Re: [amsat-bb] Heavy Use of Birds

[Bob Bruninga <bruninga@xxxxxxxxxxxxx>]

On 2 Mar 2000, Tony Langdon wrote:

>  JO> For anything other than a single channel, FM is not practical for
>  JO> amateur satellites.  The power requirements are just too strong
>  JO> and FM is not efficient nor needed for it.  
> 
> ... but is it really that impractical?  (... while FM is  somewhat less
> efficient, it's not impractical on a linear bird, just that you get
> around 1/8 of the number of QSOs on that part of the passband).

I think it is two things.  Bandwidth of 15 KHz vs 3 KHz means it displaces
5 other possible QSO's and the power factor requirement on the Satellite's
transmitter power budget is 8 times greater (the difference in peak to
average power of an SSB signal).  Thus combined, the resources needed to
support FM are 5*8 or 40 times greater, not just the 8 you mention.

Similarly on the ground, the 5 times greater bandwidth needs a 5 times
stronger signal to get a decent signal to noise ratio to hear it.  So
yes, a single FM channel is far far less effecient.

Notice, however, that I am not taking sides here.  There are reasons to do
FM in some cases.  All of the military birds use FM.  But then, their
objectives are different than Ham radio...  ;-)

de WB4APR, Bob


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