That article is reprinted in the May/ June 2005 QEX.
Thanks Ed,
Jim WE4S
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On Oct 20, 2022, 1:43 PM, at 1:43 PM, Edward McCann <ag6cx1@gmail.com> wrote:
>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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