[TowerTalk] . Re: 2006 Top Ten Chutzpah Awards

Jim Lux jimlux at earthlink.net
Thu Aug 24 01:38:26 EDT 2006

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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