Laryn, Go to the head of the class! Yes, that is exactly what we are doing by adding an impedance matcher. We are adding a tuning capability that was not included by the manufacturer. Moreover, whenever you see the adjective "broadband" describing a power amplifier, rest assured that it will not be optimized at your frequency!
Let's not forget that impedance comprises both a real (resistive) and an imaginary (reactive) component. So, an amplifier that tunes perfectly into a 50 ohm resistive load (let's assume that its impedance is 50 +j0 ohms) cannot be expected to tune as well into a duplexer that may be 50 +j15 ohms or 50 -j15 ohms. You'd be surprised to see that the dummy load you measured at 50.03 ohms on your Fluke 87 digital multimeter is actually 50 +j86 ohms when measured on a network analyzer, and might be tossed on the reject pile by a reputable manufacturer. If an experienced technician uses a network analyzer to tune a duplexer so that it is a very close match to 50 +j0 ohms, then a transmitter feeding that duplexer should also present a source impedance of 50 +j0 ohms for a perfect match. I think we all realize that such a "perfect" match is a pipe dream, but we can come pretty close. That is why I cringe when someone recommends tweaking the factory settings of a duplexer, trying to get it to match the transmitter. The duplexer is already at optimum setting, and if the transmitter doesn't match it perfectly, it is the transmitter that should be adjusted, NOT the duplexer. But, what do you do when the transmitter has no adjustments? You have two choices: You can laboriously make an impedance transformer of sorts by repeatedly altering the length of the transmitter to duplexer cable until a reasonable match is obtained, or you can use an add-on tuning device called an impedance matcher to counteract the reactive components of the PA and achieve a reasonable match in a fraction of the time. Actually, there is a third choice: You can do absolutely nothing about the problem and hope that the heat sinks are big enough and the components are capable of sustained operation in a mismatched environment. In my opinion, a lot of stations are installed and operated without a careful assessment of PA matching, and these are frequently the ones that die unexpectedly, and at the worst possible times. 73, Eric Lemmon WB6FLY Laryn Lohman wrote: > ... Are we not, by adding a Z matcher, simply adding tuning adjustments to the problem amplifier?... 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/
