The Case of the Crazy Antenna

At the power company where I work, we went from the 130MHz VHF radios to a 900MHz trunked radio system some time ago. The talk groups and system-wide access worked as advertised, but the coverage in the outlying areas was not what we expected. We had abundant complaints about not being able to "affiliate" with the system.

This turnkey system had been installed by a "Motorola shop," but in the hinterlands it seems that Motorola had hired and "authorized" a couple of Joe's Pizza and Mobile Radio (Citizens Band) shops. When the problem is poor reception, it is usually because an antenna has been removed to accommodate a low-hanging branch. But these antennas looked good.

After putting the Bird-watt meter on and finding 23 watts forward and 21 watts reflected, I compared the "bad" antennas with one I nabbed from one of the "good" areas. Hmmm. Different size. I looked at the model number on one of the spares the turnkey shop had left with us and found they were designed for cellphone frequencies.

That certainly made sense. They were much cheaper, which left the shop a greatly-enhanced profit margin. Had it checked return loss? No; it must have thought the cellphone antennas were close enough and would do fine.

After checking the transmit power on the antenna, we found that the shop had gotten a great deal on its coax, too (23 watts at the radio, 8 watts at the antenna). We replaced the coax and antennas on our vehicles in the troubled areas. Then the system worked much better.

This entry was submitted by Paul H. Dolton and edited by Rob Spiegel.

Paul H. Dolton has served as a communications technician for SCE for thirty years and served as an Electronics Tech for the US Coast Guard for 21 years before retiring. He has installed and maintained analog and digital microwave and multiplex equipment, as well as mobile radio, telephone, and EPBX systems.

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