[ARC5] Receiver Selectivity-add cap to REDUCE bandwidth?

[Kenneth G. Gordon]

On 21 May 2013 at 13:38, Bruce Long wrote:

> I think there is another way to add selectivity to an ARC5 receiver
> using a gimmick capacitor that does not rely upon regeneration.  I
> have hesitated posting until I could find the original reference but
> since I have been unable to find the reference to date here goes.
> 
> I remember seeing somewhere in a 1930's vintage radio handbook the
> fact that inductive coupling of two parallel resonant circuits has the
> opposite polarity of capacitive coupling of two resonant circuits.  If
> correct, this means if you have two inductances that are mutually
> coupled as part of a two pole resonant circuit adding a small coupling
> capacitance from the hot side of the primary side resonant circuit to
> the hot side of the secondary side resonant circuit will in fact
> reduce the total mutual coupling.

Ah HA!

I had wondered about that, but had nothing on which to base my "wonder". 
That makes absolute sense.

I had seen a similar method used in one of the ARRL's homebrew receivers, 
although in their case, they used "bottom coupling" at RF to limit the 
bandwidth.

I have a sweep generator here (which I have to fix first) and a couple of 
scopes. I think it is time for me to do some work.

I did find out that with the BC-453 and R-23/ARC-5s, the difference in 
spacing between undercoupled and overcoupled is "...about 1/4"..." at 85 
KHz. according to the military maintenance manuals on those rigs.

The amount of spacing I have added to the 1415 KHz IF can I have been 
working on is probably very close to 1/2" and is, I am almost certain, too 
much.

Thanks, Bruce, for this very valuable information. I'm going to dig out one of 
my Terman's to see if this is mentioned in there.

Ken W7EKB