[ARC5] Why an UNUN?

[Bruce Long]

I think Mike's suggestion to look at the network the original designers put into the dummy load and Neils' comment about the likely range of antenna terminal impedance, both real and reactive for the range of short to not-so-short wires that can be strung on small and not-so-small aircraft really put a strong, useful boundary on the range of antenna impedances expected by and provided for by the original engineers.  Bruce   KJ3Z

________________________________
 From: Mike Morrow <kk5f at earthlink.net>
To: arc5 <arc5 at mailman.qth.net> 
Sent: Thursday, October 25, 2012 8:48 AM
Subject: Re: [ARC5] Why an UNUN?
 

>> I didn't express myself as clearly as I should have, and meant that
>> somewhere I've seen 5 ohms as the impedance that the transmitter
>> is designed to work into.
>
> That seems as good a design center as anything, Neil.  My only point was 
> that it had to tune a range around that point, so it's hard to say 
> precisely what center point the ARC engineers chose.  I seem to recall a 
> 5 ohm figure as well, but I couldn't find it during a lineup of the 
> usual suspects.

I think the electrical characteristics of the A-61-A dummy load would be
a good indication of the "nominal" design goal for "command set" MF/HF
transmitters.  The transmitter testing set-up from the SCR-274-N manuals
show the transmitter connected to A-61-A terminal A, which is a 5 ohm
resistor in series with a 100 uuF capacitor in series with the RF ammeter.

73,
Mike / KK5F


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