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Re: Metal vs. Fiberglass Boom



I have to concur with Roy.  My M2 436CP42 is mounted in just this way and
works very well...of course if I have lost 1 or 2 dB from the 16.5 dBd
forward gain, who cares on AO-40 (I know Mike Stahl of M2 is cringing after
all his effort to design this antenna for good performance)!  I haven't
even bothered with checking the feed distance to the cross boom (per
wa5vjb's advise).  I did have to drill new holes in the antenna boom to
mount it in the "x" configuration.  Regarding non-circularity, many AO-40
operators are using simple linear yagis...so that really doesn't matter
that much, either.  If you can get around 500-1000w EiRP (25w x 13 dBi)
then the satellite will hear you in mode -US!

Concerns for F/B ratio have more bearing if you are using this as a
receiving antenna (i.e. mode -J)(however it works "jim-dandy" for me
hearing AO-27 & UO-14).  As an uplink antenna (Tx -only) it doesn't matter
much as long as you have sufficient forward gain.  With all the problems
attendant to using fiberglass: extra weight, lack of stiffness, crushing by
the antenna clamp, dressing the coax off the rear, UV deterioration...it
just isn't worth the bother in my opinion (sorry Hasan).

Now if you are mounting an array of x-yagis for moon-bounce where receiving
performance (best G/T ratio) is paramount...then you must use
non-conducting booms.  I do on my four M2 xpol-20 yagis for 2m-eme.

A final comment:  Kent knows his stuff so you can trust his analysis which
was presented at Dayton (based on actual range measurements)...after all
Kent is the VHF Antenna column writer for CQ-VHF, and conducts the
microwave antenna measurements at CSVHF...what else?  He was one end of the
"very first" 10 GHz eme contact! ...and he built my 0.15 dB NF* 2m-eme
preamp!  (*placed 2nd in NF measurements at 1999 CSVHF
conference...definitely one talented guy!

Ed, AL7EB

>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.
 



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