I think you have missed the point. If you followed this thread from the beginning, you will know that the issue concerns the source impedance of the power amplifier only. When a duplexer is properly tuned with a network analyzer and appropriate in-line attenuator pads, with the third connector terminated in a 50 ohm load, that duplexer will perform best with a 50-ohm source at the TX input. I believe that everyone agrees that, if you have a 50 ohm load (the duplexer TX input) fed over 50 ohm cable through constant-impedance connectors (N or BNC), the best match occurs only when the source (power amplifier) is 50 ohms as well.
And that is exactly the problem; few solid-state PAs present a 50 ohm source. Some come very close, and the best are far more forgiving than the majority of aftermarket amplifiers. Tube-type PAs generally do not have this matching problem, because they have tuning adjustments to match the load. As several posters have pointed out, you can (effectively) transform one impedance to another by cutting the interconnecting cable to a specific length (or multiples thereof) through a process of cutting, reterminating, testing, cutting some more, reterminating, testing, etc. Or, you can simply connect an impedance matcher and accomplish the same result in a very short time. I'll admit that sometimes I get lucky with the TX to duplexer cable, and the impedance matcher makes no worthwhile gain. In that rare case, I leave it out. I'll also admit that sometimes the impedance matcher doesn't reach an optimum match within the range of its adjustments; in that case, I probably need to put the PA on the bench and find out why it's not performing properly. The case history I cited in a previous posting highlights the fact that relatively inexpensive VHF and UHF radios often use PA modules with no following directional coupler, high SWR sensing, or mismatch detection circuitry. It is this type of radio, and stand-alone PA, that a Z matcher will benefit the most. While there is some disagreement among the members of this list as to the best way to accomplish the matching of the PA to the duplexer, I think we all agree that there is an issue to be addressed. If you really wanted to, you could take the cable and the optimally-tuned impedance matcher back to the network analyzer at the shop and determine the length of cable that produces the same result. However, I doubt very much if another round trip to the repeater site is worth less than the modest cost of the impedance matcher! 73, Eric Lemmon WB6FLY ac0y5 wrote: > Impedance matchers don't give you "a matched system in a minute or so". They > just match one side to the other. Example, a radio connected to a duplexer > that doesn't present a 50 ohm load to the transmitter, for what ever reason... 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/
