[Elecraft] OT: Vertical doublets (was: Home made Sigma-GT5 & KRC2 or SGC?)

[Don Wilhelm]

John,

I think we are mixing too many things together here - and that results in
more variables than my brain can handle!!!  It is not really possible to
answer your question directly without a LOT of work, but I can give you some
pointers.

Firstly, a length of transmission line CAN be a tuner all in itself, it will
behave that way anytime there is SWR present on the transmission line - it
will transform impedance from its input to its output.  That is one big
reason that one length line will work in a particular installation while
another will not.  The half wavelength (or multiple) feedline case is easy -
that is the only time a feedline with SWR will have the same impedance at
the input and output ends.  If one does not understand that, he (she) will
continually scratch their head wondering why a particular antenna fed with
ladder line works and another one doesn't - it depends very greatly on the
feedline length, and the characteristic impedance of the feedline is NOT the
impedance that is to be matched - these are quite different things.

First consider the antenna for its radiation pattern, then next for its
feedppoint impedance. (Or just the feedpoint impedance alone if the pattern
is not important to you.) Then separately consider the length of feedline
sufficient to reach from the antenna to the shack (this is the minimum
length, you can always add, but not subtract from this).  That (after a few
calculations) will tell you what impedance you must feed in the shack.  Then
finally, ask the question 'What kind of network do I need to transform that
impedance to the 50 ohms resistive that my transmitter needs?'.  If you want
to avoid the calculations, you can put it up and make measurements with an
antenna analyzer and proceed with 'cut and try' methods.

I don't know of any other method to do it right.  As for tuners, some tuners
are more efficient than others.  It all depends on the Q of the components
and the magnitude of the circulating current - primary losses result from
the current through the inductor(s).  Folks who want to generalize this by
saying that all tuners are lossy are technically correct, there is no free
lunch, but there are things that can be done to minimize the loss.  We all
have to figure out just how much is tolerable for our installation and
budget - lower losses usually translate to increased cost - larger wire in
the coils to reduce the series resistance, better quality capacitors, lower
loss transmission line, and the list can go on and on.

One just has to 'make your choices' and do the best with what we have based
on those choices.  There are no 'magic' answers.

73,
Don W3FPR

----- Original Message ----- 
>
> > L B Cebik's 44 foot length is chosen because of the single lobe
> radiation
> > pattern for all bands 40 through 10 - LB assumes (correctly) that any
> > impedance can be matched by some method.
>
> Understood. I'm aware of other arguments that this design is very
> inefficient because of tuner loss. Translating from feedpoint impedance
> calculated by a program such as Multinec to a tuner and what is
> matchable and what isn't with the KAT100 is an area I don't understand
> (yet).
>
> > A 44 ft dipole is not resonant on any ham band, so your question about
> > high,
> > low resonance does not compute - it is in the class of a non-resonant
> > antenna.
>
> Sorry for not being clear. I meant that given the effect of size on
> feedpoint impedance and ultimately the tuner, how should the structure
> be sized, the reasonance point for a smaller structure being higher
> frequency and a larger lower frequency. Playing with Multinec, it looks
> like the higher angle lobe that appears on 10 m with a 44' foot vertical
> doublet is reduced with a shorter hatted doublet. It looks like the
> physical structure of the Sigma 5 is resonant toward the higher
> frequency size with matching done at the antenna feedpoint. Is this
> still the design goal if ladderline is used and matching is by the
> KAT100 at the transceiver feedpoint?
>
> 73,
> John K7JG.
>
>
>