Reg, G4FGQ wrote:
"No it doesn`t! (Thus an antenna for which H=0.45 lambda can by suitable
top loading be made to have a field distribution in the vertical plane
of H=0.6 lambda.)"

Reg is right. Between two antennas there will always be differences.
But, as Richard Clark might say, "Does it make a Db of difference?" One
dB can easily be lost in measurement error.

Top loading has been around since at least 1909 when it was patented by
Simon Eisenstein of Kiev. Russia. See Fig 9-24 on page 9-17 of ON4UN`s
"Loe-Band DXing". Eisenstein shows current distribution on his patent
application. He gets the base current up as it might be in a full height
antenna.

I would believe what Terman wrote because I`ve never been able to
disprove anything he wrote. Now I look for my error in logic when
something of Terman`s seems wrong.

ON4UN says on page 9-29 of his 1994 edition of the Low-Band DXing book:
"Over sea-water the 5/8 wave has lost 0.8 dB of its gain already, the
1/4-wave only 0.4 dB." (It`s less than one dB).

Even a disappearingly small radiator produces radiation less than 1/2 dB
weaker than a 1/2-wave dipole, or a 1/4-wave vertical. In lossless
antennas, the only difference in radiated signal between the full length
antenna and a too-short antenna comes from the slight difference in
their patterns.

Short antennas have efficiency problems because they have low radiation
resistances. This low radiation reaistance goes not compare as well with
a given loss resistance as does the higher radiation resistance of the
full size antenna. However, great care can be taken with the too-short
antenna to minimize its loss resistance and get good efficiency.

You have only to consult the "ARRL Antenna Book" and compare a short
continusously loaded vertical`s performance with that of a full-size
1/4-wave vertical. In my 19th edition it`s on page 5-25:
"Fig 46-Helically wound ground-plane vertical. Performance from this
type of antenna is comparable to that of many full-size 1/4 vertical
antennas."

In 1949, I worked in a transmitting plant where two stations, KPRC, 950
KHz, and KXYZ, 1320 KHz, shared the same transmittinng tower. Both
stations had identical RCA 5-C, 5 KW transmitters. Regional coverage was
almost identical despite many more degrees in the tower at 1320 KHz than
at 950 KHz.

One of the operators at the stations was a ham. He was J.L. Davis,
W5LIT. J.L. had a new 1949 Ford with a cane pole bolted to the rear
bumper. The pole was wound nearly end to end with enameled wire to serve
as antenna for his mobile ham rig. He had no top hat at the tip of his
antenna, so sometimes when he was talking a high voltsage corona
discharge would plume from the top of his antenna. Very impressive
though no help to his QSO..

Bill Orr writes on page 78 of "Vertical Antennas":
"A helix length of about .05 wavelength or more provides good results as
a substitute for a full size quarter wavelength vertical antenna."

It worked for W5LIT.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote
ON4UN says on page 9-29 of his 1994 edition of the Low-Band DXing
book: "Over sea-water the 5/8 wave has lost 0.8 dB of its gain already,
the 1/4-wave only 0.4 dB." (It`s less than one dB).

I think you should question that conclusion. Sea water (or any path of
fixed parameters) attenuates every groundwave by the same decibel amount for
the same path, conditions, and frequency. For example, using the FCC curves
for groundwave propagation from a radiator with 1 kW of power and 120
1/4-wave radials, over a seawater (only) path at 1MHz...

- a 1/4-wave vertical produces a field of 190 mV/m at 1 mile,
and 85 mV/m at 2 miles.
- a 5/8-wave vertical produces a field of 274 mV/m at 1 mile,
and 137 mV/m at 2 miles.

This is as expected. Doubling the distance reduces field strength by 6 dB
in each case. The absolute value of the groundwave signal has no bearing on
the percentage of it that is lost as it propagates.

Even a disappearingly small radiator produces radiation less than 1/2 dB
weaker than a 1/2-wave dipole, or a 1/4-wave vertical. In lossless
antennas, the only difference in radiated signal between the full length
antenna and a too-short antenna comes from the slight difference in
their patterns.

The difference in peak gain between an isotropic radiator and a reference
dipole in free space is 2.15 dB. Practical antennas in real-world
applications can show greater than 0.5 dB losses for shortened radiators.
In my example above, the 1/4-wave radiator would need about 2 kW of input
power to produce the same field as the 5/8-wave radiator with 1 kW, over the
same path -- which is a 3 dB ratio.

In 1949, I worked in a transmitting plant where two stations, KPRC, 950
KHz, and KXYZ, 1320 KHz, shared the same transmittinng tower. Both
stations had identical RCA 5-C, 5 KW transmitters. Regional coverage was
almost identical despite many more degrees in the tower at 1320 KHz than
at 950 KHz.

