[ARC5] S+N/N ratio results.

[Geoff]

While the cover accessories for the URM-25 are often missing there is plenty 
of Internet data on rolling your own.
Im glad to see more people on here are joining the 6dB pad bandwagon I 
mentioned in this thread a few days ago.
They are also beneficial and regularly used with modern radios and test 
equipment.

Carl


----- Original Message ----- 
From: Glen Zook
To: Tim
Cc: Geoff ; ARC5
Sent: Sunday, June 16, 2013 6:01 PM
Subject: Re: [ARC5] S+N/N ratio results.


Basically the same as the Measurements 65 and that is "all over the place". 
The URM-25 came with 6 dB pads for 50-ohms and 75-ohms and those really need 
to be used.

Glen, K9STH


Website: http://k9sth.com



From: Tim <timsamm at gmail.com>
To: Glen Zook <gzook at yahoo.com>
Cc: Geoff <geoffrey at jeremy.mv.com>; ARC5 <arc5 at mailman.qth.net>
Sent: Sunday, June 16, 2013 2:15 PM
Subject: Re: [ARC5] S+N/N ratio results.



Anyone have an idea what the source impedance of a URM-25 might be?

Tim

N6CC




On Sun, Jun 16, 2013 at 12:00 PM, Glen Zook <gzook at yahoo.com> wrote:

The input impedance on the S-Line receivers is somewhat more consistant than 
the A-Line receivers including the 75A-4.  However, it is very easy to 
believe that the impedance could range from 30-ohms to 150-ohms depending on 
the band.  That is one of the reasons an antenna trimmer was included in the 
A-Line receivers.  The general coverage receivers including the 51-J, R-388 
series, etc. definitely have input impedances that can vary all over the 
place.  The nominal antenna impedance of the 75A-4 was 50-ohms unbalanced 
whereas the nominal antenna impedance on the 75A-1, 75A-2, and 75A-3 
receivers was 300-ohms balanced.  Especially the 75A-1 and, to a somewhat 
lesser extent the 75A-2 and 75A-3, are much happier with either a balun or 
antenna tuning unit matching 50-ohm coaxial cable to the receiver.

The primary reason for the matching pad on the older signal generators was 
that the impedance of the signal generator was definitely not constant. 
Therefore, a pad, usually 6 dB, was implemented to present a constant 
impedance of the input signal.  Many signal generators, including the 
Measurements and most military generators (i.e. TS-497 series) had 6 dB pads 
available for both 50-ohm and 75-ohm loads.

A matching pad is not usually required on "modern" signal generators with a 
true 50-ohm output and the signal level, in microvolts, applied to the unit 
under test should be the same.  However, the "reading" on the calibrated 
attenuator (before the matching pad) will be twice that of the applied 
voltage before the 6 dB pad.  But, during the time frame in which the 
Collins receivers were designed, the vast majority of signal generators did 
require a matching pad when feeding a 50-ohm input impedance.  Therefore, 
Collins definitely did use a matching pad when aligning the receivers.

Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: Geoff <geoffrey at jeremy.mv.com>
To: Glen Zook <gzook at yahoo.com>
Cc: ARC5 <arc5 at mailman.qth.net>
Sent: Sunday, June 16, 2013 1:26 PM
Subject: Re: [ARC5] S+N/N ratio results.


When I did all the mods to the 75A4 at National 1965-66 one of the first
things the engineers wanted to see was the input impedance. Using a new GR
1606A they recently acquired it was anything but 50 Ohms across each band
and ranged from 30 to almost 150. No problem in use since the antenna
trimmer did the matching.
I would have to believe their general coverage sets of the era were even
worse.
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