SB-200

[John Farrington]

At 04:43 PM 08/19/2000 -0700, you wrote:
>Has anyone had success in lowering 40m input SWR by varying values
>of C-31 and C-32 across L4.  I'm using the 4' RG58/U between the
>exciter and SB-200 per the manual.  Can't seem to get SWR below 2.1
>or so; other bands are fine.  Joe K7MKS


The input network coils L1-L5 have adjustable cores, but the manual
has no steps for aligning them, stating only that "The coils in the
input networks are factory aligned and require no further adjustment."
That may have been true when driving the SB-200 with tube-type
exciters, but at least some solid-state exciters are more finicky:
they don't quite match the impedance, causing SWR on the RF line
to the linear. I'm surprised you don't have that problem on just
about all the bands - I do with an ICOM exciter. I'd like to find
the instructions for tuning those coils.

There are 2 routes you can go - altering the input network in the
SB-200, or putting a small antenna tuner with an SWR meter between
the exciter and the linear. The 1st route may require a lot of cut-
and-try, and will possibly leave the SB-200 somewhat mis-matched
to exciters other than the one you're currently using;  the 2nd
route is easier and will work with other exciters, but requires a
lot of knob-twiddling on the tuner(s) and linear, plus recording of
the many settings when you change power or frequencies (especially
if you use another transmatch between the linear and the antenna).

You could also experiment with different lengths of coax between
the exciter and the linear, as there may be some fraction of a
wavelength that will match 40 meters close enough (but then it may
not match at other frequencies).

I haven't tried modifying the SB-200's RF input networks, but some
time ago received the following comments about this:
----------------------------------------------------------------

 >You should check out Rich Measures page at
 >   http://www.vc.net/~measures
 >He has detailed information on this very subject, however relating
 >to an SB-220.  The GOOD news is, the SB-200 coils are much easier
 >to get at without too much danger as long as you stay away from the
 >bottom of the amplifier.  In short, Rich advises to lower the Q for
 >the input tuned ckts a bit.  This makes them easier to deal with SS
 >rigs.  My experience has been that (for my rig) 50-60 watts drives
 >it to max....I get an honest 500 watts on all bands (as measured P-P
 >on a scope, the meter lags a bit).
 >Anyway, Rich's procedure requires that you remove some turns (L)
 >from the coils and juggle the C to reduce the input Q.  Now, and
 >this is only a guess, and I was going to try it, but you can also
 >reduce Q by padding the LC with resistance.  It would be interesting
 >to tack on, say 200-300 ohms in parallel with the input to see if
 >there is any change. Note on your schematic, that not all of the
 >input ckts are identical.  Some do not have any C.
----------------------------------------------------------------

If you come across any more information on this, or have success
with your modifications, please let me know.

73

John Farrington  KE5ZB

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