Helical-wound Monopoles
 

Cecil Moore wrote:
On Mar 29, 5:35 pm, "Richard Fry" wrote:
Quoting from Antenna Engineering Handbook, 2nd Edition by Johnson and Jasik,
page 13-18: "For a normal-mode helix whose dimensions are small compared to
a wavelength, the current distribution along the helix is approximately
sinusoidal."

But John, a helix that is 180 degrees long electrically is not small.
It is electrically double the size of a 1/4WL monopole.


"small" in Kraus's book means "physically" small, not electrically small.


Therefore it is unclear as to the source of
this belief that current would be maximum at the center of "1/2-WL" helix
whose end-end length is 1/4-WL. In reality the current maximum would be at
the base of the radiator, just as it is for a 1/4-wave linear monopole.


Take your NEC helical model and adjust the frequency to approximately
double the resonant frequency and take a look at the current
distribution.



This is no different than taking the "non-helical" antenna and feeding
it at twice the frequency.

I would imagine that the pattern of the helically loaded and the
unloaded will be quite similar at ANY frequency, until you get to where
the *diameter* of the assembly starts to be a significant fraction of a
wavelength.

What might change more is the resistive losses, although I suspect
they'll scale in proportion too. Whether you've strung 10 meters, 20
meters or 30 meters of wire in a physical 10 meter length doesn't change
the *radiation* properties a huge amount.

Helical-wound Monopoles
 

On Mar 30, 10:28*am, "Richard Fry" wrote:
I have already done an illustration based on the currents in the NEC
comparison posted earlier, showing a helix and a linear monopole each about
6 degrees in aperture (link below).

What we have here is a failure to communicate. Please forget about
your previous posting. We are not talking about 6 deg. electrically
short helicals. We are talking about comparing an 180 degree
electrically long monopole to a 90 degree long RESONANT monopole.
Here's how to accomplish what we are talking about:

1. Wind a helical that is 90 degrees long, i.e. the feedpoint
impedance is R1+j0. That helical is 1/4WL long electrically and
resonant. It may be ~1/8WL (45 deg) long physically.
2. Now increase the frequency until the helical is 180 degrees long
electrically. At something like double the frequency, it will be 1/2WL
long electrically and the feedpoint impedance will be R2+j0 where
R2R1. It may be ~1/4WL long physically.

John said his 180 degree helical outperformed his resonant 90 degree
helical. His statement has nothing to do with electrically short
helical monopoles because they are resonant.

The current maximum for a 90 degree resonant helical will be at the
base feedpoint just as it is for a 90 degree stub. The current maximum
for a 180 degree helical will be halfway up the antenna just as it is
halfway up a 180 degree stub.
--
73, Cecil, w5dxp.com

Helical-wound Monopoles
 

On Mar 30, 11:31*am, Jim Lux wrote:
I would imagine that the pattern of the helically loaded and the
unloaded will be quite similar at ANY frequency, until you get to where
the *diameter* of the assembly starts to be a significant fraction of a
wavelength.

A helical longer than a few degrees will exhibit transmission line
effects. A helical that is electrically 180 degrees long will have
essentially the same standing wave current envelope as a 180 degree
long open-circuit transmission line stub. EZNEC agrees.

John said his 180 degree electrically long helical outperformed his
electrically long 90 degree helical. The standing-wave current
envelope for the 90 degree helical is a cosine with the current
maximum at the feedpoint. The standing-wave current envelope for the
180 degree long helical is a sine wave with the current maximum point
in the middle of the helical. They would not have the same radiation
patterns. EZNEC agrees.

Again, I have modeled these conditions using EZNEC and I am reporting
the results. The "Currents" button will give the current magnitude/
phase for each segment in the helical.
--
73, Cecil, w5dxp.com

Helical-wound Monopoles
 

"Cecil Moore" wrote
The standing-wave current envelope for the 180 degree long helical
is a sine wave with the current maximum point in the middle
of the helical.

That is true ONLY if the end-to-end length (height) of a normal-mode helical
monopole occupies about 180 degrees of a free-space wavelength.

If that helix occupies only about 90 degrees of a free-space wavelength,
then no matter how much linear wire length is contained in the coils of the
helix, that helical radiator will have the radiation resistance, pattern and
directivity characteristics of a 90-degree linear monopole of the same
end-to-end height.

The length of coiled wire in a helix of any physical length makes very
little difference in the current distribution along its aperture, its
directivity, or its radiation patterns.

Please forget about your previous posting. We are not talking about 6 deg.
electrically short helicals.

Rather than suggesting that my previous posting(s) on this subject should be
forgotten, perhaps they should be re-read -- especially the link to
http://i62.photobucket.com/albums/h8...le_Current.gif .

Helical-wound Monopoles
 

P.S.

Both linear, and helical normal-mode monopoles of ~6 degrees physical
aperture (and less) can be made resonant at the operating frequency via a
suitable inductance placed either in the monopole itself, or at its
feedpoint.

But resonance so achieved does NOT mean that such monopole radiators will
have a very useful amount of radiation resistance, or that such a resonant
condition equates to the performance of a radiator that is resonant without
the need for such an additional inductance.

This reality appears to have been overlooked in some of the earlier posts in
this thread.

Helical-wound Monopoles
 

On Mar 30, 5:58*pm, "Richard Fry" wrote:
If that helix occupies only about 90 degrees of a free-space wavelength,
then no matter how much linear wire length is contained in the coils of the
helix, that helical radiator will have the radiation resistance, pattern and
directivity characteristics of a 90-degree linear monopole of the same
end-to-end height.

