[ARC5] Why an UNUN?

[Geoff]

Autotransformers can be tapped to provide many ratios which is nice when 
various ham bands are used into the same electrically short vertical.

At the lower frequencies high VSWR doesnt result in high coax losses when 
using a low loss cable to start with and over a reasonable length.

For the transmitters under discussion a unun can be used to step up the low 
impedance the TX is looking for to 50 Ohms and then coax to that antenna. Or 
wired as a balun and then feed another balun for whatever final impedance 
you want for an open wire feedline.

Since VSWR might cause HV on the various cores/windings even at low power, 
use Scotch 27 or similar HV tape on the core and Teflon insulated wire for 
the windings which is available scrap at lower prices than PVC insulation.


----- Original Message ----- 
From: "mac" <w7qho at aol.com>
To: "ARC-5 List" <arc5 at mailman.qth.net>
Cc: "Mike Hanz" <aaf-radio-1 at aafradio.org>
Sent: Wednesday, October 24, 2012 8:24 PM
Subject: Re: [ARC5] Why an UNUN?


> OK, I remember hearing a figure in that range before. Now, the command
> xmtrs were designed to operate into antennas that presented highly
> reactive loads, a relatively low resistance and a large capacitive
> component and the function of the roller inductor was to cancel out
> the latter.  Reasonable to assume, then, (but not necessarily for
> certain) that the variable link alone would allow proper loading of
> the transmitter into a purely resistive 13 ohm load or something close
> to it and a 2:1 step-up (4:1 impedance ratio)  transformer (or UNUN if
> you prefer) would then provide a very good match to 50 and 75 ohm
> resistive loads with the inductor completely removed from the
> circuit.  Not clear from the many comments  whether or not anyone has
> actually verified this?  Also, seems to be some confusion as to
> whether or not a series capacitor would still be required which should
> NOT be the case.
>
> I've never tried the UNUN approach, BTW,  series caps and the roller
> always worked FB here as I've mentioned before.
>
> Dennis D.  W7QHO
> Glendale, CA
>
> ****************
> On Oct 24, 2012, at 3:16 PM, Mike Hanz wrote:
>
>> The manual gives a range of values, John.  The upper limit is around
>> 13
>> ohms resistive, but it also expects a capacitive component.  See para
>> 1-28 in the ARC-5 manual, although the SCR-274-N and ATA manuals
>> contain
>> the same data, IIRC.
>
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