It's ok to disagree Eric. I typed up a big duplexer cavity 101 text that started to get unwieldy, but chose delete it because I don't have the time to type out and prove the theory. Actually, I don't really want to.
The point is the 50 ohm termination is provided through/via the cavity for the PA, you are hopefully adjusting the cavity for F-center of the "desired" clean output signal. > You propose to alter the load impedance > of the duplexer to (hopefully) > match the source impedance of the PA, > but that will likely not result in > an optimum match. No, with the 50 ohm termination, the cavity/duplexer Fc tuning adjustment does not modify the impedance of the duplexer or the PA. > In fact, the range of source impedance > over which a solid-state PA may vary > far exceeds the input tuning range of > a duplexer. No, the above is not accurate. There is no input tuning range adjustment of a bandpass cavity related to changing its impedance. > When using a Bird wattmeter as you suggest, > one has not fixed the problem of impedance > mismatch; one has merely lessened its > detrimental effect. No, the adjustment centers up the max transfer of energy to the provided 50 ohm termination. No impedance modifications are made. If the power amplifier canot properly match a resistive 50 ohm load, you have bigger issues to deal with. A matching network/tuner becomes a viable option. > As soon as the wattmeter is removed > from the circuit, the circuit mismatch > is worsened. Moreover, the presence > of the wattmeter in the circuit alters > and invalidates the measurement. Only if the PA will not properly operate into a 50 ohm load. > Every manufacturer of cavity duplexers > warns to NEVER tune a cavity under > transmitter power, True, but the final adjustment I mention is done after the main adjustments are complete as a very brief fine match that is often less than 1 rod turn. While I understand your sparking/stiction concerns, they do not pan out as problem for brief adjustments of an already closely matched cavity into a 50 ohm load. Most of the higher cavity circulating currents are nowhere close to the tuning plunger finger stock. I have examined many cavities I've adjusted under rated power as described and not found or experienced the problems you described. It's probalby safe to say most Cavity and Duplexer mfgrs dont want people doing coarse tuning at rated power where extended high reflected power in the cavity is a big problem. > I pointed out that when one attempts to > feed a 50-ohm duplexer over 50-ohm > cable with a 30 ohm (or 70 ohm) PA, the > PA will not be efficient and it will get > hot. Have you run into a number of power amplifiers which are not able to deliver into 50 ohms loads? > I pointed out that the logical solution is > to alter the output impedance of the PA > to be 50 ohms. The desired result would be a 50 ohm output impedance from the power amplifier. > When that is accomplished with the > recommended impedance matcher, maximum > power transfer occurs. It's logical, > and it works. Sure, but you need to follow through with my final step. The duplexer tx cavities are adjusted for min reflected power at rated output. If you do not preform the final adjustment I describe, the reflected power causes heating. The point of my previous post was to mention the reflected power might also be enough to cause excessive PA heating problems. > 73, Eric Lemmon WB6FLY The proper final adjustment of the typcial duplexer tx cavity path should be for min reflected power into a 50 ohm termination. Initial best adjustments of any bandpass type cavity are pretty much never exact or best matched to 50 ohms except when adjusted at rated power into a proper load. Most of you can easily reproduce the reflected power matching proceedure I describe with a common bandpass cavity, reflected power meter and a quality 50 ohm load. If your power amplifer is not able to match 50 ohms, you must deal with that issue aside from the final cavity adjustment. cheers Eric skipp 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/
