Topband: "Linear Loading" & 160-Meters

[Edward Swynar]

On 24th February, Jerry wrote:

"...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)..."

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Hi Jerry,

I beg to differ with you, but the "classic" Bobtail is, indeed, a
voltage-fed array. Period.

The feedpoint is at the bottom of the centre vertical element---feedline
coax is coupled to it by way of a parallel L/C network, with the end of the
coax being either tapped down the coil for the best match, or coupled to it
by way of a link coil.

The "Robert Tail Array" on the other hand (per a title in an early edition
of "THE ARRL ANTENNA COMPENDIUM"---volume 2, or 3, I believe), DOES exhibit
a low impedance feed point: it essentially looks like an inverted Bobtail,
and is fed at the point where the "counterpoise" wire meets the centre
vertical element.

The low impedance of the Robert Tail has an advantage over a classic Bobtail
in that one can hook the coax feeder directly to it without need of any
tuning network. As well, no system of ground radials is deemed especially
mandatory with the Robert Tail.

However, the classic Bobtail has an advantage in that the current node of
the centre vertical is higher up in the air than it is with the Robert Tail,
and consequently free of any nearby obstructions. Of course, the Bobtail
requires some sort of ground system, although most writers affirm that this
need not be extensive...

I've used BOTH versions on 40-meters...they were probably the best DX sky
hooks that I'd ever had the pleasure of using on that band, bar none!

~73~ de Eddy VE3CUI - VE3XZ