A reader's question about testing a VHF antenna and coaxial cable answered by one of PBO's experts
My VHF radio antenna and coaxial cable date back to 1984. Lately, while my own reception seems to be normal, I am getting comments that my transmissions are not being received very clearly. A few years ago I replaced the deck plug near the mast and this appears to be in good order. It seems a major job replacing radio and antenna, including new coaxial cable, when only one component may need replacement. How can the system be tested separately, ie the radio and the antenna? You may think that considering the age of my equipment, complete replacement is the clear solution.
Chris Ellery replies:
The radio itself can be tested by a marine electronics dealer, who should have the appropriate workshop test gear to measure the power output and modulation (speech) levels etc, and listen to the transmission on another radio.
They should also be able to visit the boat and test the antenna using an in-line SWR (standing wave ratio) meter – VHF type, not CB radio – which measures the ratio of forward and reflected power.
A new aerial typically has an SWR of 1.2:1. Anything significantly higher would suggest an aerial fault. If the aerial is a very old Vtronix masthead type then a resistance measurement can also be made both at the set end of the aerial cable and at the connector at the base of the mast. The reading should be approx 2-3ohms. Later (current) Vtronix masthead aerials read around 9.8kohms between the inner core and outer screen of the cable, as they added some components to give better lightning protection. (The glassfibre fold-down aerials used on motorboats usually read open circuit, so this test is not used on this type of aerial.)
These tests should help you find if the problem lies in your aerial, either section of the cable or in the set itself. If the set is the problem, then it’s probably time to upgrade to a new DSC/VHF radio. Repairs to older sets are not really cost-effective, as the price for a new one has in real terms come down over the last few years. Choose a unit from a reputable manufacturer and it should give you many years’ service.
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