On Sep 3, 10:29*pm, Jeff Liebermann wrote:
On Thu, 3 Sep 2009 19:11:42 -0700 (PDT), wrote:
How in the heck are you going to
get **ANY** vertical radiator to have a truly isotropic pattern?
It's impossible. An isotropic pattern is a theoretical pattern
in which radiation is equal in all directions. Such a pattern
does not exist with real antennas.

A real isotropic radiator may not exist, but one can get fairly close.
If you believe the model, the total error is 0.44 db. *See:
http://802.11junk.com/jeffl/antennas/isotropic/index.html
The NEC2 deck is under the photo labeled "main".

I once built one of these antennas on roughly 444MHz out of cardboard
and magnet wire. *The oscillator was a small crystal can oscillator
running from a 9V battery to avoid having the feed coax wrecking the
pattern. *The impedance was nowhere near 50 ohms and required a bit of
matching to get the VSWR down. *I'm now digging for the photos. *

I used a piece of string to maintain a constant radius, a tiny pickup
loop at the end of a length of coax cable running inline with the
string, and eventually going to my antique HP spectrum analyzer. *On
the 2dB/div scale, it was a fairly good approximation of an isotropic
radiator with errors mostly caused by indoor reflections and
interference with the bench. *


Sure, you can get fairly close to isotropic with the right
system, but how are you going to do it by tipping a
vertical? The likely results do not fit my idea of isotropic.

U¿ytkownik napisa³ w wiadomo¶ci
...
On Sep 3, 10:29 pm, Jeff Liebermann wrote:
On Thu, 3 Sep 2009 19:11:42 -0700 (PDT), wrote:
How in the heck are you going to
get **ANY** vertical radiator to have a truly isotropic pattern?
It's impossible. An isotropic pattern is a theoretical pattern
in which radiation is equal in all directions. Such a pattern
does not exist with real antennas.

A real isotropic radiator may not exist, but one can get fairly close.
If you believe the model, the total error is 0.44 db. See:
http://802.11junk.com/jeffl/antennas/isotropic/index.html
The NEC2 deck is under the photo labeled "main".

I once built one of these antennas on roughly 444MHz out of cardboard
and magnet wire. The oscillator was a small crystal can oscillator
running from a 9V battery to avoid having the feed coax wrecking the
pattern. The impedance was nowhere near 50 ohms and required a bit of
matching to get the VSWR down. I'm now digging for the photos.

I used a piece of string to maintain a constant radius, a tiny pickup
loop at the end of a length of coax cable running inline with the
string, and eventually going to my antique HP spectrum analyzer. On
the 2dB/div scale, it was a fairly good approximation of an isotropic
radiator with errors mostly caused by indoor reflections and
interference with the bench.


Sure, you can get fairly close to isotropic with the right
system, but how are you going to do it by tipping a
vertical? The likely results do not fit my idea of isotropic.

"An isotropic radiator is a theoretical point source of waves which exhibits
the same magnitude or properties when measured in all directions".
The only way to make the real point source is the proper tipping. Of course
it must be a monopole.
S*

"Szczepan Bialek" wrote in message
...

"An isotropic radiator is a theoretical point source of waves which
exhibits the same magnitude or properties when measured in all
directions".
The only way to make the real point source is the proper tipping. Of
course it must be a monopole.

another blabbering idiot chiming in... a monopole can't have the same
properties in all directions since it is linear, no matter how much you tip
it!

On Sep 4, 3:37Â*am, Szczepan BiaÅ‚ek wrote:
U¿ytkownik napisa³ w ...
On Sep 3, 10:29 pm, Jeff Liebermann wrote:



On Thu, 3 Sep 2009 19:11:42 -0700 (PDT), wrote:
How in the heck are you going to
get **ANY** vertical radiator to have a truly isotropic pattern?
It's impossible. An isotropic pattern is a theoretical pattern
in which radiation is equal in all directions. Such a pattern
does not exist with real antennas.

A real isotropic radiator may not exist, but one can get fairly close.
If you believe the model, the total error is 0.44 db. See:
http://802.11junk.com/jeffl/antennas/isotropic/index.html
The NEC2 deck is under the photo labeled "main".

I once built one of these antennas on roughly 444MHz out of cardboard
and magnet wire. The oscillator was a small crystal can oscillator
running from a 9V battery to avoid having the feed coax wrecking the
pattern. The impedance was nowhere near 50 ohms and required a bit of
matching to get the VSWR down. I'm now digging for the photos.

I used a piece of string to maintain a constant radius, a tiny pickup
loop at the end of a length of coax cable running inline with the
string, and eventually going to my antique HP spectrum analyzer. On
the 2dB/div scale, it was a fairly good approximation of an isotropic
radiator with errors mostly caused by indoor reflections and
interference with the bench.

