At 06:37 PM 8/23/2006, K8RI on TowerTalk wrote:
> > At 11:11 AM 8/23/2006, Keith Dutson wrote:
> >> >It is simply not true that a larger antenna transmits better than a
> >> >smaller
> >>one.
> >>
> >>That's a pretty broad statement to make. If so, how come all the big guns
> >>use very big antennas?
> >
>< snip >
>
> > If one had an unlimited budget and some time for experimentation, one
> > could
> > do better than the existing things that are available, and likely the
> > antenna itself would be physically smaller. The problem is that the value
>
>It depends on what you mean by smaller but in the normal sense, I doubt it
>would work out that way. The design programs will show a "full size" yagi
>should perform the best compared to well... yagis that are not full size.
This is true for yagis, but not for antennas in general. In general, for
instance, an all driven phased array can outperform a yagi (in which N-1
element are passively excited) of the same size.
This gets back to the old Chu-Harrington thing about the tradeoffs between
directivity, physical size, and energy stored in the field around the
antenna. The latter often manifests itself as increased loss in simple
designs, but there's no law of physics that says that it "has" to be that
way. In fact, there's some intriguing stuff that shows that if you remove
one of the assumptions inherent in the Chu-Harrington analysis (having to
do with the impedance characteristics of what you drive the antenna with)
you might even do better. DARPA is spending a few million on this in an
effort called: Adaptive Amplification to Enable Electrically Small
Antennas, which is looking at using non-Foster terminations for the elements.
http://www.ecs.umass.edu/ece/allerton/papers1998/ESE/ is one example of
early work.
>
> > The other factor driving to tall antennas is the advantage you get from
> > height above ground, particularly in the ham "legal power limit"
> > context. You can't afford to give up the "ground reflection gain" by
> > going
>
>Sure you can if the take off angle it lower than optimum for the day.
Indeed, but the question was more "why do the big guns have big antennas on
big towers", and in the general case, big towers are better than small
towers, more so than big antennas are better than small antennas. The big
gun will have multiple antennas, and can pick the antenna configuration for
conditions, so maybe physical size (when using non-adjustable antennas as
elements) implies multiple antennas, which then puts you in the "adjustable
antenna" category, and in general, an adjustable antenna of a given size
beats a non adjustable of the same size.
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