[amsat-bb] Re: Phase 4 versus Eagle

[Tom Clark, K3IO]

Ed, KL7UW said:
> Have to agree with Drew even if this reduces signal levels at extreme 
> latitudes.  I would assume that the P4 centered on the NA&SA would 
> boresight the equator with a beam chosen to match the earth's 
> apparent angular size = arcsin (8000/23500) = 19.9 deg. which 
> corresponds with an antenna with 19.5 dB.  At 2.4 GHz an 18-inch dish 
> has about 19.7 dB gain.  Such a footprint would only be 3 dB down at the poles.
Ed -- I think you calculated for the gain over a dipole and not dBi. The
Isotropic would be ~2 dB higher (22.4 dBi for a beam that is about 3.5
dB down at the edge of the earth). I derived the number in my paper on
phased arrays presented at the last (Pittsburgh) AMSAT Space Symposium;
see pages 1&2 and Table 1 in
http://mysite.verizon.net/w3iwi/electronic_scanning_antennas.pdf.  In my
EAGLE paper, I made the calculation for the altitude of a GEO satellite,
since that approximates the altitude we would get with EAGLE with a
rocket boost from GTO.

Also, strictly speaking, your number is the directivity. Directivity
refers to the beam size without considering any ohmic or scattering
losses in a real antenna. The real "gain" of an antenna is always poorer
than the directivity by a factor of typically 50% or so.

To divert any incoming questions: Yes, my symposium paper describes my
thoughts on antennas for EAGLE in an elliptical HEO orbit, where we have
to expend some effort on pointing the antenna from a spinning
spacecraft. My phased array ideas are based on the notion that
    (a) we will want to point at the earth even when we are away from
apogee, and
    (b) trying to mechanically point an antenna (or the spacecraft
itself) is wayyyyy! beyond the limits of amateur technology.

Item (b) points to one of the big advantages of the piggyback-to-GEO P4
possibility -- the "parent" bus will provide a platform, stabilized to a
degree or better, so we can use "simple" antennas (probably dishes) for
the microwave frequencies [saving me the hassle of designing a "real
world" electronically steered phased array].

To get a feeling for the "antenna farm" we might have for P4, Google
yielded a few interesting photos:

   1. http://space.skyrocket.de/index_frame.htm?http://space.skyrocket.de/doc_sdat/intelsat-14.htm
      shows the DoD IRIS payload riding in the space we might occupy
   2. http://www.cnes.fr/web/print-6086-intelsat-11-optus-d2.php shows
      an Intelsat satellite being mated to an Ariane 5
   3. Intelsat 10, launched on 2004:
      http://www.astrium.eads.net/families/daily-life-benefits/communications/intelsat-10-02
   4. The 2005 Sealaunch of IA-8:
      http://www.dailywireless.org/2005/06/23/intelsat-spotbeam-launched
      <http://www.dailywireless.org/2005/06/23/intelsat-spotbeam-launched/>

73 & Season's Greetings de Tom, K3IO