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

"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.

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

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

On Mar 31, 7:10*pm, "Richard Fry" wrote:
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.

I'm not saying that it has a benefit - just that a 270 degree
electrically long antenna can never have the same radiation pattern as
a 51 degree physical whip even if the physical length of the 270
degree helical antenna is physically 51 degrees.

To be clear on what I am saying: Up to a certain percentage of a
wavelength, the physical length of the antenna dictates the radiation
pattern. Above that percentage of a wavelength, the theory falls
apart.

It is akin to assuming that the current distribution in the top
portion of a monopole is a straight line. At some point, the straight
line assumption fails because the current distribution is actually
sinusoidal.
--
73, Cecil, w5dxp.com

On 3/31/2011 7:26 PM, Cecil Moore wrote:
On Mar 31, 7:10 pm, "Richard wrote:
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.

I'm not saying that it has a benefit - just that a 270 degree
electrically long antenna can never have the same radiation pattern as
a 51 degree physical whip even if the physical length of the 270
degree helical antenna is physically 51 degrees.

To be clear on what I am saying: Up to a certain percentage of a
wavelength, the physical length of the antenna dictates the radiation
pattern. Above that percentage of a wavelength, the theory falls
apart.

It is akin to assuming that the current distribution in the top
portion of a monopole is a straight line. At some point, the straight
line assumption fails because the current distribution is actually
sinusoidal.
--
73, Cecil, w5dxp.com

Cecil -

Do you have an EZnec file you can post? I'd like to see what you're doing.

Thanks es 73,
John

On Mar 31, 7:31*pm, John - KD5YI wrote:
Do you have an EZnec file you can post? I'd like to see what you're doing..

It is at:

http://www.w5dxp.com/helix.EZ

The 90 degree (1/4WL) resonant frequency is 10.067 MHz where the TOA
is 150 degrees.

The 270 degree (3/4WL) resonant frequency is 26.493 MHz where the TOA
is 155 degrees.

The difference in TOA is because of the two current maximum points at
26.493 MHz.

The 180 degree (1/2WL) resonant frequency is 16.6254 MHz where the TOA
is 29 degrees. Raising the single current maximum point from the
feedpoint to the midpoint of the helical monopole only moves it by
2.625 feet which is 0.0444WL (16 physical degrees) and that lowers the
TOA by one degree. Since the 1/2WL helical contains twice as much
wire as the 1/4WL helical, I don't see any advantage for the 1/2WL
helical over the 1/4WL helical except for the elevated current maximum
point which may require a less robust radial system.

The "Currents" button on EZNEC will display the current magnitude/
phase in the helical segments.
--
73, Cecil, w5dxp.com

Followup -- the link below compares the relative current distribution,
directivity and radiation efficiency of a helical and a linear radiator
system when the helical radiator described in my earlier post is operating
at the frequency of its first self-resonance, and the linear monopole height
is set for its first self-resonance at that same frequency.

It is interesting to note that linear form has better performance than the
helical form.

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

On Sat, 2 Apr 2011 17:42:49 -0500, "Richard Fry"
wrote:

It is interesting to note that linear form has better performance than the
helical form.

Hi Richard,

It is also like saying that donuts are sweeter than apples. However,
I can imagine what is driving the thread that takes us into that well
charted territory.

73's
Richard Clark, KB7QHC

On 3 abr, 00:42, "Richard Fry" wrote:
Followup -- the link below compares the relative current distribution,
directivity and radiation efficiency of a helical and a linear radiator
system when the helical radiator described in my earlier post is operating
at the frequency of its first self-resonance, and the linear monopole height
is set for its first self-resonance at that same frequency.

It is interesting to note that linear form has better performance than the
helical form.

http://i62.photobucket.com/albums/h8...inear_1st_Reso...

Hello Richard Fry,

Why is this so interesting, as it is what I expect (and I think you
expect this also)? The current*(physical length) product is more, so
given same feed current it produces more field (hence more radiated
power). This results in higher input impedance, hence reducing the 10
ohms ground loss.

The small change in shape of pattern is just due to the less isotropic
array pattern of the 0.25 lambda radiator (w.r.t. to the array pattern
of the 3 m radiator).

If it is not time consuming, I would like to see what happens when you
extend the helix until it gets its second (half wave) high impedance
resonance (current maximum in the middle). I expect some gain increase
due to small change in antenna pattern and reduced ground loss.

¡Very informative thread!

73, Wim, PA3DJS, www.tetech.nl.