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Re: Metal vs. Fiberglass Boom



I have always used a metal cross boom on my KLM antennas, even predating
Kent's confirming tests.  My roof top tripod is too close to the roof to
have dragging coax runs snagging roof shingles, getting buried in the
winter snow and frozen to the roof.

So, I mounted the antennas in the X configuration on the metal boom and ran
the coax out the boom and down the antenna booms to the coax connectors on
the antennas.  It works great, no SWR that I can measure and it doesn't
seem to affect the antenna pattern such that I can detect it.

Later when Kent presented his article in Arlington, Texas at the Amsat
Symposium, I he discussed two more requirements for making this arrangement
work.  Here is the sum total of his findings:

1.  Mount the antennas such as to keep the tips of the elements as far away
from the cross boom as possible.  Thus the X configuration.

2.  Don't let the cross boom protrude out past the antenna booms any more
than you need to fasten the antenna to the boom.

3.  Position the antenna boom such that the cross boom is not a multiple of
a halfwave length from the driven elements.

After Kent's presentation, I came home and discovered I had lucked out in
points 2 & 3, having done things that way already.  That's it.  You can run
the coax out the crossboom, but keep it close to the booms at the point
where the coax turns from the crossboom to the antenna boom.  Be careful of
the minimum turning radius specified for your coax.  My 2 meter KLM had a
mounting arrangement such that it was easy to just rotate the antenna on
its boom axis to the X position.  However the 70cm KLM had a mounting
arrangement using two U bolts passing through holes in the antenna boom.  I
just made a square metal plate and used 4 U bolts to match the 2 meter
arrangement and easily mounted the 70cm antenna in the X configuration.

I have used this arrangement since before 1985, out of necessity.  Does it
work?  You bet.  I reached Level 8 on the ZRO certificate for hearing weak
signals.  I don't have to worry with ice forming on the "trailing" coax or
coax snagging on the roof.  Additionally you can balance the load on the EL
rotor and have it stay constant throughout rotation instead of having a
constantly changing load as the EL rotor moves.

I think you would be surprised to find out how many and who are using metal
cross booms successfully for many years.


hasan schiers wrote:
> 
> 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
> > ----
> > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> 
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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-- 
73, Roy -- W0SL

E-Mail: rdwelch@swbell.net
Home Page: http://home.swbell.net/rdwelch
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