Sure, you can get fairly close to isotropic with the right

system, but how are you going to do it by tipping a
vertical? Â*The likely results do not fit my idea of isotropic.

"An isotropic radiator is a theoretical point source of waves which exhibits
the same magnitude or properties when measured in all directions".
The only way to make the real point source is the proper tipping. Of course
it must be a monopole.
S*

Excellent. I am so happy that somebody out there is not following the
pied pipers of denial.
Thanks for your input.

On Fri, 4 Sep 2009 00:56:39 -0700 (PDT), wrote:

Sure, you can get fairly close to isotropic with the right
system, but how are you going to do it by tipping a
vertical? The likely results do not fit my idea of isotropic.

I forgot to connect my comments to the original question. Sorry(tm).
You're correct. There's no way to get a good isotropic radiator
pattern with a simple vertical radiator. However, you can still get
fairly close if you make the antenna sufficiently small relative to
the operating wavelength. As the physical antenna size approaches a
point radiator, the pattern starts to look rather spherical.
Unfortunately, the gain drops, efficiency drops, and feed point
impedance drops, resulting in a rather inferior antenna.

There's also a question of how close to perfection does the spherical
pattern need to become? Within 0.1dB, 1dB, 3dB, etc???? Offhand, I
would guess anything within a few dB of spherical could be considered
isotropic, as in all the patents I noted.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

"Jeff Liebermann" wrote in message
...
On Fri, 4 Sep 2009 00:56:39 -0700 (PDT), wrote:

Sure, you can get fairly close to isotropic with the right
system, but how are you going to do it by tipping a
vertical? The likely results do not fit my idea of isotropic.

I forgot to connect my comments to the original question. Sorry(tm).
You're correct. There's no way to get a good isotropic radiator
pattern with a simple vertical radiator. However, you can still get
fairly close if you make the antenna sufficiently small relative to
the operating wavelength. As the physical antenna size approaches a
point radiator, the pattern starts to look rather spherical.


That doesn't sound right. The directivity gain of an infinitesimal electric
doublet (i.e. a dipole with infinitesimal length) is about 0.4 dB less than
that of a half-wave dipole. Its similar lemniscate-of-rotation radiation
pattern results from the symmetry about its axis. The results for monopoles
derived from these forms of dipole won't be too different. It is rather
obvious that a receiving dipole of any polarisation won't receive much
signal from the end of a transmitting dipole or monopole, however it's
oriented - there's no apparent length over which unopposed current is
flowing so there's no 'moment' in that direction.

One solution to spherical radiation is the Lindenblad array (and variants
that others have chosen to re-name) which presents finite resolved
components of the lengths of some of its dipoles in all directions ... but
the tilt of the elements has nothing at all to do with Art Unwin's 'theory',
it's simply a matter of making sure there's a resolved component in each
direction. Of course, a Lindenblad designed for a near-omni pattern
achieves this in respect of circular polarisation so it would be
ineffiecient in a system where a linearly polarised antenna is used at the
other end of the link.

I was once told a true isotropic radiator would have to be circularly
polarised because it would be so small that it could contain nothing with a
defined axis of symmetry ... that is, the antenna would have the form of an
infinitesimal sphere. The question then is 'which sense of circular
polarisation' ... which undoubtedly has nothing at all to do with Coriolis
force! The real answer is that it doesn't matter because, as you mentioned
(below) such an antenna has an infinitesimally small radiation resistance
and cannot be made to radiate.

Chris



Unfortunately, the gain drops, efficiency drops, and feed point
impedance drops, resulting in a rather inferior antenna.

There's also a question of how close to perfection does the spherical
pattern need to become? Within 0.1dB, 1dB, 3dB, etc???? Offhand, I
would guess anything within a few dB of spherical could be considered
isotropic, as in all the patents I noted.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Fri, 4 Sep 2009 16:44:48 +0100, "christofire"
wrote:

"Jeff Liebermann" wrote in message
.. .
However, you can still get
fairly close if you make the antenna sufficiently small relative to
the operating wavelength. As the physical antenna size approaches a
point radiator, the pattern starts to look rather spherical.

That doesn't sound right. The directivity gain of an infinitesimal electric
doublet (i.e. a dipole with infinitesimal length) is about 0.4 dB less than
that of a half-wave dipole.

I'll plug a series of shortened dipoles, possibly with loading coils,
into 4NEC2 and see what happens. You may be right.

As I recall, the big holes in the pattern, that are inline with the
elements gets smaller is diameter as the antenna gets electrically
smaller. The rounded circular donut pattern tends to flatten. I
wanna play with the models to be sure. This still begs the question
of how close to spherical does the pattern need to be in order to call
it isotropic? Dunno.

