"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

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

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

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

"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.

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

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.

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

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

"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