Hi,

I created a "omni-directional" vertical antenna that NEC-2 reports to
have a free-space gain 4 dBi. The shape of the antenna looks like:


------+
A |
| B
|
+-----+
C * (* = feed point)
+-----+
|
|
|
------+

The lengths can be adjusted to give the antenna a 50 ohm feedpoint
impedence. The overall length of wire forming the antenna (4A+2B+C)
is on the order of 1.5\lambda and the height (2B) is something like
\lambda. I built this antenna for 2-meters and it seems to perform
quite well. The .nec files and parameters are available from my
antenna pages at http://www.jedsoft.org/fun/antennas/omni.html.

I am sure that I am not the first to create this simple antenna,
nevertheless a google search has turned up nothing similar. Have you
seen such an antenna before and if so, what is it called? I suspect
that it belongs to some class of antennas (antennae?). I would
like to give the proper credit and name for it on my web page.

Also, if you can find a flaw in my NEC modeling of the antenna, please
tell me. The prototype that I built does have an SWR of 1.05:1 as
given by my uncalibrated meter at the design frequency.

Thanks,
--John

On 25 Sep 2006 17:40:22 GMT, (John E. Davis)
wrote:

I am sure that I am not the first to create this simple antenna,
nevertheless a google search has turned up nothing similar. Have you
seen such an antenna before and if so, what is it called?

Hi John,

A Stub Fed Doublet. A Stub Fed Dipole. A Stub Fed Short Dipole. A
Stub Fed Short Dipole with End Loading. Take your pick.

The end loads can be removed and the stub lengthened to 18.5 inches to
re-obtain resonance at a higher Z to demonstrate two principles:
1. End loading increases current into the structure to lower its
resistance;
2. Stub tuning works.

Several comments. What's with the curious cogging of the SWR computed
from NEC2? EZNEC predicts a quite smooth curve. Your measured values
suggest nearby losses. What do you do to snub common mode currents of
the nearby transmission line that is precariously close, and co-linear
with the polarization of your vertical dipole?

73's
Richard Clark, KB7QHC

On Mon, 25 Sep 2006 11:19:25 -0700, Richard Clark
wrote:
A Stub Fed Doublet. A Stub Fed Dipole. A Stub Fed Short Dipole. A
Stub Fed Short Dipole with End Loading. Take your pick.

I think I prefer "Stub Fed Doublet".

Several comments. What's with the curious cogging of the SWR computed
from NEC2? EZNEC predicts a quite smooth curve. Your measured values

I was also wondering about that. The SWR values are computed by
xnecview. It is conceivable that the choppiness of the curve is due
to numerical stability issues, e.g., using the difference of 2 small
numbers. Tonight I will dig out the xnecview source code and
investigate further.

suggest nearby losses. What do you do to snub common mode currents of
the nearby transmission line that is precariously close, and co-linear
with the polarization of your vertical dipole?

For the prototype, I tried to run the coax perpendicular to the
polarization to minimize the issue. Eventually I will use something
like a 1-1 choke balun. In fact, at the bottom of the web page I
suggest that something like that should be used. Of course I am
open to other suggestions.

Thanks,
--John

On Mon, 25 Sep 2006 11:19:25 -0700, Richard Clark
wrote:
Several comments. What's with the curious cogging of the SWR computed
from NEC2? EZNEC predicts a quite smooth curve. Your measured values

I looked into this. The feedpoint impedence values Z(f) that NEC2
reports have discontinuities or discrete jumps, causing the predicted
SWR to have the same. I imagine that this is a result of the
numerical approximations and the segmentation used. Does EZNEC report
the oscillations when the frequency increment is on the order of 0.01
MHz? What version of NEC does EZNEC use?

I also tried using the extended thin wire kernel, but it did not help.
Nor did increasing the segmentation.

Thanks,
--John

On 27 Sep 2006 02:34:40 GMT, (John E. Davis)
wrote:

Does EZNEC report
the oscillations when the frequency increment is on the order of 0.01
MHz? What version of NEC does EZNEC use?

Hi John,

I've been using various flavors of E(L/Z)NEC for more than a decade
and I've never seen such dramatic cogging of the data that was not
attributable to construction (notably fractals). Your data is
stranger yet in having correlated noise on the left, and uncorrelated
noise on the right.

