There are several causes for varying SWR readings with power level: 1) Poor diode linearity -- diodes are less sensitive at low levels, and reflected power may be very low at QRP levels. This effect shows as a low indicated SWR at low power levels, which then becomes more accurate past a given power level.
2) Harmonics -- as described below by G3OLB. 3) Parasitic oscillations -- a misadjusted amplifier can oscillate a few hundred kHz from the operating frequency, where the antenna is not resonant (or a tuner is not tuned). VHF parastics are also possible. Any significant off-frequency output can cause a high SWR reading. Needless to say, this is very serious and must be corrected. 4) Common-mode current -- antenna currents on the outside of a coax feedline, caused by feeding a dipole (etc.) without a balun, or by coax running parallel to antenna wires even with a balun, can upset SWR readings. This effect changes with the feedline length as well as the power. SWR indicators may vary in their susceptibility to this effect. On 10/10/2012 2:20 AM, Tom Boucher wrote: > My theory in these cases is that it's nothing to do with bad antenna > connections, but > more due to the rig putting out a higher harmonic content at higher power > levels. The > SWR meter sees a bad SWR at the harmonic frequency so the reading is slightly > worse. > > 73 Tom G3OLB > > N3OI wrote: <*One observation:* I use low power to set SWR with my TenTec > manual > tuner. My FT-920's SWR meter would usually shift a bit when I increased power > to 100w > and I would then quickly tweak it back down. With the K3's SWR meter (no ATU > installed), after setting the tuner at 10w or so, the reading stays the same > when power > is increased to 100w.> -- Vic, K2VCO Fresno CA http://www.qsl.net/k2vco/ ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
