Topband: Preamp power supply

[Herbert Schoenbohm]

From: "N1BUG"  .....looking for suggestions on *aggressive* power supply
decoupling / filtering for my W7IUV preamp.  The preamp works great when
powered from a battery but is totally useless on the power supply I built
for it.  Experiments  thus far have not led to an acceptable solution.

Paul,  There are a few things you could try to reduce or eliminate the
common mode noise connection:

(1) Place the power supply at the antenna and don't power it with DC down
the coax but rather run AC to the unit with separate cables. I would not run
115 VAC but rather use a couple of small  AC wall warts back to back at any
voltage 6VAC, 12VAC, or 24VAC as long as they are the same.  Use yet a third
DC wall wart at the antenna to bring the voltage back to what you need for
the pre-amp.  You will essentially have create an isolation transformer and
a balanced AC power feed to your unit.

(2) another option is to run 75 ohm "triax"to the pre-amp from the shack
using only the inner two conductors for the signal path and grounding the
outer shield at one end only.  This will create an effective "Farraday"
shield to induced currents across the length of the feedline.  Which end
gives the best rejection of common mode noise can be determined by
experimentation.  This method was used years ago to prevent induced hum from
long video cables at a TV station I used to work for. If Triax or double
shielded (independent outer shield) is not available, I have even wrapped
standard RG-6 with aluminum foil covered then covered the length with a
light wrap of black plastic tape.  The foil at one end was earthed with a
small mini-hose clamp.

(3) a third but more exotic suggestion is to put a few small solar cells and
a rechargeable 9 volt battery float at the antenna pre-amp location.  The
single transistor in the pre-amp can't take more than 50 milliamps of
current and the battery makes a great regulator without any switching noise
problems.

Good Luck,

Herb Schoenbohm, KV4FZ