Helically-wound Monopoles
 

A few years ago there was some discussion on r.r.a.a. about helically-
wound, normal-mode monopoles, and the rather common expectation that
they had higher gain than a linear monopole of the same physical
height (and with other things equal).

A recent NEC-2 analysis of this topic might be of interest:
http://i62.photobucket.com/albums/h8...r_Monopole.gif
..

Also this link to a page from John Kraus' ANTENNAS FOR ALL
APPLICATIONS, 3rd Edition: http://i62.photobucket.com/albums/h8...ndVertical.gif
..

//

Helically-wound Monopoles
 

On Mar 23, 5:55*pm, Richard Fry wrote:
Also this link to a page from John Kraus' ANTENNAS FOR ALL
APPLICATIONS, 3rd Edition:http://i62.photobucket.com/albums/h8...y-woundVertica...

Kraus' short resonant normal-mode helical antenna uses close to 1/4WL
of conductor. We know that because of adjacent turn coupling, when a
conductor is coiled into a helical configuration, more conductor is
required to maintain a constant electrical length, e.g. 90 degrees in
this case. I suspect that Kraus' helical antenna example would be
resonant at about 1.8 times the design frequency rather than at the
design frequency. Please note the last line in the Kraus quote
regarding the advantage of a helix.

Since a helical monopole is 90 degrees long at the design frequency,
one wonders if half of the helix would be 45 degrees long at the
design frequency? And if the missing half of the antenna were replaced
by a whip to return to the original resonant frequency, why wouldn't
we have a base loaded antenna with the base loading coil occupying 45
degrees?
--
73, Cecil, w5dxp.com

Helically-wound Monopoles
 

On 23 mar, 23:55, Richard Fry wrote:
A few years ago there was some discussion on r.r.a.a. about helically-
wound, normal-mode monopoles, and the rather common expectation that
they had higher gain than a linear monopole of the same physical
height (and with other things equal).

A recent NEC-2 analysis of this topic might be of interest:http://i62.photobucket.com/albums/h8..._Linear_Monopo...
.

Also this link to a page from John Kraus' ANTENNAS FOR ALL
APPLICATIONS, 3rd Edition:http://i62.photobucket.com/albums/h8...y-woundVertica...
.

//

Hello Richard,

Maybe there is confusion between gain and directivity.

When there is no change in the phase of the current, and overall
physical length 0.25 lambda, directivity will be 1.76 dBi (or 4.76
dB over perfect electrical conducting ground).

I can imagine that part of the matching can be in the helix (lots of
copper) so that the ohmic loss may be less w.r.t. to a lumped coil at
the feed point. If so, the gain of the helix can be higher.

If you can make the helix electrically longer then 0.25 lambda (so
that current maximum is not in the feed point, but for example in the
middle), directivity will not change, but ground loss will reduce as
the helix will have high Re(Zin).

Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me.

Helically-wound Monopoles
 

Since a helical monopole is 90 degrees long at the design frequency,..

It may have ~ zero reactance, as does a linear ~ 90-degree monopole, but
the helix will not have the radiation resistance of the linear version, as
John Kraus pointed out in the linked page.

Radiation resistance is a function of the end-end length of the helix and
the frequency, whether the helix is self-resonant or not. The radiation
resistance of the resonant helix in Kraus' example is much lower than that
of a series-fed, 1/4-wave, linear monopole.

Helically-wound Monopoles
 

On Mar 24, 11:57*am, "Richard Fry" wrote:
Radiation resistance is a function of the end-end length of the helix and
the frequency, whether the helix is self-resonant or not.

Yes, I thought that was the purpose of your posting. As Kraus said,
the helical has an advantage over a short straight conductor - same
radiation resistance with less reactance. The radiation resistance of
a 6" long Texas Bugcatcher coil is approximately the same as a 6"
piece of wire.
--
73, Cecil, w5dxp.com

Helically-wound Monopoles
 

Cecil Moore wrote:
... The radiation resistance of a 6" long Texas Bugcatcher coil
is approximately the same as a 6" piece of wire.
______________

We agree on that point, Cecil.

But if, as if you posted earlier, "a helical monopole is 90 degrees long at
the design frequency," are you claiming that such a short, self-resonant,
normal-mode, helical monopole has the same radiation resistance and system
performance as a self-resonant linear monopole of about 1/4 of a free-space
wavelength (other things equal)?

