[Elecraft] (K3) SWR Consistent - low or high power

[Vic K2VCO]

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/