[amsat-bb] Re: NASA's American Student Moon Orbiter...

Thu Jul 3 18:14:57 PDT 2008

Hi Ed, KL7UW

If we put AO40 at a distance of 400.000 km instead of 60.000 km
from the earth the increase of isotropic attenuation at 2400 MHz is
about 16 dB
Supposing that the AO40 antennas are looking at the earth with the
same squint angle try to remember with how many dB of (S+N)/N
you was normally receiving  the downlink of the S2 transponder.
You should remember that the AO40 antenna for the S2 transponder
was a RHCP Helix of only 5 turns with an estimated gain of 8 dBic
By the way AO40 was also equipped with the S1 transponder using
a Short-Backfire antenna RHCP designed by ON6UG with a gain of
18 dBic
Unfortunately the S1 TX has been non-functional since 2001 aug 13
but when the S1 transponder was connected to the 18 dBic Short-Backfire
the S1 downlink was really about 10 dB over the S2 transponder.
If ipotetically we put AO40 at 400.000 km using the S1 transponder
we gain 10 dB over the S2 transponder and this is like to have reduced
the isotropic attenuation to 16 - 10 = 6  dB
To compensate for the rest of 6 dB it is only necessary to double the
diameter of the receiving dish.
In my case if I increase the diameter of my dish from 1.2 to 2.4 meters
i.e from 4 to 8 ft I will receive the S1 transponder from 400.000 km
with the same signal level of the S2 transponder from a distance of
60.000 km

The problem here is the uplink depending on what band we are using
for it.
If we use the L band 1268 MHz then the difference in the isotropic
attenuation between 400.000 km to 60.000 km is again about 16.5 dB
If  I want to put into the satellite receiver at 400.000 km the same
signal level that I put on it at 60.000 km I am in trouble because the AO40
23 cm antenna was already a 18 dBic gain dish and so to compensate for
the above increase of  path loss for the uplink the only way is to increase
the power or the antenna gain or both at the ground station.
If  I   use a yagi array's every time I double the numbar of yagi's I gain 3
dB but only in theory and every time I double the power I gain 3 dB
as well.
In my situation in order to put the same signal level into the satellite
receiver at 400.000 km and 60.000 km I should increase my power from
the actual 120 watt at 1268 MHz to 240 watt to get 3 dB and I should
enlarge the actual array of 4 x 23 element yagi to 16 x 23 element yagi
to gain another 6 dB and in total 3+6= 9 dB but in the above situation
I will be still 16.5 - 9 = 7.5 dB weeker than at 60.000 km

By the way I was looking at the many pictures taken on my Spectrum Analyser
when AO40 was operational and using a 4 ft dish helix feed and an overall
noise figure of 0.7 dB for my receiving system I see that the AO40 General
Beacon was at 20 dB of (S+N)/N most of the time while the average of the SSB
traffic was swinging in the range from 10 dB picking to 20 dB over the
noise and sometime over 20 dB with LEILA.

In conclusion if AO40 is transmitting from 400.000 km using the S 1
transponder with the 18 dBic Short-Backfire we gain 10 dB over the S2
Helix antenna and if I double my dish from 4 ft to 8 ft I gain another 6 dB
so that 10+6= 16 dB and the path loss for downlink is totally compensated.
In this condition the General Beacon of AO40 would be received unchanged
at 20 dB over the noise but the average of the SSB traffic would be received
weeker and weeker because the most part of the users would not be able to
increase the EIRP to compensate for the 16 dB more path loss in the uplink
at 1268 MHz

In addition if I improve my own 1268 MHz array from 4 x 23 (24 dBi)
to 8 x 23 element yagi (+3 dB) and using 240 watt (+3 dB) than I would
compensate the path loss in uplink by only 6 dB and my SSB translated
signal in comparison to 60.000 km  will be received weeker by
16 - 6 = 10 dB but if before it was received 20 dB over the noise it still
will be audible in SSB about 10 dB over the noise.

With only my actual 4 x 23 (24 dBi) element yagi and the actual 120 watt
I shall receive my own CW from 400.000 km at about 20-16= 4 dB over
the noise.

