Checking out a VHF coax able?

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Howard

THe mast on my H-27 will be raised in a week or two. At the end of last season I was having a little problem with the VHF. Does anyone know if you can check out the coax with the mast down. Can you do it with a Ohm Meter? I don't know if the problem is with the cable, the cable ends, the antenna or the radio? If I have to replace the cable in the mast I'd like to do it with the mast on the ground. Heights give me the willies. Any advice would be helpful, Thanks,
 
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David E

Coax Testing

The A-1 way to test a coax and antenna is with a SWR meter. I think this would be possible with the mast down if the antenna clear of any metal obstructions by 6 feet. Do Not Touch metal antenna whip while transmitting, and good idea to keep clear of it by 3 feet. To test coax with an Ohm meter: 1. Take antenna off. Measure from inside center contact to antenna(ant.) whip. This should be 0 ohms (shorted). Measure from antenna whip to outside threaded ring. Most of the time this should read infinite ohms (open) [There may be some 1/2 wave antennas out there that should be 0 ohms between whip and ground connection] 2. Measure PL-259 connection between center connector and outer connector with ant. off. This should be infinite ohms.(open) 2a. If connector reads shorted, cut connector off, strip braid back and measure between braid and center wires. It is my experience that problem is usually shorted connector. 3. Short antenna connector. (A buddy with screwdriver held over center pin and base.) Measure between center of PL-259 and outer connector. This should mearsure 0 to 5 ohms assuming less than 100 foot mast. [50 ohm coax is 50 ohm impeadance to radio frequencies not to DC current-ohm meter] 3a. If ohm-meter read infinite ohms check center solder connection at antenna mount connector, try gently moving center of coax away from solder joint. If that is clean, shiny and nice cut connector off and try again. 4. Make sure that inside the antenna the center is clean, shiny and bent down enough to make contact with center of ant. mount base. Make sure center of antenna base is nice and clean. (Can clean both of these with pencil eraser, or bronze wool- make sure no scraps of cleaning utensil is left on connection points) Hope this helps.
 
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Mike H

Ohm is ok if...

You are looking for a dead short. If the problem is intermittant then an Ohm meter is of limited use. 1) How old is the coax? If 10+/- years old then I would replace w/ marine grade regardless. 2) Try replacing the connectors at both ends of the coax. But you will have to cut the coax back until the outer braid is shiny/clean/uncorroded. If the outer braid is oxidized/dull/corroded well back of the end of the coax, it may be a sign it's time to replace it. Note, you may not have enough slack to cut the coax back. 3) Then borrow or buy an SWR (Standing Wave Ratio) meter designed for 150mhz band and hook it up in-line between the radio, and the coax leading to the antenna. Transmit on the radio (with antenna attached to the other end of the coax!). If you get a high SWR - 3:1 or higher, then you have a problem with either the coax or the antenna, or both. Many things effect SWR including height above ground. Ideally the SWR should be at or near 1:1, but if your testing the antenna close to the ground you may not see a 1:1 SWR. Length of coax can also affect SWR - which is why it's important to keep the run of coax as short as practicle. Note, if you check the SWR, then transmit only long enough to get a reading. If there is a high SWR, you can quickly fry the transmitter in your radio. Some radios are self- protecting from a high swr - they autmatically refuse to xmit or shut down if high SWR is encountered - but don't assume your radio is one of those unless you know it for a fact. 4) The Antenna itself could fail, as most 150mhz marine antenna have a coil inside which if exposed to the elements will corrode. Also, the coil can be damaged by a near-miss lightening strike (a direct hit would most likely obliterate the antenna). Hope this helps Mike
 
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IslandJack

Cable is cheap

I've got the mast down on my H-27. New coax was cheap, and easy to snake with the old coax. I used to be an avionics guy and we would change them out sometimes when the radio head and the antennae were changed but the radio was still not working properly. Continuity and output tests would be fine on the ground, but pilots would report malfunctions. We used to say the FM was out. FM standing for 'f**ing magic'. It was probably a small crack in the copper wire center but it was frustrating. Granted a sailboat has a lot less vibration than a Huey, but the ease of replacement would make it a no-brainer for me!
 
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Mark MIllham

Proper check of coax, but why?

the correct way to check a coax cable is with a piece of gear called a time domain reflectometer.Unless you have a friend in the bsiness, you'll probably not get your hands on one. These can tell you shorts, crimps, loss of insulation, and insulation breakdown at a specific distance from the test set. I wouldn't waste time with it, since coax is inexpensive, and your run is relatively short( 30-40 ft I'd guess). Use the best grade cable that will run through your existing holes.At least RG-6, preferrably RG-8, if it will fit. Bigger is better for signal loss issues. Good luck
 
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