[ARC5] Easy selectivity increase for receivers.

[Richard Knoppow]

     The reason for adjustable coupling may have been to 
make alignment easier. Overcoupled transformers can not be 
simply peaked, they should be set up with a sweep 
oscillator. By moving the coils apart the transformer is 
made either critically coupled or somewhat undercoupled 
where it _can_ be just peaked and the coils moved apart 
again.  The IF transformers in the Hammarlund Super Pro 
receivers have mechanically adjustable coupling by means of 
a rod extending from the bottom of the transformer and 
operated by a cam worked by the bandwidth knob on the front. 
Mechanical variation of coupling has the advantage that the 
expansion of bandwidth with overcoupling is symmetrical.  In 
other types of variable coupling using capacitors or even 
auxiliary coils its not. The auxiliary coil method is nearly 
symmetrical and is used in the SP-600 and RCA AR-88 series.
    I have not looked at the instructions for this receiver 
but suspect the variable coupling is used mostly for 
alignment.  A combination of overcoupled and critically 
coupled transformers results in a fairly wide, fairly flat, 
pass band with steeper skirts than if all transformers were 
critically or under coupled.
    Selectivity is usually stated at 6db (voltage) down and 
skirt selectivity at 60db down.  This gives a rough idea of 
performance but says nothing about the flatness through the 
passband or rejection at very low levels. The skirt should 
really be measured at at least 80db down.

--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com
----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: <Arc5 at mailman.qth.net>
Sent: Friday, May 03, 2013 8:56 AM
Subject: Re: [ARC5] Easy selectivity increase for receivers.


> 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