Without to increase my receiving dish from 4 ft to 8 ft I will loose 6 dB
and my CW signal will be received  with a (S+N)/N= - 2 dB but this is
not a problem on CW because skilled operators are able to receive by ears
signal levels that are even weeker.

Best 73" de

i8CVS Domenico

----- Original Message -----
From: "Edward Cole" <kl7uw at acsalaska.net>
To: "AMSAT BB" <amsat-bb at amsat.org>
Sent: Thursday, July 03, 2008 8:28 AM
Subject: [amsat-bb] Re: NASA's American Student Moon Orbiter...

> The gain increase for the antenna would be deltaG = (12/4)^2 = 9, or
> in dB = 10Log(9) = 9.5 dB.  You get this gain in Tx and again in Rx
> so the total gain = 19 dB.  So this means that the spacecraft will
> need to be 32-19 = 13 dB stronger than AO-40.  So perhaps the S/C
> antenna would be larger and maybe the Tx higher power?
>
> Ed - KL7UW
>
> At 08:16 PM 7/2/2008, w7lrd at comcast.net wrote:
> >How would my 12 foot paraclips work for this exercise?
> >73 Bob W7LRD
> >
> >--
> >"if this were easy, everyone would be doing it"
> >
> >-------------- Original message --------------
> >From: "Andrew Glasbrenner" <glasbrenner at mindspring.com>
> >
> > > The moon is roughly 360,000 to 400,000 km away. By comparison, AO-40
had a
> > > apogee of about 60,000km. At 2.4Ghz, that's about 16db difference
> > each way.
> > > Put AO-40 at the moon, and if I'm doing this right, you'd need about
32
> > > times the ground station antenna both coming and going to get
> > with a few db.
> > > I'm gonna need a bigger rotor for sure! I'm sure smarter folks
> > will check my
> > > math....
> > >
> > > 73, Drew KO4MA
> > >
> > > ----- Original Message -----
> > > From: "Joe"
> > > To: "Andrew Glasbrenner"
> > > Cc: "Trevor" ; "AMSAT BB"
> > > Sent: Wednesday, July 02, 2008 6:05 PM
> > > Subject: Re: [amsat-bb] Re: NASA's American Student Moon Orbiter...
> > >
> > >
> > > > what would a sample average link budget be?
> > > >
> > > > Andrew Glasbrenner wrote:
> > > >
> > > >>As far as I can recall we are pursuing both Eagle and the P4
opportunity
> > > >>equally, concentrating on common elements until the details are
ironed
> > > >>out. Neither has been identified as a primary or secondary
objective.
> > > >>
> > > >>I agree a package on a lunar orbiter would be neat, but also that it
is
> > > >>not the best use of what volunteers we have. We need more folks
> > to step up
> > > >>to do things, AND we need to make better use of them when they do.
> > > >>
> > > >>73, Drew KO4MA
> > > >>
> > > >>----- Original Message -----
> > > >>From: "Trevor"
> > > >>To: "AMSAT BB"
> > > >>Sent: Wednesday, July 02, 2008 4:53 PM
> > > >>Subject: [amsat-bb] Re: NASA's American Student Moon Orbiter...
> > > >>
> > > >>
> > > >>
> > > >>>--- On Wed, 2/7/08, Dave hartzell wrote:
> > > >>>
> > > >>>>http://www.spaceref.com/news/viewpr.html?pid=25839
> > > >>>>http://asmo.arc.nasa.gov/
> > > >>>>
> > > >>>>Wouldn't it be fun to have a transponder on this! ;-)
> > > >>>>
> > > >>>Fun yes, but dare I say it, a waste of precious Volunteer
resources.
> > > >>>
> > > >>>All lunar orbits are inherently unstable and will impact after a
couple
> > > >>>of years. The link budget requirements would not attract a mass
user
> > > >>>base.
> > > >>>
> > > >>>I suspect the number of Technically Capable volunteers is already
being
> > > >>>thinly stretched in trying to provide both the primary
> > objective Phase-IV
> > > >>>Lite (funded by Federal Government dollars) and the secondary
objective
> > > >>>the Eagle HEO.
> > > >>>
> > > >>>73 Trevor M5AKA