On 3/3/2018 12:09 PM, Joe wrote:
BUT I may have fixed it by accident. I have been in the process or
bringing an old dead SB-220 back to life, and have put it inplace this
morning, so cabling has changed greatly in the shack. And the problem
may be gone now by just changing out all the cables?
Lots of problems like this are caused by a bad piece of coax, or coax
with a poor shield connection (or no shield connection). Under normal
operation, current on the center conductor returns on the shield, so the
field stays within the coax. When there's no shield connection, current
returns on whatever path Mother Nature finds, and there's a strong field
OUTSIDE the coax. That field radiates, and also magnetically
(inductively) couples to anything around it.
What is the RF path for antenna switching in the SB220? Many unwashed
engineers used the chassis as return rather than running coax. This
creates the same sort of problem inside the amp -- current returns on
the chassis rather than the coax shield, so it produces EM and magnetic
fields. GOOD amp designers use coax for everything in the RF path.
73, Jim K9YC
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