An amplifier which is designed for a 50 ohm system, should operate without grief as spec, else it probably should be repasired or modified for proper operation. Solid State Broad Band Power amplifiers with matching problems can be serious trouble makers on busy mountain tops.
> A circulator (isolator) is always a good > idea, but it really won't do much to match > a poorly-designed power amplifier. > When I started this thread, the issue > centered on the fact that many solid-state > PAs do not present a true 50 ohm source > impedance. Most all amplifiers should operate within rated design power levels into a 50 ohm system, or modified/repaired to do said. Doesn't matter if the impedance matching is done in or "outside the box". The external matching box you folks mention and use appears to be a working example of "thinking outside the box". (Actually I just wanted to use that silly phrase somewhere before the year was up) > James has provided an excellent example of > how an unstable PA design reacts to a > "perfect" load: The only "perfect load" is probably the non reactive termination (dummy load) used for alignment and initial testing. If the amp wont play well into a load, the amp needs attention whichever way you choose to go. > While it is possible to experiment with > various lengths of cable to achieve an > "impedance transformer" to match the PA > to the duplexer, Depending on cable length matching techniques is not good practice at the subject location. In often tends to be much trouble in problematic amplifier circuits. > it is almost always easier and quicker to > install an impedance matcher right at the > output port of the PA. It might also be practical to open the PA and modify the output section. Inside or Outside, the goal is the same. Requires a bit of amplifier 101 homework, could be the PA, could also be the often included output low pass filter network. A potential problem with outside the PA matching is the potential interaction with any internal low pass filter networks. A matching circuit might modify the location of a high current point within the power output section or following low pass filter network. It is not uncommon for parts to unsolder themselves in high current locations. I have repaired many such a problem. > When this is done, the cable length > becomes irrelevant. > 73, Eric Lemmon WB6FLY Hopefully the cable lenght should be irrelevant, but never say never. There are cases to be made for min specific cable length when desired phases required/prefered, but that's another kettle of fish. I like the match box idea, but I would take a hard mean look at any amplifier which could not dump rated power into a termination without help. cheers Eric skipp skipp025 at yahoo.com 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/
