[ARC5] S+N/N ratio results.

[Tim]

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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