[ARC5] Why an UNUN?

[mac]

On Oct 24, 2012, at 6:24 PM, J. Forster wrote:

>> 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
>
> Yes
>
>> and the function of the roller inductor was to cancel out
>> the latter.
>
> Sort of. The roller inductor was designed to series resonate the  
> antenna's
> capacitance.


Same thing...


>
>> 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.
>
> If you short out the inductor, rather than resonating it with a  
> capacitor,
> you remove a hugh-Q filter from the antenna line. This increases  
> harmonics

Separate issue and may or may not be a problem depending on the type  
of antenna  used, if feeding into a tuner, etc.

> to the antenna.
>
>> 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.
>
> If you short the coil, you don't need the cap. See above. IMO, this  
> is a

> bad idea.

Still haven't received an answer  my question...

DD

>
>> 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
>
> Best,
>
> -John
>
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