If your repeater's solid-state PA is running hot, and the power output is lower than you expect, you may have an impedance mismatch. This problem cannot be corrected by adding a ferrite isolator. That's because the isolator (assuming it's properly tuned) will present a nearly perfect 50 ohm load to the PA, regardless of whether the antenna is connected or not. But, if the PA's output impedance is not 50 ohms, the isolator is no help.
Solid-state PAs, especially the PA modules often found in mobile radios, frequently do not have stable output impedances. What's worse is that the output impedance changes with the drive level. In order to achieve a stable output impedance that is very close to 50 ohms, an impedance matcher is needed. Here's an example: I am fielding a 2m repeater that is based on the Motorola R1225 50-watt full-duplex transceiver. When I connected a Bird 43 wattmeter and a known-good 50 ohm dummy load to the transmitter port, I found that the factory power level was almost exactly 50 watts. When I hooked the rig to a duplexer, the power output dropped significantly. The radio was running hot, more so than it should when delivering 50 watts. On a hunch, I connected a VHF impedance matcher right at the TX port and then tuned it for maximum power output into my dummy load as measured by the Bird meter. Surprise! The power climbed up to nearly 70 watts, far more than the PA's rating. I lowered the drive level via the service menu in RSS until it was 50 watts when the impedance was tuned for maximum power transfer. When the duplexer was reconnected, the expected power was seen going to the antenna, minus the duplexer insertion loss. Even after several minutes of TX, the radio was relatively cool. The current draw of the radio was about 15% lower than before. Besides optimizing the efficiency of a PA, installing an impedance matcher also eliminates the need for a "magic" length of cable between the PA and the duplexer. When the PA is tuned for a 50 ohm source impedance, and the duplexer is tuned for a 50 ohm load impedance, the length of the interconnecting cable is irrelevant. I have used impedance matchers manufactured by Telewave and EMR, with similar results. These little gems cost between $80 and $100, but the benefits are well worth the cost. Just remember that the benefits are realized only at one drive level; if the drive level is reduced during a power failure, the impedance will no longer be properly matched. 73, Eric Lemmon WB6FLY Yahoo! Groups Links To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ To unsubscribe from this group, send an email to: [EMAIL PROTECTED] Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
