Ron Wright wrote: > Kevin, > > Many PAs have SWR shut down protection and they still are required to have > isolators. >
Yes, but for reasons you are not stating. VSWR shutdown protection circuits are not foolproof. Depending on the electrical distance from the active devices and the VSWR directional coupler (sampler), it is possible that damaging currents can be present back on the active components (output transistors), while the protection circuitry thinks everything is fine. In low-band, this doesn't happen (or happens to a much lesser degree) because the relative phase between the two are small, however on UHF, it can be significant; which can lead to sampling error, and PA failure. One such documented instance is the GE Mastr II UHF Station amplifier. http://www.repeater-builder.com/ge/mastrIIuhfpacomments.html I don't disagree that an isolator or circulator can be used to satisfy a site owner/manager, but it HAS to be implemented correctly to do this function. If not, it CAN make the IM situation worse. Kevin Custer
