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Re: Re: Let's build some Linear Transponders



I think we agree about all the advantages of a WIDEBAND transponder, the
wider the better, but it does not apply in our case...

On Tue, 1 Jul 2003, William Leijenaar wrote:

> > > Then I still don't know why amateurs still want one mode satellites,
> > > like a PSK31 transponder.... What is the purpose of it ??? What kind
> > > of improvement it will give to amateurs world ???
> >
> >To provide a multi-user linear transponder that can be powered by only a
> >4" solar panel (Thats all you get on a cubesat).  Such a transponder can
> >support up to 20 or more users in a single 3 KHz passband.
>
> So, you think that 3kHz is better than 50kHz ???
> Technically the differences is only the IF filter, and I think that your
> problem is the large used kristal filters to get >3kHz... But this problem
> can also be solved by much smaller sized filters .-)

You missed the point.  Yes it is JUST AS EASY to make it 50 KHz, but the
power avaiable on a 4" cubesat cannot support it if too many people get on
it.  ANd there is no way to control the number of users on the uplink.

> When the FM satellite users know FM will also work on this transponder, it
> will very soon be the newest FM bird. (this already happens on FO-20 !! FM
> station tx-ing over ssb)

It is difficult to get FM into a 3 KHz channel.  That is why it is
intended to be so narrow...

> >2) Multi-user "broadband" transpnoder (3 KHz)
> I dont understand the combination of 3kHz and broadband.... for PSK31 its
> maybe broadband ... but then its ONLY a CW and psk31 satellite...

Yes, that is the only way to get multiusers on such a small low power
satellite...

> that means only a small part of AMSAT world can work it. Then I would
> purpose a psk31 reciever (with demodulation and detection in the
> satellite) that would be a easier and cheaper choice. Propably this
> needs also less current (when the digital stuff is reduced to a minimum)

Yes, that is what is our PCSAT2 design.  The downlink is FM for the
entire 20 users in the transponder.  But the FM takes more power...
>
> >3) Will fit and be powered by a 4" cubesat.
> So a 3kHz transponder fits better than E.g a 50kHz transponder ??
> About the transponder size itself, I have a design that would fit abt. 3x in
> a 4" box haha thats not the problem with todays electronics. (ofcourse it is
> not a design for beginners)

SIZE is not the problem.  POWER from a 4" solar panel is the limiting
problem...

> > > Transponders are ideal to do tests with, like sstv, ssb, psk, and
> > > even psk31... with many stations at the same time... That most
> > > amateurs only want to talk or do just this one mode they like I
> > > know, but keep in mind that there are also amateurs who like to do
> > > tests... :-)
> >
> >Yes, but if you do SSB, or SSTV in the 3 KHz transponder then it is a
> >single USER satellite again that so many have pointed out is a
>
> A transponder can only be on/off, the modulation and mode you choose
> yourself :-) So, everyone... from a beginner to an advanced, from a
> digital freak to a dx-er can use it...

But not if it is only 3 KHz wide.  The 20 PSK-31 users will be upset at
the ONE SSB user that jams all of them...

> >Of course CW is very narrow and would be allowed  in  a small portion of
> >the passband.  But we will ddetermine that once we are on orbit...
>
> I am not against PSK31 or other interresting modes, but I don't understand
> why making a transponder 3kHz small and reduce its technical and
> experimental value. One channel or many user very close to each other just
> doesn't work. Example UO-14... or FO-20 with many users that often
> overlapping eachother because of doppler "freq walking".

I agree completely that it is MUCH BETTER to have a 50 KHz transponder
for everyone to use all linear modes.  But we just cannot support the
power needed on our small satellite design.  We could if users would all
agree to work closely to limit uplink power.  But Not only in amatueur
band, but Military and commercial as well, no one has been able to get
multiple independent users to successfully share power.  It just ends up
not working well for everyone.

So our experiment is to limit the bandwidth and see if that helps.
YES, please build a 50 KHz transponder for us.  But please make sure it
has enough power to support the number of users that will want to use it.

We think we have a good way to do that for our small 4" satellite...

See the design at:  http://www.ew.usna.edu/~bruninga/raft.html

de WB4APR


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