And if you do, could you please explain why this approach was not adopted
many decades ago for use by AM broadcast stations?

Helically-wound Monopoles
 

On Mar 24, 5:35*pm, "Richard Fry" wrote:
But if, as if you posted earlier, "a helical monopole is 90 degrees long at
the design frequency," are you claiming that such a short, self-resonant,
normal-mode, helical monopole has the same radiation resistance and system
performance as a self-resonant linear monopole of about 1/4 of a free-space
wavelength (other things equal)?

Absolutely not. I am claiming that a 1/8WL long *resonant* helical is
electrically 90 degrees long and has approximately the same radiation
resistance as a 1/8WL straight piece of wire. Radiation resistance and
linear *physical* length are correlated. Radiation resistance and
*electrical* length are NOT correlated. As I said previously
(concerning standing wave antennas) the feedpoint impedance is
associated with the electrical length of the antenna. Radiation is
associated with the physical length of the antenna.
--
73, Cecil, w5dxp.com

Helically-wound Monopoles
 

On Fri, 25 Mar 2011 04:36:26 -0700 (PDT), Cecil Moore
wrote:

On Mar 24, 5:35*pm, "Richard Fry" wrote:
But if, as if you posted earlier, "a helical monopole is 90 degrees long at
the design frequency," are you claiming that such a short, self-resonant,
normal-mode, helical monopole has the same radiation resistance and system
performance as a self-resonant linear monopole of about 1/4 of a free-space
wavelength (other things equal)?

Absolutely not. I am claiming that a 1/8WL long *resonant* helical is
electrically 90 degrees long and has approximately the same radiation
resistance as a 1/8WL straight piece of wire. Radiation resistance and
linear *physical* length are correlated. Radiation resistance and
*electrical* length are NOT correlated. As I said previously
(concerning standing wave antennas) the feedpoint impedance is
associated with the electrical length of the antenna. Radiation is
associated with the physical length of the antenna.

Cecil,
You frequently embed comments in your posts that I find especially
important to antenna basics.

I am starting a list of Antenna Axioms with this:

"Radiation resistance and linear *physical* length are correlated.
Radiation resistance and *electrical* length are NOT correlated. As I
said previously (concerning standing wave antennas) the feedpoint
impedance is associated with the electrical length of the antenna.
Radiation is associated with the physical length of the antenna.
--
73, Cecil, w5dxp.com"

As a student of antennas I do appreciate your stating the basic
principals in answering questions.

There are a lot of Hams who still think a short resonate dipole is
just as good as a full size dipole.

Helically-wound Monopoles
 

On Mar 25, 9:53*am, John Ferrell wrote:
There are a lot of Hams who still think a short resonate dipole is
just as good as a full size dipole.

That would be true if everything was lossless. Unfortunately in the
real world, a short dipole *system* almost always suffers more overall
losses than a well-designed full size dipole *system*. In general, the
shorter the dipole, the greater are the losses in the process of
transferring energy from the source to the load. Short dipoles are
indeed efficient radiators of the *energy delivered to the antenna*.
The loss problems are in the energy transfer/delivery systems, not in
the energy radiating system.

The reason that a high-Q center loading coil system has less loss than
a 100% helical system is interesting. The impedance discontinuity at
the loading-coil/whip junction yields a lossless phase shift that
contributes to the electrical length of the antenna. It is a rare
"something-for-nothing" gift from the Antenna Gods. :)
--
73, Cecil, w5dxp.com

Helically-wound Monopoles
 

Richard Fry wrote:
A few years ago there was some discussion on r.r.a.a. about helically-
wound, normal-mode monopoles, and the rather common expectation that
they had higher gain than a linear monopole of the same physical
height (and with other things equal).

A recent NEC-2 analysis of this topic might be of interest:
http://i62.photobucket.com/albums/h8...r_Monopole.gif
.

I think the difference might be in gain, not directivity...
The IR losses in the conductors (and components) would be different in a
helically loaded monopole and a lumped network matching a short unloaded
monopole. One could probably construct examples for cases where either
one has lower loss.

