Subject: Re: Build a RF Gain Control?
Date: 31 Dec 1997 18:51:16 GMT
From: jpd@space.mit.edu (John Doty)
Organization: MIT Center for Space Research
Newsgroups: rec.radio.shortwave
References: 1

In article <19971231005601.TAA19848@ladder01.news.aol.com> rickw999@aol.com (Rick W 999) writes:
> I would like your input on building one. Is it a simple variable resistor wired
> into the antenna input or is there more to it. I have a RF gain on my DX440 and
> see the usefulness. I want to add it internally to my little YB400 which
> sometimes blasts the eardrums with loud peaks. I attached a pot. from the
> radio's antenna ground to the whip. This seems to work. Would this be the way
> to wire it internally or am I way off? Thanks

You've reinvented a classic circuit. Many 1920's radios had an
"antenna shunt" as a volume control. The "antenna" lead was connected
to the wiper and the RF input, with the "ground" lead connected to
one end of the element and the chassis. Also common was antenna to
one end of the element, ground/chassis to the other, RF input from
the wiper.

2 kohm was a common value. For a small whip, you might do better
with a bit more, 5 kohm maybe. An "audio taper" control, wired
conventionally (clockwise->louder) will give you the best control
of strong signals.

This circuit disappeared with the advent of variable-mu tubes in
the 1930's. I suppose part of the reason is that it would aggravate
the mediocre noise figure of a multigrid RF tube. With transistors,
RF attenuation has made a comeback, athough the most common scheme
is to have an attenuator with discrete steps rather than a pot. I
wish my ICF-SW100 had a pot: there are only two attenuator settings,
and they are sometimes too far apart!

You're going about this exactly right: formulate a solution, try
it out, make it more permanent when it works. Have fun!

-- 
John Doty               "You can't confuse me, that's my job."
jpd@space.mit.edu