatures.
Lightning closer than about three miles away is a warning to take shelter immediately. Successive lighting strikes are often two to three miles apart. If the first stroke is three miles away, the next one could hit you.
If you don't hear it, you got hit, so never mind.
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Old 22-06-2008, 06:09   #7
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This is all very good advice, and makes sense. I'm going to do these checks in a few days. Thanks to all!
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Old 07-07-2008, 08:29   #8
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Iv'e had lightning hit my HF tuner on the roof of the building where I work and found the cover of the tuner about 50' away and the inside of the tuner blackened and several components blown apart, but no damage to cables or radio.
Lightning strikes again! At some point over this past weekend lightning hit my 35' whip again at work, which is on top of a 50' building. Blackened part of the PCB and blew off some circuit traces in the tuner, blew the GTO-15 in half, fried all the semiconductor devices in the tuner but no damage to coax, control cable or the M710 radio. Put a new board in, replaced the GTO-15...back in business. My VHF marine antenna and 2 meter ham antenna are just a couple feet away from the HF tuner but no damage to any of that.

Eric
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Old 07-07-2008, 09:37   #9
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It is possible that the coax will test perfectly good. But since it is plastic jacketed, and plastic melts from heat, and that zap may have burned some of it away...The only way to be SURE that the coax is undamaged, would be to pull it out and visually inspect it, even if it passed all other tests. If there's a melted spot in the outer jacket, water will eventually get in and it will eventually fail. I would say it is not worth the time and effort to futz around, but simply buy new cable and replcae the old one in one shot.
Coax cable usually ages too gracefully. That is, it stretches under its own weight (unless supported really carefully every few feet) and degrades, and while may folks say they use a cable "forever" an equally large number will tell you to simply replace it every five years anyhow, to make sure it is still good when you really NEED to use that radio.
If the cable that comes out is still good, great. You've got spare wiring for an emergency antenna. But unless it was really top-dollar and fairly new, just be done with it.
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