Topband: "Linear Loading" & 160-Meters

[Wayne Rogers]

While we're on the subject of Bobtails - I've been considering a design that 
is slightly different than what you normally see.

What if one were to use a cylindrical tube for the center element, and run 
the coax up the cylinder to the top and connect to the current feed point 
there?  This solves the problem of how to run the cable for a current feed. 
Would any ground plane be needed?  How would you connect it?  Would this 
work?

This for an application on 40 or 80 meters - too long a center element for 
160.

Thanks, Wayne N1WR

--------------------------------------------------
From: "Edward Swynar" <gswynar at durham.net>
Sent: Thursday, February 25, 2010 10:39 AM
To: "K4SAV" <RadioIR at charter.net>; <topband at contesting.com>
Subject: Re: Topband: "Linear Loading" & 160-Meters

> 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)..."
>
> *********************************
>
> 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
>
>
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