I'm not at all sure what all the hoop-lah following Richard Fry's
posting reproduced below is all about. What Richard wrote is accurate,
as he says confirmed by NEC simulation, and also from the
King-Middleton second-order theory of linear antennas. From the date,
it sounds like Brown's paper was a confirmation of the theory,
actually. An antenna resonant at 95% of a freespace quarter wave,
above perfect ground, would be about 150 times as long as its
diameter--a 75 meter tower about half a meter effective diameter. NEC
gives slightly different numbers, but perhaps more interesting is that
even for VERY thin wires, the resonant length is noticably shorter than
a freespace quarter wave. A wire a millionth as thick as it is long
still shows resonance more than a percent shorter than the freespace
wavelength.

It's an interesting observation, but I thought everyone (with a serious
interest in antennas) would know about it.

The effect at full-wave dipole resonance/half-wave above a ground plane
is considerably more pronounced, over ten percent for a moderately
thick antenna.

Cheers,
Tom


Richard Fry wrote in Message-ID: :

"Richard Harrison" wrote:
It is the convention to describe AM broadcast towers in electrical
degrees. Harold Ennes reprints an RCA resistance chart for heights
between 50 and 200 degrees in "AM-FM Broadcast Maintenance".

Formula given is:
Height in electrical degrees = Height in feet X frequency in kc X
1.016 X 10 to the minus 6 power.

_______________

If electrical length is defined as the physical condition where
feedpoint
reactance is zero (e.g., resonance), then the true electrical length of
an
AM broadcast radiator on a given frequency is a function of the
physical
length AND physical width of that radiator. This was proven
experimentally,
and documented by George Brown of RCA Labs in his paper "Experimentally
Determined Impedance Characteristics of Cylindrical Antennas" published
in
the Proceedings of the I.R.E. in April, 1945. It also has been proven
in
thousands of independent measurements of AM broadcast radiators ever
since.

The curves in Figure 3 of Brown's paper show the feedpoint reactance
terms
of the base impedance of an unloaded monopole of various lengths and
widths,
working against a nearly perfect ground plane. Those values cross the
zero
reactance axis at physical heights ranging from about 80 degrees (for
the
widest radiator) to about 86 degrees for the most narrow.

Brown calculated height in degrees as (Physical Height in feet x
Frequency
in kHz ) / 2725 . Brown's equation, the one in the Harold Ennes quote
above, and the one that the FCC uses in their published data all define
only
the relationship of the physical length of the radiator to its
free-space
wavelength in degrees at that frequency.

But clearly these lengths in degrees do not define the self-resonant
length
of that radiator. The self-resonant length, invariably, will be
shorter by
several percent. This fact is easily confirmed by simple NEC models,
for
those who want to probe into George Brown's data.

Tables relating a single value of base impedance as typical for towers
of
various electrical heights (only) must be read with an understanding of
the
above realities. For example, Ennes' list shows a tower of 90
electrical
degrees to have zero reactance. But Brown's 1945 paper and a great
amount
of later field experience shows that this is incorrect, for the
conventional
use of this term.

RF