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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I’m laughing with a wry smile – I spent over 20 years of my career in Motorola product development and have seen your experience played out more than once. The caliber of the typical “third-party” installation guys was hard to guarantee, but as you experienced; occasionally at the level of fresh-hires direct from the Dominoes Pizza delivery routes. Lacking a thorough understanding of a radiating system, to say the least. In Engineering, product design and antenna design always went hand-in-hand, and the antenna performance was always engineered to the Nth degree in the anechoic chambers.But “fool-proofing” the antenna connection was not always done, and too often, the only guarantee that the proper antenna P/N was used would be the fine-print in the Field Service manual. So many looked alike, it could have been easily overlooked; either accidentally, or intentionally, as you suggested.But more likely, it was “accidentally” due to the limited technical skills of the installers.
From the information provided, the antenna wasn't part of the turn-key package. Was the antenna (and coax, for that matter) part numbers specified in the kit?
It's always a good idea, but it's essential when you have a niche area, such as 900MHz trunked radio, that you clearly specify your technical requirements to your vendors. Otherwise, you can be taken for a ride (by accident or not) by suppliers who don't understand what you need. At JEM Engineering, we test test every single serialized antenna that we produce. We have near-field and far-field chambers, plus several analyzer types, to cover testing antennas from 30 MHZ to over 40 GHz. We even offer testing services to our competitors.
Unfortunately upgrading an amateur radio license to extra is not enough. I have met at least one individual who passedd the test simply by going through the sample examinations a couple hundred times and memorizing all of the correct answers.
But understanding about feed line losses and antenna matching is "a very big deal" in the area of making a radio communication system work correctly. My guess is that the installers of the poor performance systems did not have the correct instruments, or did not use them, or perhaps did not understand the results. That does happen. And there are even some extra-class amateurs who claim that reflected power does not matter. So even that level of license is not a guarrantree of excellence.
As W.K. suggested, I believe the installers lacked the knowledge and/or equipment to properly test the antenna system (assuming they tested it at all).
On the story itself, you could have had an ideal feedline with a poorly matched antenna, and detected the Fwd/Rev power reported based only on where the wattmeter was inserted into the feedline.
A matched dummy load used during feedline testing would have been a more accurate test.
The proper antenna certainly would have been properly kitted at the time of sale. Point being, however, that if a service call was made (after point-of-sale) that other similar looking, compatible-fitting, but actually WRONG antennas might have been pulled from the truck’s inventory, and installed on site. It sounds like that was the situation.
Very believable because it is so reflective of the everyday world we're all a part of anymore. Actually there are many communications technicians that don't have a full appreciation and understanding of antennas. Amateurs used to be the backbone of commercial radio because their motivation to learn went well beyond the limits of earning a living. Using "modern" testing methods, rote memorization can now earn an Extra class license, thus the loss of prestige that it once carried.
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