[amsat-bb] Re: S-band continued.

[John B. Stephensen]

The preference for U/V is my recollection of what published in the AMSAT
journal a couple of years ago.

In the San Diego meeting, there was discussion about using the V, U, L, S, C
and X bands. The spacecaft is too small for a decent HF antenna and 24 and
47 GHz ground station equipment is too expensive. We also had a goal that
Eagle be usable over 75% of its orbit with a minimal size ground station.
This requires that Eagle have antennas that can cover approximately 90
degrees if the ground station is at the subsatellite point or approximately
120 degrees for ground stations at the Earth's limb. It can be done with
simple antennas consisting of 2 or 3 dipoles having 3-5 dBic of gain at V
and U band. At higher frequencies, the path loss is much higher so you must
go to an electrically steerable array with just enough gain to cover the
Earth at apogee (appx. 17 dBic). Gain must be as high as possible in order
to make RF power requirements at the satellite low enough for AMSAT to
afford.

The minimal size ground station antennas were defined as small
commercially-available CP yagis at V and U band and a 2-foot diameter dish
at higher frequencies.  It was clear from published measurements and the
experience of AMSAT members that S-band noise levels are increasing
dramaticly at many locations and each 3 dB increase in noise floor doubles
the power required at the satellite. The NTIA recently did a study of noise
levels near the 136-138 MHz satellite band, found that they have decreased
significantly, and this conforms the experience of hams on 2 meters. This
tips the balance towards U/V for the main transponder serving class 0
(analog) and class 1 (low-speed digital) users. It was also the only
combination that would allow the use of fixed antennas for class 1 users.

If the satellites are to be usable for emergency communications in the
future, they must also support class 2 (medium-speed digital) and class 3
(high-speed digital) users. This also allows amateurs to experiment wth new
modes such as digital voice and streaming video. Microwaves are required
here because sufficient spectrum isn't avaialable below 450 MHz. Since the
satellite is small, only one pair of steerable microwave antennas will fit.
Since the satellite is very expensive, the two frequecies must be the best
(and safest) possible choice for the 2010-2030 time frame. The uplink band
must be one where RF power can be generated inexpensively on the ground. L
was preferred, but is a probem as the EU is putting GPS in that band so S
and C were the possible uplinks because of the ablity to use WiMax power
amplier ICs. The downlink band must have as little noise as possible so that
the cost of solar panels on the satellite is minimized. S has the same noise
problem for digital or anaog signals, so C and X awere the possible
downlinks. Luckly, WiFi is below the 5830-5850 MHz C-band downlink
allocation. S/C leads to less expensive ground stations than C/X as less
gain is required so antenna pointing is easier and semiconductor devices are
less expensive and more efficient.

73,

John
KD6OZH

----- Original Message ----- 
From: "Emily Clarke" <emily at clarke-design.com>
To: "John B. Stephensen" <kd6ozh at comcast.net>; <Amsat-bb at amsat.org>
Cc: <allan_gm1sxx at hotmail.com>
Sent: Sunday, September 10, 2006 03:39 UTC
Subject: Re: [amsat-bb] Re: S-band continued.


>
> What data are you basing this on?
>
> At 08:17 PM 9/9/2006, John B. Stephensen wrote:
> >The most requested transponder was U/V (especially among members outside
the
> >U.S.) and AMSAT did respond by increasing resources for it on Eagle.
>