[ARC5] Selectivity: SCR-274N from T.O. 16-40SCR274-5

[jeepp]

All,  
Yep,  Ken has it correct and both the Army and Navy manuals are in agreement.  BTW, and not to toot the horn, that coil mod really does about halve the original B/W of the 80 and 40 meter receivers.  Lastly, I"ll again point to WA2EJT's article of using the BC-453 or R-23 as a Q5-er for receiver IFs outside the tuning range of the receiver.  One does not, for example, need either a BC-946 or an R-24 to mate with receivers such as the BC-348.  The aforementioned LF receivers will do the job.  Doing that here.
Jeep. K3HVG


Sent from my Verizon Wireless 4G LTE smartphone

-------- Original message --------
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com> 
Date: 03/16/2016  11:48 PM  (GMT-05:00) 
To: ARC-5 List <arc5 at mailman.qth.net> 
Subject: [ARC5] Selectivity: SCR-274N from T.O. 16-40SCR274-5 

Table 7, page 67

Paraphrasing from the above:

Measurements taken at 190 KC for BC-453 (A or B), average of two values 
of 190 KC to that frequency giving standard output at double (2X or 6dB, and 
1000X or 60dB) the 190 KC input signal when the tuning is centered on 190 
KC, first on one side, then on the other and averaging the measurements. All 
rods pulled UP.

1.7 KC at 6 dB down, 5.5 KC at 60 dB down.

For BC-454, 7.5 KC at 6 dB down, 25.8 KC at 60 dB down.

For BC-455, 9.8 KC at 6 dB down, 70.6 KC at 60 dB down.

Full IF passband selectivity is then double those figures above.

I am not going to bother to type out the exact words on that page as it 
amounts to several paragraphs, but you can download the two manuals and 
see for yourselves.

I have the two manuals mentioned here as PDFs and furthermore, I have an 
original hard-copy of the SCR-274N manual. It, too, says substantially the 
same as those I have quoted.

Lastly, I believe Jeep said the same thing as I have here in one of his articles 
published in ER Magazine concerning how to improve the selectivity of the 
BC-454 by sliding the two coils in the IF cans further apart.

I.e., the selectivity values given in the manuals are 1/2 the full selectivity of 
the IF passbands, or from center to one side, only, of that IF passband.

Ken W7EKB
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