In the 1970s, I worked in west Africa on a variety of radio communication systems. One day I was told that the Nigerian Navy’s base in Calabar was unable to communicate with its ships at sea. Apparently their HF (high frequency, 2 to 30 MHz) kilowatt amplifier had failed. I was told nothing else about the problem.
Lack of detail was normal. Communication and travel then in most of Africa was difficult. I grabbed a Simpson 260 VOM (volt-ohm-milliammeter), a standing wave bridge, a small toolkit, service manuals, and headed to the airport. The 500 mile trip by air to Calabar went smoothly.
When I arrived at the Navy base, I was shown to the radio room, where new HF SSB (single sideband) transceivers and a kilowatt linear RF amplifier waited. The operators demonstrated low receive signal strengths and when they tried to transmit, the amplifier immediately turned off its output stage’s plate current high voltage power supply.
I removed the coaxial cable feedline from the final amplifier’s output connector and replaced it with a dummy load. The amplifier behaved normally. I measured the resistance to ground of the coaxial feedline’s center conductor; it was a few Ohms. This is not necessarily bad; antenna couplers and some antennas with a DC path to ground will display a low resistance to ground, but for most antennas, it indicates a problem with the feedline or antenna.
Check the simplest possible failure point first
I asked to see the antenna. We walked to a nearby field, and — guess what? The antenna was lying on the ground!
Problem solved, or at least we knew what was wrong. I wish that all problems were so easily solved.
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© Russ Bellew · Fort Lauderdale, Florida, USA · phone 954 873-4695