On Mon, 2 Jun 2014, gareth wrote:

"Michael Black" wrote in message
news:alpine.LNX.2.02.1406021612540.11397@darkstar. example.org...
On Sat, 31 May 2014, gareth wrote:
Sorting through the junk box (50 years + and growing!) I am amazed at
the number of ex-valve-consumer-radio 2-gang tuning condensers (not
capacitors in them thar days!) that I have in the junk box, which was the
germ of an idea for preselection without coil switching - ie, one twin
gang
condenser giving two tuned circuits for each band in the HF range.
It was covered in "73", at least, two articles, the same author, in "73".
One was definitely in 1964, but I can't remember which. The first one
used one of those large air variables, the other used one of those mylar
insulated cariables as seen in small transistor radios.
A websearch says the small one was in "73" in December 1964, W6SFM was the
author of that and the earlier article. Both definitely have an
explanation of how it works, two coils with quite different inductance,
one coming into use while the other is mostly insignificant as the
variable capacitor changes value, and vice versa.
I ahve definitely seen the technique in a QST converter article, so I
suspect the idea may date quite far back, but not sure. That article
didn't give any real explanation of how the tuner worked, as if we were
supposed to know from past experience.
That said, it's only one tuned circuit. Good for a lot of things, but
unless you cascade a few, you won't get very good bandwidth.

Interesting, and perhaps something akin to the Z Match ATU, but not
quite what I had in mind which was two ganged tuned circuits top
coupled by a capacitor.

Ah. So how do you expect to get this to be multiband without
bandswitching?

As any general coverage shortwave receiver will tell you, you can
generally get 2:1 tuning range, so of course on the upper bands fewer
coils are needed. I always assumed receivers that could tune the whole sw
band but needed crystals every 500KHz just relied on that.

Michael