Topband: "Linear Loading" & 160-Meters

[K4SAV]

Edward Swynar wrote:
> >From the sound of things, the antenna you modelled there was the INVERTED
> Bobtail array (or, as it was dubbed in an early edition of the ARRL's
> "ANTENNA COMPENDIUM", the "Robert Tail Array")...
>
> The classic Bobtail, per se, is voltage fed---at the tip of the centre
> vertical element---and has a very high impedance feedpoint, whereas the
> Robert Tail is fed at the juncture of the centre vertical wire & the
> counterpoise wire linking all three elements together (hence the lower
> feedpoint impedance)...
>   
No I didn't model a Robert Tail.  It was a Bobtail.  There are two 
methods for feeding a Bobtail, either voltage fed at the bottom of the 
center element or at the top of the center element (a low impedance 
point).  There should be insignificant gain difference between the two.  
Feeding at the top is physically more difficult to implement because the 
feedline must be brought away from the other wires at 90 degrees and you 
need a good feedline choke at the top.  (Although that is no problem for 
a model.) I didn't model the voltage fed option because there is some 
effect on gain depending on how large the ground radial system is, that 
it is fed against and how that radial system is sloped up the meet the 
feedpoint.   That's just some more variables I didn't want to include.

Although, before building one of these, I would recommend modeling it as 
a voltage fed antenna, because that is most likely how someone would 
implement a 160 meter version.  A quick look at this, showed a large 
difference in feedpoint impedance between the shortened version 
(linearly loaded) and the full size version.  It looks like the voltage 
at the feedpoint of the shortened version should be almost twice that of 
the full size version, and voltage fed, this ends up being a very large 
voltage at 1500 watts. I didn't do an analysis to prove it, but this 
impedance may be a function of exactly how the elements are folded. 
Anyway, shortening an antenna this much is not looking like a good option.

Jerry, K4SAV