Kevin: I thought I would pass on a few comments to you about GE's MII UHF PA's.
I worked as an engineer for GE Mobile Radio while the MII product line was in full production, and then left GE for a higher paying job as Senior Staff Engineer with a major oil company from which I retired in 1995. I received my ham license in 1957 as a Technician and later as an Amateur Extra Class - W5VAY. Enough history:
The lower wattage power amps, eg: 10w, 20w and 40w: work satisfactorily into high Q loads; however the 75 w and 100 watt amps do not. In the early 80's GE had an engineering team go to a customer in Tulsa, OK to find out why they were losing so many PA's. On a limited sampling, we found that the output impedance of these two amps were anywhere from 40 ohms reactive to as high as 70 ohms reactive. It all depended on the PA transistors that were used at the time of manufacture. This was the start of GE's recommendation that a Z matcher be used between the PA and a High Q load. This solved some problems but not all. A High Q load presents a 50 ohm load at only the resonant frequency of the load. High reactive currents were still occurring in the PA assembly. Another GE customer that used many UHF stations in south Texas was continuing to lose PA's. We finally reverted to the use of a ferrite isolator between the PA and the High Q load. The use of the isolator cured most all of the MII failures. Since this was an expensive solution to the PA problems GE never adopted this cure and stayed with the recommendation of the use of a Z matcher.
For those hams that are going to put a 75 w or a 100 w UHF repeater in service, I would urge them to use a ferrite isolator with a 50 ohm dummy load and a low pass filter between the PA and the High Q duplexer. This will save them many problems replacing PA transistors and solder links in the PA. It also protects the PA from High SWR caused by damage to the transmission line and antenna.
The MII continuous duty PA is still one of the best PA's available. It has an EIA continuous rating, which means key down for 24 hours with no degradation in the published specifications at the end of 24 hours when properly terminated in a 50 ohm load.
Retired Engineer and Ham
Editor's note: Now you know why Motorola included a circulator in every MICOR UHF Base Station, Repeater Station, and Mobile radio set.
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