When I run the model at the resolution you suggest (0.01MHz), yes, I
do see some very fine cogging. It is on the order of 0.5 Ohm reactive
up tick which, for example, translates to a shift from 1.60:1 to
1.62:1 on a generally downward slope. When I run again with the
double precision math, this goes away. Both curves do not exhibit a
flat bottom, but rather an asymptote.

You can observe this quite readily on the download, free version of
EZNEC for yourself.

73's
Richard Clark, KB7QHC

On Tue, 26 Sep 2006 22:36:53 -0700, Richard Clark
wrote:
I've been using various flavors of E(L/Z)NEC for more than a decade
and I've never seen such dramatic cogging of the data that was not
attributable to construction (notably fractals). Your data is
stranger yet in having correlated noise on the left, and uncorrelated
noise on the right.

Please look at the ftp://space.mit.edu/pub/davis/misc/nec/swr.png
for a plot of the SWR using a spacing of 0.01 MHz. I suspect that the
noise that is showing up may be due to truncation error. I believe
that spacings of higher values, e.g., 0.2 MHz result in a different
sampling of the noise.

The version I am using (see
http://packages.debian.org/stable/hamradio/nec) contains this warning:

This version contains code which hasn't been extensively tested for
errors, which was input by hand from a report -- use with care. The
numerics are currently only SINGLE PRECISION.

If EZNEC were available for linux, I would look into it. Also, can it
be driven in "batch" mode without a GUI?

Thanks,
--John

On 27 Sep 2006 06:52:46 GMT, (John E. Davis)
wrote:

Please look at the ftp://space.mit.edu/pub/davis/misc/nec/swr.png
for a plot of the SWR using a spacing of 0.01 MHz. I suspect that the
noise that is showing up may be due to truncation error. I believe
that spacings of higher values, e.g., 0.2 MHz result in a different
sampling of the noise.

Yow! That is a lot of trash.


The version I am using (see
http://packages.debian.org/stable/hamradio/nec) contains this warning:

This version contains code which hasn't been extensively tested for
errors, which was input by hand from a report -- use with care. The
numerics are currently only SINGLE PRECISION.

Hi John,

Yes, this confirms the shift to double precision in EZNEC lowering
artifacts in the fine data.

However, I think it goes beyond simple matters of single or dual
precision math. When I was designing Fourier Analysis packages while
I was on contract to HP, I discovered there was a world of variability
in math library's transcendental functions.

Microsoft's product was abysmal, whereas Borland's was superlative. A
telling example is that for the transform of a sine wave into the
frequency domain under Microsoft math libraries, the noise floor was
at 60 to 80 dB below the fundamental peak with harmonics. When I
switched to Borland math libraries, there was a single bin response
and the noise floor plunged to 200dB down!

For others following this description, they may wonder at the
terminology of noise floor for a simple sine wave transform. Fourier
Analysis is done by parts through FFTs and this departs from classical
Fourier which is continuous. Because FFTs are discrete (bound by an
arbitrary start and stop), this injects spurious responses into the
transform. On top of that, rounding errors attributable to series
expansions of the transcendentals would give lower accuracy -
statistical (largely uncorrelated) noise. The sine wave transform was
a method of self-validation of the library used and Microsoft failed
big time (especially considering the 10:1 cost differential for the
more expensive M$).

If EZNEC were available for linux, I would look into it. Also, can it
be driven in "batch" mode without a GUI?

There have been various reports of success and failure when Windows
emulators have been used. I cannot report any personal experience
because my Linux machine is largely confined to Server development
(XAMMP/Wiki/MySQL/RubyOnRails).

73's
Richard Clark, KB7QHC

John E. Davis wrote:
I created a "omni-directional" vertical antenna that NEC-2 reports to
have a free-space gain 4 dBi.

