Wow,

Cannot believe Wheeler has such a following, what he was dealing with at the
time was low frequency loop antennas for navigation (e.g. LORAN, aircraft
Omni stations) down around 300 to 500 KHz. If I remember correctly he made a
statement in his paper that antenna was less than 1/6 of a wavelength in
length.

I was interested because my design requirements for our customer was for a
fairly wide bandwidth but with a shorten radiator rod to be canceled in a
ordinary looking plastic housing for aesthetic reasons.

Homer J.



"Jim Lux" wrote in message
...
Art Unwin wrote:
On Apr 7, 10:48 am, Jim Lux wrote:

Homer J wrote:

Here is a reference to a technical look at short loop antennas from the
1950's by Wheeler.

H. A. Wheeler, "Fundamental Limitations of Small Antennas", Proc. IRE,
vol. 35, pp. 1479-1484

More specifically, the December 1947 issue

From a practical standpoint, this paper is quite useful, although it
mixes effects of the matching network in with the antenna, which the
more rigorous analyses don't. It also doesn't provide any backup for
its assertion of the validity of the "radiansphere" or "radianlength",
hence the equations might not be valid over all possible antennas.

Wheeler's 1975 paper ("Small Antennas", IEEE Trans Ant & Prop, V AP-23,
#4, July 1975, pp462-469) revisits some of the stuff in the earlier
paper and provides more backup and describes the limitations of the
"radian sphere" model (which he defines as the volume within which the
reactive power density is higher than the radiation power density).

Of particular interest to would-be miracle small antenna builders is
that he specifically mentions the problems if there is anything
conductive or magnetic within the empty space oustide the "antenna" but
within the radiansphere (defined as lambda/2pi). The latter paper also
discusses some electrically small antennas (for 15 kHz, lambda=20km)



To be more specific is Wheeler refering to small FULL wave
antennas or smal fractional wave antennas.?

Any size antennas... Wheeler's analysis essentially points out that when
talking about "size" of an antenna, it's not just the mechanical
dimensions of the metal that counts. You also have to account for stored
energy in the fields around the antenna.

here's the abstract from the 1947 paper:
"A capacitor of inductor operating as a small antenna is theoretcially
capable of intercepting a certain amount of power, independent of its
size, on the assumption of tuning without circuit loss. The practical
efficiency relative to this ideal is limited by the "radiation power
factor" of the antenna as compared with the power factor and bandwidth of
the antenna tuning. The radiation power factor of either kind of antenna
is somewhat greater than
1/(6*pi)* A*b/L^2
in which Ab is the cylindrical volume occupied by the antenna, and L is
the radianlength (defined as 1/(2*pi) wavelength) at the operating
frequency. The efficiency is further limited by the closeness of coupling
of the antenna with its tuner. Other simple formulas are given for the
more fundamental properties of small antnenas and their behavior in a
simple circuit. Examples for 1-Mc. operation in typical circuits indicate
a loss of aboute 35 db for the I.R.E. standard capacitive antenna, 43 db
for a large loop occupying a volume of 1 meter square by 0.5 meter axial
length, and 64 db for a loop of 1/5 these dimensions.
"
(forgive the typos when I copied it)



I would suggest that nobody attempt to argue the applicability or not of
this paper from just the abstract. Get the paper, and the one from 1975
(and the ones by the other authors he cites: Chu, Fano, Wait, etc.). The
maturity of the analysis of this sort of problem has advanced
significantly over the last 60 years.