Eric. Thanks for the great explanation. I did not realize the output impedance is affected by the output level.
This could really mess you up if you got every thing set and then adjusted for your final output power. Glad I did not mistakenly do this. Just lucky since I didn't know it mattered. With all these variables it's a wonder these things work at all! -Tim > -----Original Message----- > From: Eric Lemmon [mailto:[EMAIL PROTECTED] > Sent: Saturday, October 02, 2004 11:51 AM > To: Repeater-Builder@yahoogroups.com > Subject: Re: [Repeater-Builder] Duplexer story update (Discussion) > > > Tim, > > You stated the problem quite well, even for a Saturday morning! > > Since power transfer between RF circuits is maximum when the source and > load impedances are matched, that happy state is what we seek to > achieve. If an RF power amplifier's output impedance was a stable 50 > ohms at all drive levels, we would not be having a problem with > impedance matching and cable lengths. > > Solid-state PAs, especially those at the low end of the price scale, are > notorious for unstable output impedances. They seem to work great when > feeding a nice 50 ohm resistive load, but they sometimes go bonkers when > presented with a highly reactive load- such as a duplexer TX cavity. > Tube-type PAs, such as many of those made by GE, are far more docile in > this regard because they have tuned output stages. > > The tuning instructions for most commercial duplexers invariably call > for the use of a network analyzer or a high-end spectrum analyzer with a > return-loss bridge. These instruments present a precise 50 ohm source > to the cavity under test, so that the bandpass peaks and notch depths > are adjusted to present a 50 ohm load to the transmitter at the precise > frequencies of interest. Remember that cavities are resonant circuits > by definition, so they should NEVER be detuned once they have been > properly adjusted. > > Okay, we use high-quality 50 ohm coaxial cable, such as RG-400 or > RG-214, to connect our 50 ohm cavity to the power amplifier- and we find > that the power loss through the duplexer is far more than it should be. > The correct action is to adjust the PA so that it 50 ohms. If that is > not possible, perhaps because the PA is a broadband solid-state unit, > then the next best action is to transform its output impedance to 50 > ohms. One way to do this is to create an impedance transformer of sorts > by adjusting the cable length of the TX-to-duplexer jumper, or by > installing an impedance matching device (Z-matcher) at the PA output > connector. In any case, the duplexer cavity should NOT be detuned to > fix this mismatch. > > Some solid-state PAs settle down if a ferrite circulator is installed at > the output. While a circulator is often provided as a means to present > the PA with a constant load impedance, it also serves to shunt incoming > RF to a dummy load, thereby greatly minimizing the possibility of > intermodulation. When a circulator is paired with a dummy load, many > manufacturers call it an isolator. Most high-tier transmitters being > made today have at least one circulator built-in to the PA, especially > UHF stations. A ferrite circulator will generate a second harmonic, > since it is a nonlinear device, so there must be a low-pass or notch > filter following the circulator. > > Finally, be aware that changing the drive level on a solid-state PA will > usually change its output impedance, thereby upsetting the impedance > balance achieved earlier. This must be considered on repeater systems > that switch to a lower power level when operating on backup batteries. > > 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/
