Dan Jacobson wrote:
"Just curious, what is the result of computer modeling for the ideal
shape of Yagi elements?"

Advantage of a Yagi is gain in a small space due to close element
spacing. This also results in lower radiation resistance as the size
goes down with increased coupling and gain. To get a more convenient
Yagi drivepoint impedance (5 ohms is not usually convenient), a folded
dipole is often used as the driven element. That can multiply the
impedance by 4. See "Multi-Element V-H-F Beam Antennas" section of
Editors and Engineers "The Radio Handbook", William I. Orr, W6SAI,
Editor 15th Edition.

The tapered elements, large at the boom and small at the tip as shown in
the "ARRL Antenna Book", are mechanically sound but the taper is the
reverse of that needed for wide bandwidth.

Cones which grow in size from feedpoint to open ends have an advantage
of more uniform characteristic impedance.

Kraus says on page 229 of his 1950 edition:
"Whereas the characteristic impedance of a biconical antenna is uniform,
the impedance of antennas of shapes other than conical is nonuniform."

Terman says on page 921 of his 1955 edition:
"A particularly effective manner of utilizing the thick antenna
principle to obtain a wideband is to employ a cone antenna. Excitation
at the apex starts a spherical wave, as illustrated in Fig. 23-56, which
spreads out along the cone and in adjacent space without reflection
until reaching the end of the cone."

I don`t know what a computer program indicates as an ideal shape for a
Yagi`s driven element. The conical element shape may be a candidate for
computer optimization. Kraus and Terman have suggestions about the apex
angle and length of the cone.

Best regards, Richard Harrison, KB5WZI

On Sat, 3 Apr 2004 11:25:34 -0600 (CST),
(Richard Harrison) wrote:
Kraus says on page 229 of his 1950 edition:
"Whereas the characteristic impedance of a biconical antenna is uniform,
the impedance of antennas of shapes other than conical is nonuniform."

Hi Richard,

This analysis is derived from Schelkunoff's work that was later (1952)
offered in his "Advanced Antenna Theory." This material is offered in
a more accessible form in "Antennas and Radiowave Propagation," Robert
E. Collin, 1985 with the following observation:
"In his book Schelkunoff shows .... the biconical antenna theory
provides a theoretical basis for assuming a sinusoidal current
distribution on thin-wire antennas."

Basically, the biconical antenna theory is the basis for the
fundamental treatment of antenna as a non-terminated transmission line
by which all thin-wire antennas may be shown to devolve from (as the
degenerate forms of infinitely small radius cone sections). Such
treatment allows those interested to observe both the SWR relationship
(E/I distribution) of thin-wire elements compared to what you
correctly identify as the uniform impedance of the large cone angle
forms and both be mathematically consistent with such diverging
characteristics (which is to say those who oppose the discussion of
Antenna as transmission line are unaware of this work).

However, as to its application in a Yagi-Uda design, it has to be
particularly unrewarding as nearby elements preclude a full-blown
conic development without the attending physical interference.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
"This analysis is derived from Schelkunoff`s work that was later (1952)
offered in his "Advanced Antenna Theory"."

Kraus acknowledges this at the beginning of his 1950 "Antennas" chapter
8:
"This problem (input resistance and reactance) is most simply approached
by Schelkunoff`s treatment of the biconical antenna (S.A. Schelkunoff,
"Electromagnetic Waves"1943) which will be outlined in the following
sections."

As Reg Edwards says, these guys all copied each other`s work. But, they
nearly always gave each other the credit due.

Other than a few discones, which have 180-degrees as one of their vertex
angles, I don`t see many amateur conical antennas despite the obsession
with bandwidth and SWR. Skeletal designs might retain desirable
characteristics without extreme cost and complexity.....Nah, the plain
thin wire dipole is too simple and easy.

Best regards, Richard Harrison, KB5WZI