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R: Cross Boom (Long)



Joe,

Look please at a circularly polarized microwave feed horn for EME
type W2IMU or VE4MA

They are made using a lenght of circular waveguide wich is closed at
one end and is open to the other end.

Two probes mounted at 90 degrees one to the other are fitted in the
same plane near the close end of  waveguide.

Each probe is supplied with the same amount of power and with the
same phase.

To produce the necessary delay of 90 degrees in one of the linearly
polarized wave travelling inside the guide a set of metallic screw
is diametrically fitted at 45 degrees in front of  probes to produce CP in
the far field at the open end of  the guide thereby demonstrating that metal
posts are producing an effect to the electric and magnetic field inside the
tube where the electromagnetic wave is travelling.

Metal of the above posts is not a dissipating material and since the energy
cannot be destroyed it is reflected producing the above effect.

Going back to yagi the probes replaces dipoles at X position and posts are
repacing the metallic boom in some extent obviously.

Even dielectric material produces similar effects if immersed in to electro
magnetic field and infact teflon plates are fitted at 45 degrees to the
probes in to a waveguide feed horn to act as polarizer and produce the
necessary 90 degrees delay in one of the linearly polarized wave to get CP
in the far field outside the tube.

73" de i8CVS Domenico

----- Original Message -----
From: Gary "Joe" Mayfield <gary_mayfield@hotmail.com>
To: i8cvs <domenico.i8cvs@tin.it>
Cc: AMSAT-BB <amsat-bb@AMSAT.Org>
Sent: Saturday, March 08, 2003 3:22 AM
Subject: Re: [amsat-bb] Cross Boom (Long)


