[Amps] Fw: High SWR = Baluns actually

[Phil Clements]

> >The other was where the output of said transmitter would arc in it's
> >output tuned "tank" sections, due to self resonance of the unused coils.
> >I designed a replacement without that problem, that also added a lo-pass
> >filter for harmonics. We didn't think of where to get such a bandswitch
> >that would short out all the un-used open coils, but my boss told me to
> >tell them the whole story, and they found the bandswitch addition that
> >shorted out the coils, and we swallowed my design.
> 
> REPLY:
> 
> FWIW - I had a similar unwanted resonance problem in the unused coils,
> but mine was caused when the unused coils WERE shorted out. I rewired
> it so they were no longer shorted out and the problem went away.



This problem reared its ugly face when 160 meters went QRO, and the WARC
bands were added. An 80-40-20-15-10 meter design was very forgiving. You
could use military/commercial band switches from WW II, and with even a not
so perfect layout, you came out just fine.

I built several QRO amps, attempting to go 160-10 meters, and the band
switch always arced and melted down; usually on the 40-meter tap. The
problem is the Tesla effect. It can happen on any section of the tank coil.
I burned up several brand new RF Switch Co. band switches trying to follow
instructions from articles and the ARRL Handbook before I caught on to what
was going on. If you are trying to build a single or dual 8877 amp, look at
how Alpha did it with the 77DX/SX tank circuit. This will give coverage from
1.8 to 30 MHz with no fires or meltdowns. It is a product of choosing the
right band switch, and layout. You will not se this problem if you omit 160
meters and/or the WARC bands.

(((73)))
Phil, K5PC