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

[Don Wilhelm]

John,
The answer to your question does not have a simple answer.
If you know the antenna feedpoint impedance, you can use a program like TLW
(I believe you can download it from the ARRL site) to try different feedline
lengths that will be practical for your installation and see if you can find
a length that will be within the range of your tuner.

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.  Longer lengths will develop lobes
on the higher bands - and for a vertical that should translate to a
(desirable) low angle of radiation, but ground reflection will determine
just how low that angle really will be.  Your proposed capacity hatted
dipole would have different feedpoint impedance than LB's 44 foot wire
dipole, but the radiation pattern (meaning = no extra lobes) should be
similar.

As far as tuner loss is concerned, that depends on the tuner characteristics
and how it is used.  Commonly used T match tuners can have multiple settings
that will provide a match, and some of those settings are quite lossy.  If
properly used, the tuner loss can be kept to a minimum.  A properly
configured L network usually has a lower loss than the T match tuners, so
perhaps a KAT2 or KAT100 would be a better choice than a T match tuner.

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.

73,
Don W3FPR

----- Original Message ----- 

> Greetings:
>
> Any suggestions on the optimum dimensions of a ladderline-fed
> Sigma-GT5-like hatted vertical doublet that the KAT100 will tune
> efficiently between 10 and 40 m? A Sigma-GT5-like design posted on
> Usenet by AB6SJ takes advantage of the availability of aluminum tubing
> in 6 foot lengths so it is a bit bigger than the GT-5.
>
> W4RNL suggests a 44 foot horizontal doublet fed by ladderline is
> an optimal doublet length for 10-40m. It's tempting to turn this doublet
> vertical and shorten it with bar hats like the GT-5 if it is efficient.
> But at least one antenna guru (whose call I can't find at the moment)
> has stated repeatedly that W4RNL's doublet design is very inefficient
> because of tuner losses. Who is right? What are the tuner efficiency
> tradeoffs of having the structure resonance low, high or mid-frequency?
>