Re: Antenna physical size
- From: Art Unwin <ArthurUnwin@xxxxxxxxx>
- Date: Fri, 7 Mar 2008 17:29:51 -0800 (PST)
On Mar 7, 11:46 am, Jim Lux <james.p....@xxxxxxxxxxxx> wrote:
ArthurUn...@xxxxxxxxx wrote:
I have searched quite a bit for evidence that states that performance
of antennas can be rated by it's size. Formulas do not refere to
radiator size or volume
and aparture is referenced to gain. I understand that sort of thinking
based on Yagi design
but the idea that all small radiators are inefficient is rather
ludicrouse. My work, based on
the sciences of the masters, show that a efficient radiator can be any
size,shape and
configuration as long as it
is in equilibrium . Period
No where can I find reference to "size" in what the masters state
Regards
Art
The work by Chu (Journal of Applied Physics, p1163, v19, Dec 1948) and
subsequently by Harrington (IEEE Trans Ant & Prop, V18#6, Nov 1965,
p896) , Thiele (IEEE Trans on Ant and Prop, v51, #6, June 2003, p1263)
and later others, discusses fundamental limits on performance. Watch
out, though, for the assumptions in the constraints (e.g. whether the
device attached to the feedpoint is reciprocal), and, of course, where
the boundary of the system is.
Watch out also for the definition of "Q", which in this context is the
ratio of stored to disspated/radiated energy, not the ratio of center
frequency/bandwidth.
In short, there is a tradeoff between Q, directivity, and size. And,
because high Q implies high stored energy, for physically realizable
antennas with loss, efficiency is in the mix too.
Googling "chu harrington limit" often turns up useful stuff.
Googled Chu harrington and find that his work is basically empirical
around known arrangements.
When he brought the question of Q into the picture he made the
statement that small antennas
are usually of a low impedance which is correct empirically with
respect to existing designs but it is not exclusive
when dealing with all radiators that can be made that comply with
Maxwells laws. As I have said before it is implicite in Maxwells laws
that a efficient radiator can be any size shape or configuration as
long as it complies with Maxwells law.
In my case my small antenna can have any impedance value for
equilibrium and it is quite easy to have a resistive impedance in the
hundreds of ohms as well as minuit impedances. I conform to 50 ohms
purely because of component availability. As another aside my small
antennas
have a much wider bandwidth than any other available! As far as gain
or energy transmitted that all depends on what frequencies get thru
the bandpass filter and in no way directs out of pass energy to be be
redirected to band pass status and augment energy transmitted. Stored
energy has no relationship to Q in my mind since it goes around or
circulates as with a tank circuit energy that lies within the pass
bandof the tank circuit filter.
To summate, my antenna design is considered small yet complies with
Maxwells laws and yet does not have a narrow bandwidth or low
impedance thus Chu's comments cannot be inclusive of all radiators.
Best regards
Art
.
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