I created a somewhat similar antenna with 20+ dBi gain.

http://www.qsl.net/w5dxp/SUPRGAIN.EZ
--
73, Cecil http://www.w5dxp.com

"John E. Davis" wrote in message
...
Hi,

I created a "omni-directional" vertical antenna that NEC-2 reports to
have a free-space gain 4 dBi. The shape of the antenna looks like:


------+
A |
| B
|
+-----+
C * (* = feed point)
+-----+
|
|
|
------+

The lengths can be adjusted to give the antenna a 50 ohm feedpoint
impedence. The overall length of wire forming the antenna (4A+2B+C)
is on the order of 1.5\lambda and the height (2B) is something like
\lambda. I built this antenna for 2-meters and it seems to perform
quite well. The .nec files and parameters are available from my
antenna pages at http://www.jedsoft.org/fun/antennas/omni.html.

I am sure that I am not the first to create this simple antenna,
nevertheless a google search has turned up nothing similar. Have you
seen such an antenna before and if so, what is it called? I suspect
that it belongs to some class of antennas (antennae?). I would
like to give the proper credit and name for it on my web page.

Also, if you can find a flaw in my NEC modeling of the antenna, please
tell me. The prototype that I built does have an SWR of 1.05:1 as
given by my uncalibrated meter at the design frequency.

Thanks,
--John

Hi John

I really got interested in the configuration you show for the antenna.
But, when I looked more closely to the Elevation Plane Pattern that looks so
narrow (high gain), I realized that the antenna is very much the same
pattern as a basic full wave center fed wire.
The graduations on the plot graph was misleading to me.
Perhaps there is something special about this antenna that I am missing.

Jerry

In article %4WRg.9158$Wi1.6469@trnddc06,
Jerry Martes wrote:

Hi,

I created a "omni-directional" vertical antenna that NEC-2 reports to
have a free-space gain 4 dBi. The shape of the antenna looks like:


------+
A |
| B
|
+-----+
C * (* = feed point)
+-----+
|
|
|
------+

The lengths can be adjusted to give the antenna a 50 ohm feedpoint
impedence. The overall length of wire forming the antenna (4A+2B+C)
is on the order of 1.5\lambda and the height (2B) is something like
\lambda. I built this antenna for 2-meters and it seems to perform
quite well. The .nec files and parameters are available from my
antenna pages at http://www.jedsoft.org/fun/antennas/omni.html.

I am sure that I am not the first to create this simple antenna,
nevertheless a google search has turned up nothing similar. Have you
seen such an antenna before and if so, what is it called? I suspect
that it belongs to some class of antennas (antennae?). I would
like to give the proper credit and name for it on my web page.

I really got interested in the configuration you show for the antenna.
But, when I looked more closely to the Elevation Plane Pattern that looks so
narrow (high gain), I realized that the antenna is very much the same
pattern as a basic full wave center fed wire.
The graduations on the plot graph was misleading to me.
Perhaps there is something special about this antenna that I am missing.

I believe that this antenna can probably be placed in the general
class of center-fed collinears. Other antennas in this class include
the center-fed fullwave, the classic Franklin antenna, and the EDZ
(extended double Zepp). The "Super-J" is a somewhat-similar design,
but is end-fed rather than center-fed.

The center-fed collinears of this sort tend to have a high (and/or
rather reactive) feedpoint impedance. They're usually fed through a
section of transmission line - often shorted at the end and fed via a
tap partway up the section... the "universal stub".

Based on the dimensions you posted, it looks to me as if this antenna
is pretty close to being an EDZ, but with the ends of the radiators
bent back sideways. I'd guess that by bending the ends sideways, and
fiddling with their lengths (and that of the matching section) you've
been able to match the 50-ohm feedline impedance without needing a
shorted/tapped matching section.

The elevation pattern of the antenna shows a hint of the high-angle
secondary lobes which characterize an EDZ.

So, I'd conclude that you've developed a variant on the EDZ (or
something partway between an EDZ and a center-fed fullwave) which
yields slightly lower gain than an EDZ but has a simpler matching
section.

The one thing I'd watch out for, with this design, is the folded-back
ends of the radiating arms. This design puts these high-voltage,
high-impedance points right at the mast, and this might make this
antenna more subject to mast/antenna coupling and de-tuning than a
traditional EDZ or full-wave center-fed.

The old ARRL VHF handbook has quite a bit of information on these
sorts of collinears, and has a nice writeup on the "universal stub"
matching technique (not very well known these days, but quite useful).

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!