[1000mp] Tuner quirk

[Kok Chen]

Mark Tetrault <tetrault@mv.mv.com> wrote:
> My Mark V so far has been perfect. Yesterday I stuck a B&K TVI filter
> in line. Now the tuner doesn't want to tune 160, 80/75, and 10 meters.
> It tunes the other bands ok, but slower.
> 
> I tried another filter, Bencher TVI, same results.
> 
> I switched coax jumpers to the filter, same results.
> 
> Pulled the filter and jumper back off line, tunes fine again!
> 
> Does the MP/ MP Mark V not like TVI filters?


Here is my weird conjecture.

The rig itself puts out a small amount of harmonics (well, that
is why we use a TVI filter in the first place :-).  Most TVI
filters are not absorbtive (i.e., they do not dump the harmonics
into a dummy load, like, say, a diplexer).

The harmonics get reflected back to the rig.  It is not converted
to heat (no frequency selective dummy load), and it is not converted
to some other frequency (lack of non-linear elements in the TVI filter),
and it does not go to the antenna port (or the filter isn't very good).
Ergo, it is reflected.

If the reflected harmonic components are larger than the reflected
signal of the fundamental, the tuner no longer has decent phase
relationship of error (reflected fundamental) vs reference (forward
fundamental) signal to drive the tuning algorithms.

I did say weird conjecture, didn't I? :-)

73

Chen, AA6TY

P.S., I remembered reading about the use of a diplexer (lowpass port
to the antenna and highpass port into a dummy load) as an effective
TVI filter.