> Dom,
>
>      Thanks for replying again.  It is nice that we can discuss this
without
> flaming attacks!  There is a problem with your model.  It was touched on
> before.  Your model is a wonderful far field example.  It is not a good
near
> field example.  Let me guide you through a couple of thought processes.
>
>       1. Suppose I have 5 element by 5 element circular yagi.  To make
this
> easier to picture let say the elements are on the same boom separated by a
> quarter wavelength to make circular polarization.  I think we will both
> agree in the far field this works pretty well, and is how most of us get
> circular polarization (we do it electrically if not physically).
>
>      Now let's say I measure the field in front of one of the reflectors,
> but behind the other.  Will the polarization pattern be circular?  Now
let's
> move our probe forward, and measure between the driven elements (in front
of
> one behind the other).  Will the pattern be circular?  Let's move our
probe
> forward again so we have the driven element and a reflector behind the
probe
> in one plane (2 elements) and a driven element, reflector and one director
> behind the probe in another plane (3 elements).  I think we would all
agree
> that it is tough to make a circular antenna with 2 elements in one plane
and
> three in another.
>
>      We are not destroying the circularity of the pattern, because it has
> not yet been completely established.  Your high speed rotating dipole is
> circular immediately, where as the yagi is not.  This model is closer (I
> know it is not perfectly correct) to reality than the rotating dipole.
>
>      2. Another way to look at it.  Suppose I have two separate vertically
> polarized yagi antennas.  It is generally accepted that I can mount them
> together with a steel cross peace and they will work fine (many antennas
are
> 'stacked' this way).
>
>      Now let's say I go goofy and turn each antenna to 0 degrees so the
> support and the elements are in the same plane.  I think we will all agree
> the antenna performance will suffer at this point.  So, we know 90 degrees
> works and 0 degrees doesn't.
>
>      What happens at 85 degrees?  I will argue it will be very hard to
tell
> 85 degrees from 90 degrees.  Where do things go bad?  The best way to find
> out is to experiment.  The experiment has been done.  The results say I
can
> mount the antennas at 45 degrees without problems as long as I avoid the
> very sharp nulls at every 1/2 wavelength (from the driven element) along
the
> boom.
>
>     Can I mount the antennas one at 45 degrees and the other at -45
degrees
> along a crossboom (conductive or nonconductive) and produce circular
> polarization in the far field?  Yes, if I feed them correctly they can
> produce circular polarization in the far field, but not the near field!
> Imagine measuring the polarization directly in front of one of the
antennas,
> would it be circular?  If I shrink the length of my crossboom to zero the
> pattern will be circular as well.
>
>     The pattern will not be circular when the cross piece (conductive or
> nonconductive) is short allowing for coupling between the ends of the
> elements.
>
>     Hopefully, I have made it more clear.  All the actual data I have seen
> agrees with me.  Mounting them in an X, avoiding the small areas near 1/2
> wavelength works VERY well.  Mounting them as a + does not.
>
> 73,
> Joe
> ka0yos@amsat.org
>
>
> ----- Original Message -----
> From: "i8cvs" <domenico.i8cvs@tin.it>
> To: "kgshutt" <kgshutt@cox.net>; "Roy Welch" <rdwelch@swbell.net>; "hasan
> schiers" <schiers@netins.net>
> Cc: "AMSAT-BB" <amsat-bb@AMSAT.Org>
> Sent: Friday, March 07, 2003 6:09 PM
> Subject: R: [amsat-bb] Cross Boom
>
>
> > Hi Ken,
> >
> > I aegree with you.
> >
> > Suppose to have an ideal AC sincronous motor in condition to be supplyed
> > with AC at a frequency of 144 MHz and its shaft to rotate 144 million
> turns
> > for every second.
> >
> > Now ideally mount mechanically a dipole for 144 MHz  at the end of the
> shaft
> > and connect the dipole electrically in parallel to the motor and both to
a
> > 144 MHz generator.
> >
> > The dipole starts to rotate sincronously with the shaft of the motor 144
> > million turns for every second radiating with circular polarization.
> >
> > Now the dipole can be seen as a solid propeller intercepting every thing
> is
> > introduced in it and into the radiated circularly polarized field.
> >
> > In real life the above example is replaced by two crossed dipoles feed
> > with the same amount of power but 90 degrees out of phase so that
> > the resultant vector rotates 2 x pi x f  for every second producing CP
> > like the above propeller in the ideal example.
> >
> > I cannot realize any condition to prevent that the rotating field will
not
> > see a conductive material introduced in whatever position X or + or
> > any other place along the direction of CP radiation.
> >
> > If the field experimentation demostrates that using a metallic boom X