I was once told a true isotropic radiator would have to be circularly
polarised

"Near isotropic circularly polarized antenna"
http://www.google.com/patents?id=saMgAAAAEBAJ
CP satellite antenna used on Intelsat V. I've been looking at the
patent for a while trying to figure out how it works.

Yeah, it should be CP because that would correctly fit the definition
of the field being identical along the sphere, in all possible
measurement antenna orientations. Note that the isotropic simulator I
posted is *NOT* circularly polarized. If you plug the deck into 4NEC2
and instead of looking at the total gain in the 3D window, look at the
vertical and horizontal gains individually, you'll see something
really ummm.... interesting.
http://802.11junk.com/jeffl/antennas/isotropic/isotrop2-vert.jpg
http://802.11junk.com/jeffl/antennas/isotropic/isotrop2-horiz.jpg
Needless to say, that the polarization is not even close to being
uniform over the sphere. (I'll add these to the menu as soon as I can
figure out what the latest JAlbum update broke in my photo
collection).

Drivel: Just got 4NEC2 setup on my new computah (Dell Optiplex 755
E8500 with 4GB). A messy tower and antenna simulation, that took over
an hour on my old PIII/1GHz clunker, now takes about 4 minutes. I'm
happy (for now).




--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

In article ,
Jeff Liebermann wrote:

I forgot to connect my comments to the original question. Sorry(tm).
You're correct. There's no way to get a good isotropic radiator
pattern with a simple vertical radiator. However, you can still get
fairly close if you make the antenna sufficiently small relative to
the operating wavelength. As the physical antenna size approaches a
point radiator, the pattern starts to look rather spherical.

The difference in pattern between a half-wavelength dipole,
and an infinitesimally-short dipole (i.e. one whose length
approaches a point source) is actually quite small. Both are
torus-shaped patterns, with a deep null along the axis of the antenna
(theoretically, the null is infinitely deep directly along the axis).

An infinitesimally-short dipole has a maximum gain of 1.76 dBi.

A half-wavelength dipole has a gain of 2.15 dBi.

There really isn't much to distinguish the two, as far as the pattern
and gain go.

Unfortunately, the gain drops, efficiency drops, and feed point
impedance drops, resulting in a rather inferior antenna.

Yeah, the low radiation resistance and high reactance of the short
dipole are its biggest drawbacks.

--
Dave Platt AE6EO
Friends of 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!

"Dave Platt" wrote
...
In article ,
Jeff Liebermann wrote:

I forgot to connect my comments to the original question. Sorry(tm).
You're correct. There's no way to get a good isotropic radiator
pattern with a simple vertical radiator. However, you can still get
fairly close if you make the antenna sufficiently small relative to
the operating wavelength. As the physical antenna size approaches a
point radiator, the pattern starts to look rather spherical.

The difference in pattern between a half-wavelength dipole,
and an infinitesimally-short dipole (i.e. one whose length
approaches a point source) is actually quite small.

A dipole is always the two monopoles and never a point source. Only monopole
is a point source.
S*

"Szczepan Bia³ek" wrote in message
...

"Dave Platt" wrote
...
In article ,
Jeff Liebermann wrote:

I forgot to connect my comments to the original question. Sorry(tm).
You're correct. There's no way to get a good isotropic radiator
pattern with a simple vertical radiator. However, you can still get
fairly close if you make the antenna sufficiently small relative to
the operating wavelength. As the physical antenna size approaches a
point radiator, the pattern starts to look rather spherical.

The difference in pattern between a half-wavelength dipole,
and an infinitesimally-short dipole (i.e. one whose length
approaches a point source) is actually quite small.

A dipole is always the two monopoles and never a point source. Only
monopole is a point source.
S*

Nonsense. Only a point can be a point source.

The principle of the infinitesimal electric doublet is the hypothetical
result of making the lengths of the elements of a balanced dipole
vanishingly small. This is useful to quantify the characteristics of the
limiting case but, because of its inherent axial symmetry, it still has the
form of a dipole and the same kind of radiation pattern with linear
polarisation and no radiation in the directions aligned with the ends of the
dipole (for the reason I gave earlier in this thread).

Monopole antennas are developed from dipoles by substituting one of the
elements, often using a 'reflection' of the remaining element in a ground
plane. Their characteristics are different from those of the parent dipole
because of this substitution but they still have the same kind of
axially-symmetric radiation pattern, with linear polarisation and no
radiation in the direction of the end of the monopole.

A point source is a hypothetical 'device' that radiates equally in all
directions. Obviously this could not be realised using a monopole because
that would provide the wrong radiation pattern.

A polarisation can be assigned to a point source, for the sake of comparison
with real antennas (which is how the point source is used), just as a point
source can be considered as transmitting or receiving a signal - but that
doesn't mean a physical antenna can be made that has the same
characteristics, that can be made to transmit or receive.

Chris