[amsat-bb] Re: Antennas?

Bruce Robertson broberts at mta.ca
Sun Mar 25 18:38:37 PST 2007

Quoting Stargate <stargatesg1 at verizon.net>:

> Howdy all,
> 	Still building up my station but cash is starting to get tight!
> I was considering the M2 436CP42 U/G and M2 2MCP22 antenna's.
> I think I'm down to the build-it, instead of buy it stage, so I was
> wondering if anyone has links
> to websites with PROVEN designs for a CP beam such as these?
> 	I have seen a few sites but I hate to build a CP antenna without prior
> recommendations from someone else.
> So, What have you? :-)
> Station so far...
> FT-847 (computer controlled)
> G-5500 rotor (computer controlled)
> LVB tracking board
> Orbitron tracking software
> ARR MSP432VDG-160 preamp
> ARR MSP144VDG-160 preamp


I know this doesn't answer your question, but I'm going to suggest that you
consider not building CP antennas at this juncture. 

I take it this is a beginning station aiming to acquire reliable links with
LEO satellites down to the horizon. If that's the case, given your existing
investment in the G-5500, I think you can get away with building
medium-length linear yagis for each band and calling it a day. As a recent
thread indicated, short linearly-polarized yagis on a az-only rotor have
done very nicely for many of us. Because of your elevation rotor, you can
afford the more narrow beamwidth of longer yagis and enjoy the improved
gain. At that point, you simply don't need the additional 3 dB of gain on
the downlink, and on the uplink you might come close to transmitting too
powerful a signal, even at the lowest setting of your rig.

In any case, this has been my reasoning in upgrading to a 7 element 2m
horizontal to match my 8 element 70cm vertical on a new az/el setup. I
believe that the complexity of CP switching and phase-line building is
sufficient that without great care I'm likely to end up with something no
better than the linearly polarized model. By the launch of P3E, I do hope
to have a longish (8 el?) CP yagi (this is why I'm going to the bother of
the az/el system and the longer antenna), but I want to carefully record
the performance of the current 7 element model before adding the second set
of elements and the phasing system.

I'd throw together a 5-6 el. 2m yagi and a 8 el. 70cm and enjoy your
station ASAP.

73, Bruce

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