[amsat-bb] Re: HF Satellite Relay

[Robert Bruninga]

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 at 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 at usna.edu>
> To: <amsat-bb at 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 Sigma
> Pr = ------------------------------
>            (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 and 
> the 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 target 
> disc multiplied
>                by the reflectivity coefficient of 50%
> 
> Computing:
> 
>                  / 2                     2
>                /\                10.71
>  Ar  = ----------  =  ----------- = 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 39.2
> Pr = --------------------------------------- = 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
> 
> 
> 
> 
> 
> 
> 
>