> > is best than + there are for sure good reasons because between two bad
> > conditions one is the best.
> >
> > Only ghost are able to cross metal  and the electromagnetic waves are
> > reflected.
> >
> > 73" de i8CVS Domenico
> >
> >
> > ----- Original Message -----
> > From: kgshutt <kgshutt@cox.net>
> > To: i8cvs <domenico.i8cvs@tin.it>; Roy Welch <rdwelch@swbell.net>; hasan
> > schiers <schiers@netins.net>
> > Cc: AMSAT-BB <amsat-bb@AMSAT.Org>
> > Sent: Friday, March 07, 2003 7:35 PM
> > Subject: Re: [amsat-bb] Cross Boom
> >
> >
> > > Hello Everybody,
> > >
> > > Dom,  your comment is spot on.  I don't see how anyone can expect to
> > insert
> > > a conductive boom into the rotating electro magnetic field and expect
it
> > not
> > > to have any effect on the radiated wave, regardless of x or + element
> > > orientation.
> > >
> > > 73 de Ken - K5GUU
> > >
> > > ----- Original Message -----
> > > From: "i8cvs" <domenico.i8cvs@tin.it>
> > > To: "Roy Welch" <rdwelch@swbell.net>; "hasan schiers"
> <schiers@netins.net>
> > > Cc: "AMSAT-BB" <amsat-bb@AMSAT.Org>
> > > Sent: Friday, March 07, 2003 12:19 AM
> > > Subject: R: [amsat-bb] Cross Boom
> > >
> > >
> > > > Hi All,
> > > >
> > > > Mounting a crossed yagi for CP in X or + configuration over a
metallic
> > > > boom produces the same effect over the circularity.
> > > >
> > > > When two crossed dipoles are feed by the same amount of power but
> their
> > > > components are 90 degrees out of phase the resultant vectors of
> voltage
> > > > and current rotates by  2 x 360 x f  every second producing CP
> > > >
> > > > The rotor of a AC sincronous motor rotates by the same phisical
> > principle
> > > > of magnetic rotating field discovered by Galileo Ferraris.
> > > >
> > > > Try now to put your finger in to the squarrel cage of the motor in
> both
> > > > X or + position and see what happen to it !
> > > >
> > > > Your FINGER or your METALLIC boom are the same thing.
> > > >
> > > > 73" de i8CVS Domenico
> > > >
> > > > ----- Original Message -----
> > > > From: Roy Welch <rdwelch@swbell.net>
> > > > To: hasan schiers <schiers@netins.net>
> > > > Cc: <amsat-bb@AMSAT.Org>
> > > > Sent: Thursday, March 06, 2003 9:54 PM
> > > > Subject: Re: [amsat-bb] Cross Boom
> > > >
> > > >
> > > > >
> > > > > hasan schiers wrote:
> > > > > > I participated in the original discussion of this many months
ago
> > and
> > > I
> > > > > > think that the article referred to is being "under quoted". It
is
> > NOT
> > > > just
> > > > > > mounting the antenna in the "X" configuration. That is NOT what
> the
> > > > article
> > > > > > says. It also REQUIRES careful placement of the driven element
> > > distance
> > > > to
> > > > > > the cross boom. Doing the "X" alignment is a "necessary" but not
> > > > > > "sufficient" condition to get the results that the article
> claimed.
> > > > > >
> > > > > > Two conditions have to be met:
> > > > > >
> > > > > > 1. X configuration
> > > > >
> > > > > The article does say this.  Mounting the elements in a +
> configuration
> > > can
> > > > > "see as much as a 10db loss in gain and the circularity would be
> nil."
> > > > >
> > > > > > 2. Distance from the feed to the cross boom
> > > > >
> > > > > The article does say to avoid mounting the boom 1/2 or 1 full
> > wavelength
> > > > > from the driven element.
> > > > >
> > > > > As a third requirement the article says to "saw off any excess
cross
> > > > boom."
> > > > >
> > > > > I think I said all three of these in my original reply.
> > > > >
> > > > > > Many people have testified anecdotally to the metal cross boom
> > > working.
> > > > The
> > > > > > work of the WA5 in the article made careful measurements. I
> wouldn't
> > > > > > hesitate to try it, but follow the instructions in the article
> > > > assiduously.
> > > > > > The half-baked representations I have seen discussed lately are
> > > unlikely
> > > > to
> > > > > > produce the "tenths of dB" performance hit described by the
> original
> > > > author.
> > > > >
> > > > > The author's test was done about 1993. I have used this
> configuration
> > > > since
> > > > > 1981 or 1982.  It has worked fine here and I have had reports of
it
> > > > working
> > > > > fine for others.
> > > > >
> > > > > > My comments aren't to discourage the metal cross boom approach.
> The
> > > > article
> > > > > > convinced me. However, let's not forget the second requirement.
> > > > >
> > > > > I didn't omit the second requirement.
> > > > >
> > > > > > ...hasan, N0AN
> > > > >
> > > > > ----
> > > > > Sent via amsat-bb@amsat.org. Opinions expressed are those of the
> > author.
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> > > >
> > > >
> > > > ----
> > > > Sent via amsat-bb@amsat.org. Opinions expressed are those of the
> author.
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> > > ----
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> >
> >
> >
> >
> >
> > ----
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> >
> ----
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