People have an extremely strong tendency to simplify the mass of
incoming data into simply digested and understood binary categories: Is
it good, or is it evil? Does the antenna work, or doesn't it? And here
the binary choice is between a 136 foot dipole and a shorter dipole.

The answer here, as it is to so may binary questions, is that it behaves
in some ways like one, some ways like the other, and some ways like neither.

The helically wound antenna can be made resonant. A 136 foot dipole is
resonant, but a shorter dipole isn't, unless loaded.

It will be inefficient, which is also usually characteristic of a short
dipole and not a 136 foot one. A short dipole with a properly designed
matching network could be made to be more efficient than the helically
wound antenna.

The input resistance at resonance will be between that of a 136 foot
dipole and a straight dipole the length of the helical antenna, unless
the loss is exceptionally high.

The bandwidth will be between that of a 136 foot dipole and one the
length of the helical antenna, unless the loss is exceptionally high.

The pattern will be more like that of a short dipole than that of a 136
foot dipole, although you wouldn't be able to tell the difference. It
would even be hard to measure using professional equipment.

Roy Lewallen, W7EL

James barrett wrote:
Hi, I just read instructions on building a "helically wound" antenna
using a broom stick.
http://www.hard-core-dx.com/nordicdx.../bromstik.html

After reading this, I had an idea. I'm doing more google searches for
this type of configuration, but wanted to ask here as well to get some
opinions. What if I took two broom sticks, and wound each with about
68 feet of wire. Each stick making half of a dipole antenna. Then
attach 450 ohm ladder line (or 50 ohm coaxial). How would this behave
when connected to a tuner? Would it behave like a 136 foot dipole, or
would it behave like a shorter dipole?

Jim

Roy Lewallen wrote:
People have an extremely strong tendency to simplify the mass of
incoming data into simply digested and understood binary categories: Is
it good, or is it evil? Does the antenna work, or doesn't it?
...

Uhh, yeah, that sums me up pretty much. You think I should be ashamed?

JS

The answer here, as it is to so may binary questions, is that it behaves
in some ways like one, some ways like the other, and some ways like
neither.
..
..
..

Roy Lewallen, W7EL

If one were to find lossless material (superconductors?) for the short
antenna and it's corresponding matching network, what would happen as
the antenna became shorter and shorter compared with the half-wave
dipole? Would it simply approach an isotropic radiator?
Alan

Alan Peake wrote:

If one were to find lossless material (superconductors?) for the short
antenna and it's corresponding matching network, what would happen as
the antenna became shorter and shorter compared with the half-wave
dipole? Would it simply approach an isotropic radiator?
Alan


If room temperature super-conductors were available, do you even realize
the shape antennas would take? My gawd man, share some of that material
here! The thought alone is inspiring!

Regards,
JS

John Smith wrote:
Alan Peake wrote:

If one were to find lossless material (superconductors?) for the short
antenna and it's corresponding matching network, what would happen as
the antenna became shorter and shorter compared with the half-wave
dipole? Would it simply approach an isotropic radiator?
Alan


If room temperature super-conductors were available, do you even realize
the shape antennas would take? My gawd man, share some of that material
here! The thought alone is inspiring!

Regards,
JS
Don't know what shape it would be but I'm sure I wouldn't recognize it!
Alan

Alan Peake wrote:
"---what would happen as the antenna became shorter and shorter compared
with the half-wave dipole?"

Terman answers that question on page 871 of his 1955 opus:
"The directive gain of the elementary doublet =1.5." For a resonant wire
of 0.5 lambda, the gain is 1.64.

There`s not much difference in directivity as the doublet shrinks to a
vanishingly small size. The gains shown are power ratios, not dB`s.

Comparison antenna is the isotropic of which Terman says:
"Although an isotropic radiator of coherent waves does not exist because
it cannot satisfy Maxwell`s equations, the properties of such an
imaginary antenna are easily visualized, and the concept of an isotropic
radiator is often found useful in the analysis of antenna systems."
(Page 871 in the 1955 opus.)

Best regards, Richard Harrison, KB5WZI

Alan Peake wrote:

If one were to find lossless material (superconductors?) for the short
antenna and it's corresponding matching network, what would happen as
the antenna became shorter and shorter compared with the half-wave
dipole? Would it simply approach an isotropic radiator?
Alan

No. The answer can be found in any antenna textbook, because the
lossless short dipole is a very good platform to illustrate a number of
principles without the confounding additional consequences of loss.

Briefly,

-- The pattern of an infinitesimally short dipole is very similar to
that of a half wave dipole. The difference is due to the triangular
current distribution of the short dipole as opposed to the sinusoidal
current distribution of the half wave dipole. Because the patterns are
very similar and both antennas radiate all the applied power, the gain
of the two antennas is nearly the same. The short dipole's pattern is a
little fatter so it has slightly -- about a half dB -- less gain. But
the pattern of even an infinitesimally short dipole retains the basic
two-lobed dipole shape with around 1.7 dB gain over isotropic in its
favored directions.

-- The input resistance of the very short lossless dipole is very low
and the capacitive reactance very high. The resistance approaches zero
and the reactance negative infinity as the length approaches zero.
There's no comparison to an isotropic radiator, since the latter is a
purely fictional source with no even theoretical physical realization
and therefore no definable input characteristics.

-- The Q of the short dipole is very high, so the reactance varies very
rapidly with frequency. A matched short antenna would have an extremely
narrow bandwidth.

Most of these properties of the dipoles can easily be observed with the
free EZNEC demo program from http://eznec.com, and much more information
about the properties of the short lossless dipole can be found in any
antenna text.

Roy Lewallen, W7EL

Roy Lewallen wrote:

...
-- The Q of the short dipole is very high, so the reactance varies very
rapidly with frequency. A matched short antenna would have an extremely
narrow bandwidth.
...

Roy Lewallen, W7EL


And, here is where a DLM antenna is nice, keep the coils of low Q and
bandwidth is "surprisingly wide."

Regards,
JS

On 4 Dec, 01:28, John Smith wrote:
Roy Lewallen wrote:
...

-- The Q of the short dipole is very high, so the reactance varies very
rapidly with frequency. A matched short antenna would have an extremely
narrow bandwidth.
...

Roy Lewallen, W7EL

And, here is where a DLM antenna is nice, keep the coils of low Q and
bandwidth is "surprisingly wide."

Regards,
JS

Yup,
My 160M antenna came at at a resistive 200 ohm plus resonance and with
a bit of fiddling
I now connect the coax direct and cover the whole band. Not sure if I
would have been better off with keeping the high resistive impedance
and using a transformer but snow is on the way so beggars can't be
choosers.
Regards
Art KB9MZ........XG (uk)

Roy Lewallen wrote:
Alan Peake wrote:


If one were to find lossless material (superconductors?) for the short
antenna and it's corresponding matching network, what would happen as
the antenna became shorter and shorter compared with the half-wave
dipole? Would it simply approach an isotropic radiator?
Alan


No. The answer can be found in any antenna textbook,
....
etc.

Roy Lewallen, W7EL


Thanks Roy. Unfortunately, since I retired, I no longer have access to
Jasik, Kraus etc. So, thanks for the answer. I should have realised that
a dipole of any length is still a dipole and as such will not radiate
off it's ends. Mind you, Eznec shows the average dipole, less than
half-wave above ground, goes pretty close to an isotropic radiator for
all practical purposes
Alan