I have a new power oscillation problem that is manifest only on 40 m when using an attic dipole. Basic station configuration is Kenwood TS-590S, KAT500, KPA500. As drive power is slowly increased and output power rises above about 100 W the power output falls to zero then slowly rises again. If drive power is increased the frequency of the oscillation increases but the peak output power does not change.
At onset the oscillation frequency is 3.5 Hz. While KPA500 output power is cycling TS-590S exciter power remains constant, KPA500 remains keyed, and neither LP-100A, KPA500, nor KAT500 indicate any SWR transient. KPA500 and KAT500 do not fault. This 40 m attic dipole has been in use for several years and this problem is new. I have set any arbitrary 250 W limit for this antenna and have, in the past, experienced no significant problems running that power level. What could cause KPA500 to stop power output and then resume? The KPA500 will output 500 W into a dummy load on the same frequency with no oscillation so that seems to rule out a KPA500 hardware issue. Next suspect would be RFI but what feedback mechanism would cause KPA500 to drop to low or zero power while drive remains constant and key line remains active. Why would I have RFI now if I didn't before? Next suspect would seem to be some sort of breakdown in the antenna system and that breakdown heals when power is removed. But how could any breakdown pull power to zero and not fault KPA500 or KAT500? Next step will be visual inspection of the attic antenna. I'm posting in case anyone has seen a similar problem and has any suggestions as to the cause. Early draft notes and some scope traces here - https://www.dropbox.com/scl/fi/rxidnm4bkv9pissfyln00/Power-Oscillation-40-m-draft-1.pdf?rlkey=446vn8bjoe2bv2xn4un86zpd2&dl=0 Thanks and 73, Andy, k3wyc ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
