Eric I fully agree with your assessment of the impedance problem except the dummy load statement. I have never seen a good quality dummy load (Bird or other similar quality) measure anything like 50+-j86 ohms. I have seen some Heath cantennas read more than 100+-j ohms impedance. About the worst Bird I have seen is 75+-j20 ohms and this was sent back to Bird. Generally if a good Fluke digital meter reads 50 Ohms at DC the frequency response will be very close to that up to about the upper usable freq level. This observation if from 40+ years using dummy loads all over the world from many manufactors. Tom Manning, AF4UG <[EMAIL PROTECTED]>
Eric Lemmon wrote: > 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/ 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/
