After an indirect lighting strike my VHF is being repaired and the antenna (mast top)is going to be replaced. But what about the coax cable? How do I test it? I don't want to replace it if I don't have to because it would require stepping the mast (H31).
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use a VSWR meter
A VSWR meter will tell you if the cable/antenna is OK. Need to make sure what you buy covers the VHF frequency band. Got mine at a local electronics store, can give you maker/model if it will help.
A VSWR meter will tell you if the cable/antenna is OK. Need to make sure what you buy covers the VHF frequency band. Got mine at a local electronics store, can give you maker/model if it will help.
Del
Del, That would be great. What is the VHF frequency band? When you say local electronics store, do you meen Radio Shack?
Del, That would be great. What is the VHF frequency band? When you say local electronics store, do you meen Radio Shack?
Be careful
There are some antennas that will measure like a DC short using an ohm meter. The antenna and coax aren't really shorted and can lead you on a wild goose chase. It is best to disconnect the antenna when measuring the coax for a short. Oh yes, disconnect from the radio too.
There are some antennas that will measure like a DC short using an ohm meter. The antenna and coax aren't really shorted and can lead you on a wild goose chase. It is best to disconnect the antenna when measuring the coax for a short. Oh yes, disconnect from the radio too.
Just replace it
if it was hit by lightning sufficient to damage the radio. There can be places inside the cable which carbonized that you cannot see or otherwise detect that will short the radio signal. It might even seem to work ok but the signal can be reduced in a way that is not readily apparent. Checking the resistance between the center conductor and the shield is only one factor. You could maybe check it with a megger but I doubt that many sailors even know what that is. If the center conductor is open somewhere, you might damage the transmitter the first time you key the mike. Open resistance measured at one end will not tell you if this is the case. You could hire a tech instead but that will cost more than the coax and probably take even more time than just pulling a new wire. I would just replace the coax along with the antenna. Check the grounding while you are at it. My preference it to insulate the coax shield from the mast and bonding system. Another possibility for you to get assistance is to find a ham radio guy where you live and pick his brain. He might even have some spare coax hanging around.
if it was hit by lightning sufficient to damage the radio. There can be places inside the cable which carbonized that you cannot see or otherwise detect that will short the radio signal. It might even seem to work ok but the signal can be reduced in a way that is not readily apparent. Checking the resistance between the center conductor and the shield is only one factor. You could maybe check it with a megger but I doubt that many sailors even know what that is. If the center conductor is open somewhere, you might damage the transmitter the first time you key the mike. Open resistance measured at one end will not tell you if this is the case. You could hire a tech instead but that will cost more than the coax and probably take even more time than just pulling a new wire. I would just replace the coax along with the antenna. Check the grounding while you are at it. My preference it to insulate the coax shield from the mast and bonding system. Another possibility for you to get assistance is to find a ham radio guy where you live and pick his brain. He might even have some spare coax hanging around.
The odds are that unless it was a direct hit,
the coax is OK. But the only way that you will know is to use a SWR meter and see how much energy is being refected back when you transmit. Find you a local ham and he will have the meter.
the coax is OK. But the only way that you will know is to use a SWR meter and see how much energy is being refected back when you transmit. Find you a local ham and he will have the meter.
Use Ohm meter and 50 Ohm Termination
Remove the antenna from the transmission line by unscrewing the RF connector. Place a 50 termination (purchased from Radio Shack)on the transmission line where the antenna is connected. (Make sure the transmission line connector and 50 Ohm connector belong to the same connector series BNC,PL 259)Measure the DC resistance from the coax center conductor to the oustside shield at the input to coaxial line. It should be close to 50 ohms say 55 Ohms. If ist is not, you may have an open or shorted coaxial line. The open or short would reflect all the transmitters power back into the radio and damage the radio.
Remove the antenna from the transmission line by unscrewing the RF connector. Place a 50 termination (purchased from Radio Shack)on the transmission line where the antenna is connected. (Make sure the transmission line connector and 50 Ohm connector belong to the same connector series BNC,PL 259)Measure the DC resistance from the coax center conductor to the oustside shield at the input to coaxial line. It should be close to 50 ohms say 55 Ohms. If ist is not, you may have an open or shorted coaxial line. The open or short would reflect all the transmitters power back into the radio and damage the radio.
more data
Marine VHF is 156-163 mHz. I bought a Para Dynamics PDC-9, VHF/UHF Dual Bander SWR/Power Meter. It measures VSWR and output power. I got it at a place called Tedco Electronics. Probably can do a web search and find other places to buy one.
Marine VHF is 156-163 mHz. I bought a Para Dynamics PDC-9, VHF/UHF Dual Bander SWR/Power Meter. It measures VSWR and output power. I got it at a place called Tedco Electronics. Probably can do a web search and find other places to buy one.
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