There might also be a difference in the losses in the ground plane,
although, intuitively, I suspect they would be small. The current
distribution just isn't that different between the two cases


It would be interesting to run some cases where you use "wire" (1cm
diameter conductors on your helix are pretty big... I'd try something
like 1mm (18 AWG) or maybe 2mm (12 AWG)..

As I recall, NEC does figure out the losses accounting for skin effect,
etc., although it might not deal with the "proximity effect" from
adjacent turns.

Helically-wound Monopoles
 

"Jim Lux" wrote
It would be interesting to run some cases where you use "wire"
(1cm diameter conductors on your helix are pretty big... I'd try
something like 1mm (18 AWG) or maybe 2mm (12 AWG)..

Here are the base feedpoint impedances for that NEC model
of a helix for the suggested conductor diameters...

1mm = 0.12 -j 2260 ohms
2mm = 0.12 -j 2170 ohms

Helical-wound Monopoles
 

On 3/23/2011 3:55 PM, Richard Fry wrote:
A few years ago there was some discussion on r.r.a.a. about helically-
wound, normal-mode monopoles, and the rather common expectation that
they had higher gain than a linear monopole of the same physical
height (and with other things equal).

A recent NEC-2 analysis of this topic might be of interest:
http://i62.photobucket.com/albums/h8...r_Monopole.gif
.

Also this link to a page from John Kraus' ANTENNAS FOR ALL
APPLICATIONS, 3rd Edition: http://i62.photobucket.com/albums/h8...ndVertical.gif
.

//

Somehow, I think there is a difference. I think that they are being
shown to be the same in the computer model is not valid in the real
world. That said, in real world use, the differences do seem to be
insignificant.

And, that said, I use a helical wound, half-wave electrical length -
quarter-wave physical length, monopole in lieu of a 1/4 wave
physical-length and physical length antenna. And, in personal
experience, this DOES provide increased performance over the 1/4 wave.

In most real world restrictions, the helical wound versions always are
an advantage in real world physical size ...

Regards,
JS

Helical-wound Monopoles
 

On 3/23/2011 3:55 PM, Richard Fry wrote:
A few years ago there was some discussion on r.r.a.a. about helically-
wound, normal-mode monopoles, and the rather common expectation that
they had higher gain than a linear monopole of the same physical
height (and with other things equal).

A recent NEC-2 analysis of this topic might be of interest:
http://i62.photobucket.com/albums/h8...r_Monopole.gif
.

Also this link to a page from John Kraus' ANTENNAS FOR ALL
APPLICATIONS, 3rd Edition: http://i62.photobucket.com/albums/h8...ndVertical.gif
.

//

Somehow, I think there is a difference. I think that they are being
shown to be the same in the computer model is not valid in the real
world. That said, in real world use, the differences do seem to be
insignificant.

And, that said, I use a helical wound, half-wave electrical length -
quarter-wave physical length, monopole in lieu of a 1/4 wave ELECTRICAL
length and physical length antenna. And, in personal experience, this
DOES provide increased performance over the 1/4 wave.

In most real world restrictions, the helical wound versions always are
an advantage in real world physical size ...

Regards,
JS

Helical-wound Monopoles
 

On Mar 29, 12:30*pm, John Smith wrote:
And, that said, I use a helical wound, half-wave electrical length -
quarter-wave physical length, monopole in lieu of a 1/4 wave
physical-length and physical length antenna. *And, in personal
experience, this DOES provide increased performance over the 1/4 wave.

Unlike the original example, that would produce a quite different
current distribution on the antenna. I suspect the half-wave helical
wouldn't require as good a radial system as the standard 1/4WL
monopole since the current maximum point is halfway up the helical.
Did you end-feed the beast or center-feed it? Did you have a good
radial system?
--
73, Cecil, w5dxp.com

Helical-wound Monopoles
 

"Cecil Moore" wrote
I suspect the half-wave helical wouldn't require as good a
radial system as the standard 1/4WL monopole since the
current maximum point is halfway up the helical.
_______________

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."

John Kraus also assumed sinusoidal current distribution along the helix in
his Fig 8-72 (see clip).

This current sinusoid exists along the aperture of the helix, and not along
the spiral conductor itself. 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.

