[ARC5] Why an UNUN?

[J. Forster]

> 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.

> 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
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.

> 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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