[amsat-bb] Re: HF Satellite Relay
Robert Bruninga
bruninga at usna.edu
Wed Jun 30 06:45:42 PDT 2010
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
>
>
>
>
>
>
>
>
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