----- Original Message -----
From: "Jim Lux" <firstname.lastname@example.org>
> >73 de Mike, W4EF......................................
> I'm not sure that some sort of adjustable loading network in the middle of
> a fixed element might not work just as well. When all is said and done,
> what changing the tip lengths would do is just change the reactance of the
> element, and a single motor driving a vacuum cap or low loss inductor at
> the center of the element might work just as well. Folks have done this
> sort of thing with 2 states and relays as the actuator (say, to reverse a
> beam, or to switch between CW and phone subbands on 80/75).
Yes, for an 80/75 meter design you are probably correct that a set of
variable lumped elements would be just a flexible in terms of adjusting
the center frequency and pattern of the antenna. It would just come
down to which scheme was cheaper to implement and more reliable.
I think the main advantage of a scheme like this (whether it be
implemented with SteppIR elements or continously variable lumped
elements) is that it would save on crane time (assuming you make it
reliable). You put the antenna up and leave it, but you still have plenty
of flexibility available to optimize the performance. Even if you don't
have a great model, doing empirical cut and try iterations would be
useful and practical when using some sort of motor drive element
tuning setup from the comfort of the shack. I think Force 12 has some
sort of accordion like adjustable inductor scheme on one of their
verticals that allows for continuous tuning over some range (the motor
drive stretches or compresses the length a fixed air inductor instead
of using a tapped roller contact). This is of course just another method
that can be exploited to achieve continuously variable tuning.
73 de Mike, W4EF....................................
> I've done a bit of modeling of fixed length elements with variable
> reactances in a 3 element yagi scheme, and it looks like it wouldn't have
> performance that is markedly different from the SteppIR, assuming that you
> can manage the losses. That is, all my modeling has dealt with lossless
> networks. AND, this is most important, it's really only viable for single
> band designs. (like your proposed 80 meter mini-behemoth)
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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