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

[Bruce Long]

Hi Kenneth:
I took some time and had a quick look at my 1943, first edition, tenth impression copy of Terman's Radio Engineers" Handbook
Page 167 in the Circuit Theory chapter, Figure 30 has the information you need but you have to be very careful using the given formulas.

At first glance the formula for complex Mutual L top coupled C seems to indicate the two coupling mechanisms add instead of cancel  but close inspection of the circuit diagram indicates the formula applies for the case where the coil polarities are opposite.  Indeed the text states mutual L coupling is positive, capacitive coupling is negative.  
But to my way of thinking the formulas he gives for effective coupling coefficient in the presents of complex, mutual L top coupled C do not have an explicit indication of what portion of the formula represent

s the capacitive coupling.   However if you compare to the formula for mutual inductive (alone) coupling the extra (capacitive coupling) term becomes apparent.

I have another copy of Termans radio engineers handbook which is in a box in storage somewhere.  I am sure I did not see this complex coupling stuff in that copy of terman.  that is why I gave complete publication date, edition imprint information.  I just bought the 1943 copy of terman two months ago because i needed the reference for use at work and it was cheaper and easier to order another copy on ebay rather than root through the stuff i have in storage.

It also looks possible to reduce coil coupling by introducing one or two small auxiliary loop coupling coils with proper- opposing polarity maybe with a large value series resistor - watch out for resistor parasitic capacitance- to limit the loop current and therefore limit the coupling effect.  However a single small coupling cap would be alot easier if the coil polarities are aiding not opposing.

In any even there seems to be some fruitful possibilities here.  I'd be happy to know someone is looking into it.    bruce   KJ3Z


________________________________
 From: Kenneth G. Gordon <kgordon2006 at frontier.com>
To: Bruce Long <coolbrucelong at yahoo.com> 
Cc: Arc5 at mailman.qth.net to my way of thinking the formulas he gives for
Sent: Tuesday, May 21, 2013 4:50 PM
Subject: Re: [ARC5] Receiver Selectivity-add cap to REDUCE bandwidth?
 

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