At 01:45 PM 2006-05-02, S. J. Blackwell wrote:
>I am going to go ahead with the bobtail. The real question I had was whether
>the phasing could be maintained properly when feeding the two bobtails with
>the the usual tapped parallel ckts. I had thought that at resonance the
>phase shift would not be a problem if the tuned ckts on both elements were
>similar and at the same resonant frequency. I still have not figured out how
>to model this on EZNEC, but hope to evenyually do so. Stiil low on the
>learning curve.
In general, feeding phased arrays at high impedance points can be a
problem. End-fed half-waves and bottom-fed bobtails have the same issues.
Normally, with verticals fed at a current loop (e.g. 1/4 wave
bottom-fed or 1/2 wave center-fed), setting the currents at the
feedpoint brings the currents in the rest of the radiator to
approximately the same phase angle and gives the desired pattern.
With high impedance feeds, setting the phase angle of the feedpoint
voltages may work. An EZNEC model gives a fairly good result. I'm
not sure how easy it would be in practice to keep the currents in the
two bobtails at the desired relative phase angle.
Probably the easiest way to feed phased bobtails is to drive at the
low impedance points at the top of the center wire. Then you can use
normal amateur practices for phasing networks.
I have attached 3 EZNEC models for Sam: Single Bobtail, 2 Bobtails
with Voltage Feed, 2 with Current Feed.
If anyone else is interested, I can send them a copy, too - reply off list.
73, Terry N6RY
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