Helical-wound Monopoles
 

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

Helical-wound Monopoles
 

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

Helical-wound Monopoles
 

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

Helical-wound Monopoles
 

On Apr 1, 6:57*am, Cecil Moore wrote:
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.

I just modeled the 1/4WL helical vs the 1/2WL helical with 4 elevated
radials and copper wire losses. The 1/2WL helical gain is 1.23 dB
higher than the 1/4WL helical gain and the TOA is 2 degrees lower for
the 1/2WL version.
--
73, Cecil, w5dxp.com

Helical-wound Monopoles
 

On 1 abr, 13:57, Cecil Moore wrote:
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.

Hello Cecil, for me the radial / counterpoise issue is a big advantage
of electrically (near) half wave structures (think of JOTA). As we use
100W maximum, high voltage at the feed point isn't a breakpoint mostly
(I have some CW Tesla coil experience).


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

Helical-wound Monopoles
 

On 4/1/2011 6:57 AM, Cecil Moore wrote:
On Mar 31, 7:31 pm, John - 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


Thanks, Cecil.

John

Helical-wound Monopoles
 

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

Helical-wound Monopoles
 

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

Helical-wound Monopoles
 

On 4/2/2011 6:14 PM, Richard Clark wrote:
On Sat, 2 Apr 2011 17:42:49 -0500, "Richard
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


Mr. Clark -

Your posts are occasionally informative, but usually not, as
demonstrated here. You have not offered anything of technical substance
in so many of your posts. It is obvious that you are knowledgeable in
the subject, but you seem to have a problem communicating that
knowledge. We could all benefit from your knowledge, but please do so
with direct technical information rather than the example above.

Please,
John

Helical-wound Monopoles
 

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.