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Re: 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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> > >
> > >
> > > ----
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> > ----
> > 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.
> Not an AMSAT member? Join now to support the amateur satellite program!
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----
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