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R: Metal vs. Fiberglass Boom



Hi Joe K0VTY,

A crossed yagi antenna connected for circular polarization 
has two components at 90 deg out of phase wich resultant is
a vector that rotates in the space 2 x 3,14 x f  every second
or 360 degrees x frequency in Hz  every second.

Now if you have  the 2 meter antenna in + configuration and
the 70 cm antenna in X configuration or both in X configuration
or both in + cofiguration the isolation between the antennas is
the same because the above 2 meters and 70 cm vectors rotates.
like a two bladed propeller.

The same is true if you use a metal boom,no matter if you put the 
elements in X configuration or + configuration because in any
case the EM rotating vector of circular polarizazion sees the metallic
boom or better is obscurated by it,or cross it one time for every
cycle of the exiciting frequency.

To demostrate that X is better than + configuration is necessary
to demostrate first for what physical law the resultant EM rotating 
vector on X configuration should not see the metallic boom crossing
 it every cycle of the exciting frequency.

73" de i8CVS Domenico  
 


----- Original Message ----- 
From: joseph Murray <k0vty@juno.com>
To: <schiers@netins.net>; <amsat-bb@AMSAT.Org>
Sent: Wednesday, June 26, 2002 2:32 PM
Subject: Re: [amsat-bb] Metal vs. Fiberglass Boom


> Hi All
> 
> I'll add one more tidbit on cross booms that I had to learn.
> My desire to use mode J ( 145 up and 435 down ) and the receiver de sense
> problem had me scratching my head.     Antenna isolation can improved the
> de sense problem.   Aligning the 2 meter antenna elements in a X
> configuration and the 70 cm elements in a + increases the isolation .  
> Unless you are using the non metal cross boom this is difficult to do
> when using circularly polarized antennas.
> 
> Joe  K0VTY
> ========================
> On Wed, 26 Jun 2002 06:33:17 -0500 "hasan schiers" <schiers@netins.net>
> writes:
> > It's not quite as simple as portrayed.
> > 
> > You CAN use a metal boom, but the manner of mounting and placement 
> > of the
> > antennas is somewhat critical. There was a raging discussion about 
> > this very
> > topic with empirical data presented some time ago on the sig. Search 
> > the
> > archives.
> > 
> > To simply say " the purests say" or reduce it to the following:
> > 
> > >Mount crossed yagis so
> > > that they are not in the same plane as the cross-boom; X style 
> > rather than
> > > + style
> > 
> > is not a faithful characterizations of the care that needs to be 
> > taken when
> > using a metal boom with our satellite antennas. One really needs to 
> > read the
> > article and follow its directions carefully. One person who has is 
> > Roy
> > Welch, who frequents the mailing list, perhaps Roy can point to the
> > downloadable source.
> > 
> > In most cases (purists or not), if care is not taken about placement 
> > and not
> > just X versus + mounting, the circularity will be degraded. (not to 
> > mention
> > gain and f/b) The subject is much more complicated than Richard 
> > appears to
> > represent and I would urge that anyone contemplating using a 
> > conductive
> > boom, read the empirical work done by a satellite enthusiast with 
> > decent
> > test equipment and follow closely his recommendations.
> > 
> > Will it work "well" with a metal boom? Yes, if you follow the 
> > article. Would
> > I recommend it? No. Not if you can find fiberglas easily (which I 
> > had no
> > trouble doing).
> > 
> > 73
> > hasan schiers, N0AN
> > 
> > schiers@netins.net
> > or use amsat.org or arrl.net remailer
> > ----- Original Message -----
> > From: "Richard W L Limebear" <Richard@g3rwl.demon.co.uk>
> > To: <amsat-bb@AMSAT.Org>
> > Sent: Wednesday, June 26, 2002 1:50 AM
> > Subject: Re: [amsat-bb] Metal vs. Fiberglass Boom
> > 
> > 
> > > "Tom & Judie Webb" writes:
> > >
> > > > but is a nonconductive
> > > > (fiberglass or PVC) REALLY necessary verses a metalic crossboom?
> > >
> > > NO.
> > >
> > > Purists say it will upset the antenna pattern. Sure, it will, to 
> > some
> > extent,
> > > but there is one measure you can take to lessen this. Mount 
> > crossed yagis
> > so
> > > that they are not in the same plane as the cross-boom; X style 
> > rather than
> > > + style. My yagis are all mounted in the centre of their booms 
> > too, rather
> > > than all forward of the cross-boom and getting balance problems.
> > >
> > > My system does this; it works ok. Even heard one of the Mars 
> > probes out
> > > beyond the moon with it. I might lose a dB or so but there are 
> > other
> > > things we can't change too (foliage, neighbors with shotguns, 
> > etc).
> > >
> > > This is *amateur* radio; do the best you can and you'll be 
> > surprised at
> > how
> > > well stuff works usually.
> > >
> > > 73
> > > Richard W L Limebear G3RWL
> > > g3rwl@amsat.org
> > > FOC # 1188
> > >
> > >           So many beautiful girls ..... (sob) so little time
> > > ----
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> > > To unsubscribe, send "unsubscribe amsat-bb" to 
> > Majordomo@amsat.org
> > 
> > 
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> > 
> 
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