I just modeled a 5.25' long helical using EZNEC at the 270 degree 3rd
harmonic frequency of 26.5 MHz. Both helical and whip are modeled as
lossless.

If I understand you correctly, the 270 degree helical should have a
TOA equal to a 5.25' whip. The TOAs differ by 6 degrees. The maximum
gain of the 5.25' whip is -0.25 dBi. The maximum gain of the 270
degree helical of the same length is +0.29 dBi, a difference of 0.54
dB.

The 5.25' whip is1/4WL resonant at 45.3 MHz with a maximum gain of
-0.25 dBi at a TOA of 27 degrees. The 5.25' helical at 45.3 MHz has a
gain of -3.13 dBi at a TOA of 24 degrees, a difference of 2.88 dBi and
3 degrees.
--
73, Cecil, w5dxp.com

Helical-wound Monopoles
 

"Cecil Moore" wrote:
I just modeled a 5.25' long helical using EZNEC at the 270 degree 3rd
harmonic frequency of 26.5 MHz. Both helical and whip are modeled as
lossless. If I understand you correctly, the 270 degree helical ...

A normal-mode helical with a radiating aperture of 5.25' is not a "270
degree" radiator on 26.5 MHz. It is a ~ 51 degree radiator on that
frequency.

Helical-wound Monopoles
 

On Mar 31, 10:58*am, "Richard Fry" wrote:
A normal-mode helical with a radiating aperture of 5.25' is not a "270
degree" radiator on 26.5 MHz. *It is a ~ 51 degree radiator on that
frequency.

This is making no sense to me so I fear we have some sort of semantic
problem. I'm now not sure what you mean by "a radiating aperture of
5.25 feet". "The IEEE Dictionary" says: "In some cases, the aperture
may be considered to be a line." I was assuming that the 5.25 feet
aperture was akin to a line of straight wire 5.25 feet long or a 5.25
foot long (end to end) helical monopole. If that is not the case,
please enlighten me on your definition of "aperture". EZNEC says my
5.25' (end-to-end) physically tall helical monopole is electrically
270 degrees long.

I assumed that 5.25' is the length of a straight wire or the physical
end-to-end length of the helix itself (not the linear length of the
wire). The velocity factor of a helix is a function of the helix
geometry and *varies widely with diameter and turn spacing*. The helix
I designed using EZNEC has a current maximum at the feedpoint, a
current minimum 1/3 of the distance up the helix, a current maximum
2/3 of the distance up the helix, and a current minimum at the end of
the helix. That's 270 electrical degrees any way you cut it because
*there is always 90 electrical degrees between the current maximum and
current minimum in a standing wave*.

The requirement that a 5.25' tall helical monopole has to satisfy to
be 270 electrical degrees long on 26.5 MHz is to have a velocity
factor of 5.25/27.85 = 0.1885 which is a piece of cake. The 5.25' is
the actual end-to-end height of the helical monopole and the 27.85' is
3/4 of a wavelength in free space at 26.5 MHz.

Note that the velocity factor is the distance a traveling wave travels
in the helical medium in unit time compared to the distance a
traveling wave travels in free space in the same unit time.

Richard, this is giving me a headache - what am I missing?
--
73, Cecil, w5dxp.com

Helical-wound Monopoles
 

On Mar 31, 10:58*am, "Richard Fry" wrote:
A normal-mode helical with a radiating aperture of 5.25' is not a "270
degree" radiator on 26.5 MHz. *It is a ~ 51 degree radiator on that
frequency.

Sorry, I just noticed you are talking about physical length rather
than electrical length. Do you agree that the helical is 270 degrees
long *electrically* because there are two current maximum points and
two current minimum points on the helical antenna that is 5.25 feet
long?

FP-Imax-////////////////////-Imin-////////////////////-
Imax-////////////////////-Imin

The Imax points are 3.5 feet apart. They are not very far apart
compared to wavelength (~0.1WL) but they are far enough apart to raise
the take-off-angle by 6 degrees for my particular helical according to
EZNEC. With everything else being equal, when a 5.25 foot helical
antenna has more than one current maximum point on the antenna, it
will raise the take-off-angle by an amount correlated to the
percentage of a wavelength spacing between the two current maximum
points.

Conclusion: What you have said seems to be a fact for antennas with
only one current maximum. The presence of two (or more) current
maximum points on the antenna modifies the take-off-angle according to
the laws of radiation physics which is demonstrated by NEC using the
method-of-moments algorithms. A 5.25' end-to-end helical is not the
same as a "~51 degrees radiator on 26.5 MHz" when it has two (or more)
current maximum points separated by, e.g. 0.1WL. The two take-off-
angles are 20% different just as they should be.
--
73, Cecil, w5dxp.com

Helical-wound Monopoles
 

Cecil -

The link below shows the NEC-2D results for the 3-m monopole whose geometry
I posted earlier -- at its frequency of first self-resonance, and at 3X that
frequency.

We don't disagree as far as current distribution is concerned, but maybe in
the belief that such a helix at an operating frequency that is 3X its first
resonance has a practical benefit for users.

The reason that it may not is traceable to the radiation resistances at each
frequency w.r.t. a fixed amount of antenna system loss.

http://i62.photobucket.com/albums/h8...d_Harmonic.gif