----- Original Message -----
From: "Bill Tippett" <btippett@alum.mit.edu>
To: <towertalk@contesting.com>
Sent: Thursday, March 17, 2005 4:18 AM
Subject: Re: [TowerTalk] How can a SteppIR Perform So Well?
> K6IF wrote:
>
> >I think the answer basically boils down to this: boom length is a key
> contributor to gain.
>
> >The SteppIR design removes the constraint to solve for "acceptable"
> bandwidth. This allows the antenna to be configured for "optimal" gain at
a
> given frequency with a fixed feed impedance of 22 ohms. It turns out that
> if you get to optimize the elements for gain absent a requirement for
> bandwidth, and you have four elements on a 32 foot boom, you get pretty
> close to the optimal amount of gain possible from a 32 foot boom.
>
> I agree with most of what you say, but the SteppIR suffers 2
> major constraints on 10 meters...element placement is fixed and number of
> elements is 4. I just did a quick look with YO7 and optimized a 4
> element Yagi on a 32 foot boom with equal spacing, 99% weighting on gain,
> 0% on F/R and 1% on SWR over the limited frequency range of 28.399-28.401.
<snip>
> bottom line is you can only do so much with the restriction of 4 elements
> on a 32' boom...you really need 5 elements to approach optimum gain on 10m
> for that boom length.
And in fact, this is quite close to what is theoretically expected... The
usual statement is that maximum directivity is possible with 1/4 wavelength
separated elements in an end fire array. A array 1 wavelength long (32
ft=10m) with .25 lambda spacing would be 5 elements.
In a passive array, where you depend on mutual coupling to get the
excitation in the elements, the spacing or number of elements might have to
vary, because you've got another thing to take into account: propagating the
power among the elements.
I assume YO takes into account dissipative losses in the antenna, because
that's where the rubber really meets the road on superdirective arrays.
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