If the tower was 90 degrees at 950 kHz it would have been 125 degrees at
1320 kHz. The FCC efficiency for 90 degree towers is 190 mV/m at 1 mile for
1 kW, and about 210 mV/m for 125 degree radiators. So the 1320 kHz signal
was launched with a greater groundwave, but that advantage would be lost as
the signal propagated over whatever the ground conditions are for the path
(higher freqs have greater losses). Using the FCC curves and a conductivity
of 8 mS/m, the 5 mV/m contour should be about 35.5 miles away for the 5kW
950 kHz station, and about 27.5 miles away for the 5kW 1320 kHz station.
But close in probably few would know the difference.

Bill Orr writes on page 78 of "Vertical Antennas":
"A helix length of about .05 wavelength or more provides good results
as a substitute for a full size quarter wavelength vertical antenna."

Was 0.05 lambda the pitch of the helix? If so, how many turns? How were
the two installed? How were the antennas oriented, and In which direction
from the antennas was he comparing them?

+ + +

And thanks for some serious comments on this subject.

RF

Richard Fry wrote:
"Was the 0.05 lambda the pitch of the helix? If so, how many turns?"

I`ll quote Bill Orr for accuracy:
"Resonance can be established at a given frequency by the use of a
short, helically-wound element (Fig. 14). Treated bamboo poles (J L`s
choice), PVC plastic tubing, or fiberglass quad antenna spreaders can be
used as a form on which to wind the helix. Diameter for the helix must
be small in relation to length and a practical design makes use of a one
inch (25,4 mm) winding form. A helix length of about .05 wavelength or
more provides good results as a substitute for a full-size quarter
wavelength vertical antenna.

The amount of wire required for the winding depends upon helix length
and pitch (turns per inch). In general, a half-wavelength of no. 14
Formvar-coated wire is spirally wrapped on the form, with spacing
approximately equal to the wire diameter. This amount of wire
approximates a auarter-wave resonance"

There are helical antennas of two types. The "axial mode" invented by
John D. Kraus which radiates in the direction of the coil axis and the
"normal mode" helical antenna which radiates in directions perpendicular
to the coil axis, as does a short straight wire. Carried to extremes,
the pitch could go to zero,in which case the coil becomes a loop, or the
coil is stretched out to a straight wire. The helical antenna referred
to by Orr, is the normal-node helical antenna.

While the axial-mode helix is a broad-band antenna, the normal-mode
helix is a high-Q antenna and has restricted bandwidth. Orr has
something to say about the high-Q normal-mode helix:
"In order to prevent any high voltage discharge, a 12-ibch (30 cm)
diameter wire top hat is attached to the helix. Antenna resonance can be
adjusted by varying the size of the hat, or by adding a small extra
inductance at the base of the antenna."

There was also a question about directive gain which often brings a
surprised response. Terman is my source for directive gain. On page 871
of his 1955 edition of "Electronic and Radio Engineering" he gives the
directive gain, not in decibles, of 1.5 for the directive gain of the
elementary doublet. It is not isotropic. It is however infinitesimally
short. In the same Table 23-1, Terman gives the gain of the full
half-wave dipole as 1.64. There is precious little difference in
directivity or gain, which are two sides of the same coin, more or less.
Maybe Art can make a high-gain antenna of very short elements if he can
just get them to take a lot of current and not waste much to loss

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote
Richard Fry wrote:
"Was the 0.05 lambda the pitch of the helix? If so, how many turns?"

I`ll quote Bill Orr for accuracy:
etc

Thanks. Maybe I'll model that in NEC and see what it shows. Or did you do
that already?

While the axial-mode helix is a broad-band antenna, the normal-mode
helix is a high-Q antenna and has restricted bandwidth.

The VSWR bandwidth of a normal-mode helix depends on its design. Below is a
link to one that, with branch feed, has a bandwidth of 12 MHz in the FM
broadcast band (see the text at the bottom of the first column of p 1).

http://www.dielectric.com/broadcast/brochures/DCR-M.pdf

Terman is my source for directive gain. On page 871
of his 1955 edition of "Electronic and Radio Engineering" he gives the
directive gain, not in decibles, of 1.5 for the directive gain of the
elementary doublet. It is not isotropic. It is however infinitesimally
short. In the same Table 23-1, Terman gives the gain of the full
half-wave dipole as 1.64.

The 1.5 and 1.64 are multipliers. Multiplying power by 1.64X is a gain of
2.15dBi, that is, 10*log(1.64).

RF