TowerTalkers:
Further to a recent topic that provoked significant discussion a couple
weeks back, coincidentally, a colleague sent me a 2005 paper "Short Low-and
Medium Frequency Antenna Performance" [IEEE Antennas and Propagation
Magazine Vol 47, No.2, October 2005] wherein the topic of interest was on
voltage and current at the ends of an antenna, amongst other performance
parameters, antenna performance of a vertical.
I quickly add to Jim Lux' comment that there might be a thousand such
articles on the topic of voltages and currents on antenna extremities, now
there are only 999 +(n x 1000s) to consider.
If anyone wants a copy of the article, email me at AG6CX1 at Gee Mail and
I’ll be happy to send you a copy.
Admittedly, the paper concerns itself with a top-loaded vertical, not a
Yagi, as the question was initially phrased. But the concepts outlined
resonated well with the then-state of knowledge in broadcast band days,
before NEC and such, even though this was published in 2005. I was
surprised to see almost every possible antenna design consideration
addressed in rather simple terms.
Appendix A addresses current and voltage distributions. And there are some
errata to be wary of.
But you'll chuckle at some of the admonitions and warnings, including:
“The ground plane must be carefully chosen in order to obtain optimum
performance, and it must be free of obstacles up to a half-wavelength, for
several reasons, including: for personnel protection, due to high intensity
fields close to the antenna when the power is higher than 1kW;”
and,
in your search for "high-fidelity" bandwidth to offer "good-quality speech
transmission" you'll note another of the author’s comment:
“In the low-frequency band, bandwidth is quite scarce for any top-loaded
antenna type and must be carefully evaluated in order to obtain
good-quality speech transmission. In this band, this kind of antenna is
practically the only choice, due to the antenna’s size. In the low end of
the medium-frequency band, it is quite difficult to obtain a high-fidelity
bandwidth.”
The abstract follows: [ Remember we are looking at something from 2005 !]
Lately, short antennas have attracted the attention of the broadcast and
communication communities. This kind of antenna has been used since the
1920s. Top-loaded monopoles are the logical antennas to be used in order to
get a low-profile antenna and performance according to the needs of the
broadcaster and for communication. In this paper, top-loaded monopoles have
been studied exhaustively using the transmission-line technique. Improved
expressions for the antenna's radiation resistance have been obtained,
taking into account the top-base current relationship and under different
top-loading conditions. This idea, of using an equivalent transmission-line
technique, has been used since the 1920s in order to obtain the antenna's
input reactance. Using this old idea, the novel approach here permits
obtaining the near- and far-field expressions from the current distribution
on the antenna structure. A near-field calculation is used to determine the
surface-current density on the ground plane. The power dissipation is
calculated from the artificial and natural ground-plane surface-current
densities, and the ground plane equivalent loss resistance is obtained. In
all cases, as a first approximation, a half-wavelength ground-plane radius
has been used, because this is the maximum distance covered by the ground
surface current under the antenna, closing the antenna's electric circuit.
Beyond this distance, the ground currents do not return to the antenna
generator, and are taken into account in the surface-wave propagation
calculations. The half-wavelength ground-plane surface is partially
occupied by the metallic radial ground system, and the remainder consists
of the natural soil. Artificial ground-plane behavior is paramount in
obtaining the best performance for a short antenna. This kind of antenna
could perform very close to a standard quarter-wave monopole, if it has
optimum dimensions. For these reasons, a short antenna and the
corresponding artificial ground plane have been analyzed, modifying the
number of radials and their lengths, in order to achieve optimum
performance, or to obtain maximum field strength with several soil
conditions for the Earth's surface. A very simple and efficient antenna can
be obtained, giving to the broadcast and communication communities a
product that can fulfil the required performance of radiating a
high-quality AM signal or digital transmission in the MF band, and good
speech quality in the LF band.
Ed McCann
AG6CX
Sausalito
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