[amsat-bb] Re: CP Antenna questions

[i8cvs]

----- Original Message -----
From: "Greg Dolkas" <ko6th.greg at gmail.com>
To: "AMSAT BB" <amsat-bb at amsat.org>
Sent: Monday, September 24, 2012 1:18 AM
Subject: [amsat-bb] CP Antenna questions

> Hi folks,
>
> I've just obtained a nice, old, but disassembled, KLM 144-150-16C 2m
> circular antenna.  I think I have all the pieces (yea!).  That, plus a
> tree trimmer, and I should be in great shape for an upgrade.
> The old antenna is a linear (vertical) 8 element beam of unknown pedigree.
> It's been working
> fine, but I couldn't pass this one up.
>
> Three questions:
>
> 1.  The antenna does not have a polarity switch.  The instructions are for
> RHCP ("as used by Oscar-7" to give you an idea of it's age!).  Given that
> I don't (yet) have a polarity switcher, is that still the best
> polarization?

Hi Greg, HO6TH

Switching from RHCP to LHCP is the best particularly with LEO satellites.
RHCP was nice with OSCAT-10 and OSCAR-13 because they were spin
stabilized over the Z axis wich is not on the actual LEO sats.

>
> 2.  I have a 2m preamp that I have mounted some 10' away from my current
> antenna, on the mast; I'd like to put it closer.  Is it ok to mount the
> preamp on the antenna itself, between the elements?  I'm guessing not.
> How about on the cross-arm, a couple of feet from the antenna boom?

10' is only about 3 meters long and if you use a good coax cable with
low losses at 144 -150 MHz the attenuation will be fraction of a dB
and the Noise Figure of the preamplifier will be not deteriorated
or increase significantly.

>
> 3.  The instructions recommend routing the cable out the back of the beam,
> and looping it over to the support mast.  But that won't work well as it's
> rotated in elevation (it will get caught on the stuff below).  My plan is
> to run the cable forward and then sideways out at the mount point in the
> middle.  Will this have a significant effect on the polarization?  The
> cross-arm is a wood closet pole, not metal, but I'm thinking that the
> antenna should be mounted in the X orientation instead of +, to lessen the
> effect of the cable shield on the pattern.  Good idea, or not?

For the above question read please the following document:

http://www.g6lvb.com/fibermetalboom.htm

>
> Thanks,
>
> Greg  KO6TH

73" de i8CVS Domenico