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Re: Re: Re: Cross boom length?, (Metal Cross Boom)



William, Joe, etal:

Kent's experiments and measurements of mounting of crossed yagis to a metal
boom are very interesting to read.  Of course this is a compromise and if
you want "perfection" with a x-yagi whether that be used in circular pol or
each linear pol, independently (the norm for 2m and 70cm eme) then use of
non-metallic cross-booms are required.

I have my 2m and 70cm satellite antennas mounted in the "X" configuration
on opposite ends of my aluminum elevation boom, and they seem not
particularly bothered (beam is normally oriented and impedance not
affected).  But for 2m eme my four x-yagis are mounted on an "H-frame" with
4-foot fiberglass ends so that nothing distrubes the near-field of the
array.  

My satellite coax cables are dressed along the boom and the cross-boom for
the satellite antennas.  The eme cables are dressed back of the antenna
reflectors at right angles to the boom for 6-feet and then to the center of
the array on a 45-degree angle (attached to 1-inch aluminum tubing for
support).

For satellite operation I have designed for arm-chair SSB operation with
plenty of signal over the noise level (typically +10 to +30dB).  On eme I
need to operate at times with signals buried more than below the
noise...then every 0.1 dB counts!  The weakest eme signal I have copied was
running -29dB (and required software coding/decoding to detect).

I guess the message I would give is that for satellite operation losing a
little signal gain is OK (if you are careful in your construction to follow
Kent's directions).  But if you are not using adequate antennas, preamps,
low-loss cabling that achieve a comfortably good signal then maybe one
needs to not take the shortcuts :-)

73's Ed



At 06:56 AM 3/20/2006 -0800, William Leijenaar wrote:
>Hi Joe and other AMSATs,
>
>You are right for linear polarisation. It can also be
>seen in the measurement results.
>
>For circular it is some different. Even when you use 2
>yagis, each 90 degrees ofset from each other, the RF
>radiation is still in all directions.
>
>Even when mounting the x-yagi in a 45 degree angle to
>the elevation boom, the horizontal angle will be
>reflected by the metal boom, and finally make the
>radiation patern elliptical. 
>
>From the measurements you can see that the yagi 90
>degrees to the mast does best. The 45 degrees is half
>worse, and the yagi parallel with the boom is worst
>off all. For both systems, in matter off angle to
>mount the x-yagi on the metal elevation boom, you
>would get the same result.
>
>The idea of mounting the x-yagi on the tip of the
>metal mast might be a solution, but I never tested
>this.
>
>Just my idea,
>73 de William
>---
>
>
>Ooo!  One of my favorite topics.
>
>I do not mean to dis-respect William, who is one cool
>dude!
>
>When it is actually measured it turns out metal is
>just fine if you do it 
>right.  Metal is cheaper and stronger as well.  Check
>out what happens when 
>someone actually measured the results at:
>
>http://www.g6lvb.com/fibermetalboom.htm
>
>Antenna guru Kent Britain, WA5VJB has done the leg
>work for us on this.
>
>73,
>Joe
>kk0sd
>Tired of spam?  Yahoo! Mail has the best spam protection around 
>http://mail.yahoo.com 
>----
>Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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> 
73's,
Ed - KL7UW  
=========================================
http://www.qsl.net/al7eb - BP40iq 
144-EME: FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
432-EME: FT-847, mgf-1402, 1x21-ele (18.6 dBi), 60w
=========================================
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