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

[Bill Cromwell]

Well ya know,

I recall some printed discussion about rf filters - probably in one of 
the ARRL handbooks - that icluded something called a Kohn filter. The 
filters under discussion were for use between the antenna and receiver 
and would protect a CW receiver from an SSB receiver on the same band an 
at the other end of the table at Field Day. Or vice verse. I have always 
intended to build one (or more) and I wondered if they would be useful 
in an IF strip. The signals are coupled in and out of the ones I saw 
differently but that top or bottom coupling cap is as described here. 
Since I intend to build a filter like that I will also investigate  
using it with those IF cans in the IF strips of my 40 and 80 meter 
command receivers. I already had in mind to build a separate 
Q-multiplier on a spare IF can and not make the IF amplifier(s) 
regenerate - a la a Drake or Heath outboard Q-multiplier.

I think my sweeper is functional.

73,

Bill  KU8H

On 05/21/2013 04:38 PM, 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.  IIRC this also causes an out of band notch at the frequency
> where the mutual inductive and the top coupled capacitive coupling have
> equal and opposite signs and complete cancellation results.
>
> I am really busy at work at the moment so i cannot do much more with this at this time but maybe someone here will be interested enough to take
> the ball and run with it.  I see two ways to proceed, an experimental approach
> and a modelling approach.
>
> If you have a sweep generator and can generate a graphical image of the IF filter passband shape it ought not be too hard to add a small gimmick
> or other small value capacitance from the hot side of the primary to the hot side of the secondary and see what happens   I expect minor
> re-tuning of both sides will be necessary to judge the true effect.
>
> Alternative you could model a two pole resonant band pass filter with mutual
> inductance coupling in a circuit modelling software and then add the
> small coupling cap and observe the effect.
>
> If anyone is interested in this second approach i might be able to find
> time to come up with a circuit model for a mutual inductance two pole
> bandpass filter that you can use as a starting point for your
> considerations.
>
> Good luck    bruce    KJ3Z
>
> Sorry for the double email Kenneth   forgot to click "reply all" the first time
>
>