[ARC5] ARC-5/274-N SAelectivity

[Richard Knoppow]

   I am cudgeling my memory. I know at least one ARC type 
receiver used overcoupled IF transformers to broad band it. These 
transformers had a means for moving the windings closer to get a 
critically coupled condition so that the transformers could be 
simply peaked for alignment, the coils  then being moved apart. 
The coupling definitely affects selectivity. Look at the 
selectivity curves for the Super Pro series, which used moving 
coils in two of its tranformers. The SP-600 and some other 
receives, use switched secondary coils to vary the coupling. The 
mechanical type has the advantage that the curves are ideal, i.e. 
expand symmetrically. The secondary coil method is very close to 
this.
    I don't have a web site at hand that shows the effect of 
varying the coupling but probably a search will find the curves. 
As coupling is increased the gain goes up until at critical 
coupling it is maximum. As it is increased the curve becomes 
double peaked and widens. Gain at the middle frequency drops. For 
a perfectly symmetrical system the gain of the two peaks is 
identical and their distance from the center frequency is 
identical. As coupling is decreased from critical the gain drops 
but so does the bandwidth. Note, in the over coupled condition 
the skirt selectivity is improved because the slope of the curve 
becomes greater. A combination of over coupled and critically 
coupled stages can result in a fairly flat passband with petty 
good skirt selectivity, as seen in the Super Pro and also some 
simpler receivers like the HQ-120 and 129 where the coupling is 
fixed. Overcoupled stages that are not variable are a PITA to 
adjust, a sweep generator being ideal but there are ways around 
it. This is the reason the transformers in the receiver I am 
thinking of are made variable for adjustment. They could just be 
left critically coupled for narrower selectivity.

On 9/27/2020 3:09 PM, Bill Cromwell wrote:
> Hi,
>
> Some hams have moved the windings farther apart inside the cans 
> to increase selectivity. I am not sure how effective that 
> really is. My approach has been computer sound card based DSP 
> and more specifically using the DSP in SDR software for sound 
> cards. The SDR software has the advantage of managing that 
> tuning rate while narrowing the bandwidth and reducing noise 
> (as a function or the narrower passband). I use quisk.
>
> My approach does not require any modifications to the radio:)
>
> 73,
>
> Bill  KU8H
>
> bark less - wag more
>
> On 9/27/20 5:13 PM, Doran Platt wrote:
>> You all mention selectivity, or lack thereof.  Have any of you 
>> all tried the modification to the IF transformers?  I done 
>> virtually all mine (except, of course, the low freq. models) 
>> and a few more for others.  Bandwidth is reduced by about 50% 
>> with very little losses measured.  Sure makes things better on 
>> 80 and 40.
>> Jeep K3HVG
>>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL