|To:||"Michael Tope" <W4EF@dellroy.com>,"Al Williams" <firstname.lastname@example.org>,"towertalk" <email@example.com>|
|Subject:||Re: [TowerTalk] 2 element 40m steppir yagi|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Thu, 07 Oct 2004 11:37:58 -0700|
At 10:39 AM 10/7/2004 -0700, Michael Tope wrote:
What I was thinking about for my small suburban lot was a big IR driven element attached to a mast pipe in concert with a single regular IR parasitic element cantilevered on a short boom off the mast. This would give 2 element yagi performance on 13.8 to 54 MHz and rotary dipole performance on 6.9 to 13.8 MHz, and still be reasonably "neighbor friendly". The big IR driven element could be grounded to the mast with a relay and used as tuneable top loading for a 160 meter shunt-fed tower. For eighty meters I would probably just use an inverted-vee.
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).
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)
One minor complexity (although one that is not hard to manage) is that most of the Yagi design programs don't address the concept of a variable loading reactance in the elemnet. For my purposes, I just used a general purpose optimizer in 4nec2 and let it grind to find the loading X for each element. One would also need to do some sort of sensitivity analysis to see how critical the tuning of those reactances is, but, to a first order, I can't imagine its any better or worse than the sensitivity to the lengths of the elements in a SteppIR. It's not going to change the gain much, but the F/B will be radically affected.
Where the SteppIR concept has real value is when you need to accomodate BIG frequency changes (like band to band octave steps). Trying to do this with a variable network and a fixed length element makes for many compromises. Either the element is much too short, and losses in the network eat you alive; or, the element is too long, and likewise.
Sure, one could conceive of a scheme where you have elements with switches along the element to add and remove segments at the end, and then a variable tuning network at the feedpoint to do the fine control. Such schemes have even been described in the professional literature at least as far back as the 60s. The problem is the complexity. You'd have many actuators to worry about, instead of just one, as in the SteppIR.
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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