The current distribution along the aperture of both of these forms of
radiators has a sinusoidal shape. The current at the top of both of these
radiators must be zero. The portion of a sinusoidal waveform at the
operating frequency, beginning with zero current at the top, that can exist
along the aperture of radiators that are physically short in terms of
wavelength, as in my NEC comparison, appears to be a straight line with zero
current at the top and maximum current at the base of the radiator.

With essentially identical current distribution along the aperture of both
radiator forms, it should be expected that the helix and linear monopoles in
this discussion should have essentially identical radiation resistances and
patterns.

This has been shown to be true in the NEC comparison in the OP, and is
supported by the quoted statements from well-respected authors of antenna
engineering textbooks.

Helical-wound Monopoles
 

On 3/29/2011 5:35 PM, Richard Fry wrote:

The current distribution along the aperture of both of these forms of
radiators has a sinusoidal shape. The current at the top of both of
these radiators must be zero. The portion of a sinusoidal waveform at
the operating frequency, beginning with zero current at the top, that
can exist along the aperture of radiators that are physically short in
terms of wavelength, as in my NEC comparison, appears to be a straight
line with zero current at the top and maximum current at the base of the
radiator.

Yes. This is shown in various editions of the ARRL Antenna Handbook and
the ARRL Handbook itself.

With essentially identical current distribution along the aperture of
both radiator forms, it should be expected that the helix and linear
monopoles in this discussion should have essentially identical radiation
resistances and patterns.

This has been shown to be true in the NEC comparison in the OP, and is
supported by the quoted statements from well-respected authors of
antenna engineering textbooks.

Thanks, Richard.

73,
John

Helical-wound Monopoles
 

John - KD5YI wrote:
On 3/29/2011 5:35 PM, Richard Fry wrote:

The current distribution along the aperture of both of these forms of
radiators has a sinusoidal shape. The current at the top of both of
these radiators must be zero. The portion of a sinusoidal waveform at
the operating frequency, beginning with zero current at the top, that
can exist along the aperture of radiators that are physically short in
terms of wavelength, as in my NEC comparison, appears to be a straight
line with zero current at the top and maximum current at the base of the
radiator.

Yes. This is shown in various editions of the ARRL Antenna Handbook and
the ARRL Handbook itself.

With essentially identical current distribution along the aperture of
both radiator forms, it should be expected that the helix and linear
monopoles in this discussion should have essentially identical radiation
resistances and patterns.

This has been shown to be true in the NEC comparison in the OP, and is
supported by the quoted statements from well-respected authors of
antenna engineering textbooks.

Thanks, Richard.

73,
John


The next step would be to run it plugging in some reasonable number for
the wire resistivity. The patterns should be quite similar. I theorize
that it will show that for same power in at the feedpoint, the "gain"
will be slightly less for the helically loaded one (because there's a
longer wire, so more resistance, for essentially the same current
distribution in the wire).

Then, the question would be whether the helically loaded unit has a
lower loss in a matching network at the base.

Helical-wound Monopoles
 

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.

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.

Not true. Any monopole that is electrically 180 degrees long will have
the current maximum point in the middle and a normal mode helix is no
exception. You can easily model such with EZNEC. For any 180 degree
antenna, at the feedpoint, the reflected voltage will arrive in phase
with the forward voltage. The reflected current will arrive 180
degrees out of phase with the forward current.

Zfp = (Vfor+Vref)/(Ifor-Iref) is a current minimum

Take your NEC helical model and adjust the frequency to approximately
double the resonant frequency and take a look at the current
distribution.
--
73, Cecil, w5dxp.com

Helical-wound Monopoles
 

On Mar 30, 7:01*am, Cecil Moore wrote:
On Mar 29, 5:35*pm, "Richard Fry" wrote:
But John,

Richard, I'm sorry. I have no idea why I typed "John" there. Maybe a
senior moment?
--
73, Cecil, w5dxp.com

Helical-wound Monopoles
 

"Cecil Moore" wrote

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

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).

This link shows that even though the length of wire used in the helix is
3.14 X the length used in the linear monopole, the current distribution
along their apertures essentially is the same, as will be the directivity
and radiation pattern of both versions.

This same equivalence would apply to the current distribution, directivity
and pattern of a linear, 1/4-WL monopole and a helically-wound monopole that
was 1/4-WL in aperture, but contained 1/2-WL of coiled wire.

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

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

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.