[ARC5] Easy selectivity increase for receivers.

[Kenneth G. Gordon]

On 3 May 2013 at 4:46, Fuqua, Bill L wrote:

> If I recall the low frequency receiver IF transformers are constructed
> differently than the higher frequency  ones.

Absolutely correct: the R-23 and R-24 IF transformers are built considerably differently: one 
can manually change the coupling by pulling up or pushing down on fibre rods connected to 
the top coils. There are internal stops to limit the excursion in either direction.

The higher frequency ones are fixed in position.

> I'll have to take a look.
> Reducing the coupling should increase the selectivity somewhat.
> Perhaps even putting a shorted copper strap turn between the two coils.
> Sounds likes there could be some experimentation here. However, I
> don't expect a lot of improvement  at MHz IF frequencies.

Well, according to the article in ER magazine, moving them apart doubled the selectivity. For 
one thing, in order to achieve the wide bandwidth they exhibit in stock configuration, they are 
over-coupled.

> I liked the
> regeneration approach, Q multiplier more or less. Worked well for me in
> the 60's.

It does work fairly well, but as with any regeneration method, it works less well for strong 
signals than it does for weaker ones. This is pretty much the opposite of what one really 
wants.

> Maybe after Dayton Hamvention and after I get some other
> projects finished, such as my completely restored 1970 hot rod VW
> Beetle.

You mean "Herbie" ? :-)

Ken W7EKB