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
..

//

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

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.

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

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?

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

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.

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.

"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