Re: [amsat-bb] Re: Let's build some Linear Transponders

["William Leijenaar" <pe1rah@xxxxxxxxxxx>]

Hi AMSATs,


> > 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.  The
>improvements are:

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 .-)

>1) Not FM, no capture effect
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)

>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... 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)

>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)

When the problem would be power budget I think it is much much better than 
those power wasting FM and digital sats. They even use power while they "do 
nothing". A lineair transponder only uses power when someone uses it, in the 
standby mode it can save power into a battery. Then with a lineair 
transponder you can use SSB what means ~4x less antenna power needed 
compaired to FM. Some 10...50mW would propably be enough for good ssb 
reception. Take care that in ssb it is not constant 50mW carier like in the 
power wasting FM modes.

Even when the power budget would still be negative you can turn the 
transponder on/off at intervals to charge the battery... so there are many 
possibilities...

> > 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

With my comment I mean a transponder with abt. 50kHz bandwidth, where many 
stations can work at the same time in different kind of modes.

>problem...Yes it can support them, and yes we may experiment with special
>passes, but my experience is that it is better to have a satellite comm
>system be in one mode ALWAYS so that users know what to expect instead of
>being dissapointed in changing operations..

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...

>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".

>Bob
>

73 de PE1RAH, William
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