Domenico,
Thanks for the calculations. You confirmed it will not work for Hams.
That is about what I was getting and I wanted to make sure before I scratched the idea off the list of possibilities. I just wanted to check-it-out since the purpose of the large buckyball in space was for "HF REFLECTION", I wanted to see if it had any possibilities for amateurs.
Apparentlly it will only be good for HF megawatt radars... Thanks Bob, WB4APR
-----Original Message----- From: i8cvs [mailto:domenico.i8cvs@tin.it] Sent: Tuesday, June 29, 2010 9:53 PM To: Bob Bruninga; AMSAT-BB Subject: Re: [amsat-bb] HF Satellite Relay
----- Original Message ----- From: "Robert Bruninga" bruninga@usna.edu To: amsat-bb@amsat.org Sent: Monday, June 28, 2010 7:44 PM Subject: [amsat-bb] HF Satellite Relay
Heard today of a Passive HF relay satellite being proposed. Wondered if Hams could relay off of it.
It's a 10m diameter sphere. I assumed a 10m signal and 1000 Watts And antenna gains at both ends of 10 dB. Unless I made a dumb error, it looks impossible? I get a received signal of -170 dBm Compared to a good HF receiver of -122 dBm So its 48 dB down in the noise. Going to narrow band, could improve things, but the Doppler
of
+/- 600 Hz would make that difficult.
Anyway, if someone else wants to double check the link
budget
using the radar range equation, go for it.
The beauty of this system is that it is perfectly spherical,
so
the reflection coefficient would be constant within 1 dB.
That
is the advantage over trying to use the ISS or other large rocket body... They vary by 20 dB making communication by reflection impossible.
Oh, and it would be in space for 30 years or more. So with something that reliable, it would be worth developing an
amateur
capability to use it. It is not designed for comms, but as a calibration sphere
for
over the horizon radars that have LOTS more power and LOTS
more
gain than we do.
Bob, Wb4APR
Hi Bob, WB4APR
I have assumed that the altitude of the Passive HF relay satellite over the earth is 1500 km and as we know the diameter of the sphere is 10 meters. Also I assumed that the reflectivity coefficient of the sphere
is 50%
The 28 MHz Round Trip Isotropic Attenuation using the concept
of Radar
Equation is as follows:
Pt x Gt x Ar x SigmaPr = ------------------------------ (4 x 3.14 x R^2)^2
where :
Pr = received power
Pt = transmitted power = 1watt
Gt = gain of a 28 MHz isotropic antenna = 1 in power ratio
Ar = Aperture of the isotropic antenna at 28 MHz in square
meters.
R = Radius of a sphere wich distance from the earth is 1500^3
i.e
the distance from the Passive HF relay satellite andthe earth expressed in meters.
Sigma = Surface of the target in square meters i.e. of the
Passive
HF relay satellite as seen as a radar targetdisc multiplied by the reflectivity coefficient of 50%
Computing:
/ 2 2 /\ 10.71Ar = ---------- = ----------- = 9.13 square meters 4 x 3,14 4 x 3,14
Sigma = 5^2 x 3.14 x 0.5 = 39.2 square meters
1 x 1 x 9.13 x 39.2Pr = --------------------------------------- = 4.47 ^ -25 watt (4 x 3.14 x 1500000^2) ^2
1
Round trip attenuation = 10 log --------------- = 243.5 dB 4.47^
-25
Link budged calculation:
Assuming that we are using a good HF receiver with a NF= 8 dB equivalent to 1539 kelvin we must consider in addition that the receiver sensitivity is limited by the external available noise power.For quiet,rural locations the galactic noise is the limiting factor and at 28 MHz the noise temperature is around 29.000 kelvin so that reducing the
Noise Figure
belove 8 dB at 28 MHz do not improve too much the S/N ratio.
With the above data the noise floor of this receiver for SSB
into a
bandwidth of 2500 Hz can be calculated as follows:
Noise Floor = KTB = 1.38 x 10^-23 ( 1539 + 29.000 ) x 2500 = -
151dBW
or - 121 dBm
TX power 1000 watt.............................+30 dBW TX Antenna gain....................................+10 dBi
Transmitted EIRP .................................+40 dBW Round trip attenuation 1500 km..........- 243.5 dB
Received power Pr on isotropic antenna on the earth ..............................-203.5 dBW RX antenna gain....................................+ 10 dB
Available power at RX input............... - 193.5 dBW RX noise floor...................................... - 151 dBW
Signal received with a S/N ratio.......... - 42.5 dB
So according with Bob calculations the signal is 42.5 dB under the noise and so it is not detectable.
Best 73" de
i8CVS Domenico