At 04:24 PM 3/16/2005, Bill wrote:
>Dan,
>
>I'm not trying to pick a fight and I know that I am going to regret asking
>this question but...
But hey.. that's why it's a "discussion list"... from discussion comes
understanding and enlightenment.
>How can a 4 element multiband yagi with a compromised boom length deliver
>gain approximating that of a well designed 5 element monobander? This
>doesn't seem to make sense to me. (Regardless of who made the antennas.)
The well designed 5 el monobander is, by definition, a compromise design of
some sort.. either feedpoint impedance bandwidth, or F/B, or forward
gain. An enormous amount of effort has gone into coming up with "useful"
combinations of these that are still implementable by fixed elements.
It's entirely possible to get very high directivity (and possibly gain,
depending on losses) with a physically small antenna, at the expense of
narrow impedance bandwidth. That's the whole Chu-Harrington tradeoff in a
nutshell. What the SteppIR allows you to do is to create an antenna design
that has higher Q (and therefore, potentially higher gain, assuming the
higher Q doesn't kill you with dissipative loss), because the bandwidth
only needs to be 10-20 kHz, as opposed to a few hundred kHz. I can't find
my reference that relates the increase in Q to gain, so I can't tell you
what a factor of 10 will do for you.
I'll note that an antenna fitting in a 24 foot diameter sphere at 14.1 MHz
has a "Chu Q" of something like 1.7, which is quite low. Such an antenna
would have a "normal gain" (i.e. non-superdirective) of about 5
dBi. Clearly, there's a lot of room for jacking up the Q, considering that
typical Yagis have Qs of 70 or so, and this is why the typical gain is up
around 8-9 dBi for such an antenna. Harrington says in his paper that
doubling the gain (3 dB) is possible for antennas on the order of 2
wavlengths long, and that as the antenna gets shorter, the potential
increase is even greater (assuming the losses don't bite you first).
Harrington also comments that the maximum gain will be achieved with an
endfire array with elements spaced a quarterwave apart (but not uniformly
weighted in either phase or amplitude).
>Bill, W5VX
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