# Re: FM vs SSB

• Subject: [amsat-bb] Re: FM vs SSB
• From: "William Leijenaar" <pe1rah@xxxxxxxxxxx>
• Date: Mon, 28 Jul 2003 23:12:39 +0200

```Hi AMSATs,

>If we assume an average efficiency of 80% for FM and 25% for SSB and AM
>then
>the table for DC power input is:

I think you mean the average efficiency of an class-c amplifier, but in no
way the efficiency of information transmission. (take in mind the "no spoken
word" efficiency :-)

>Unprocessed     Processed       RF
>Avg.    PEP    Avg.    PEP     BW      Mod.
>45      59      41      48       3      SSB
>45      59      41      48       6      DSB
>46      46      42      42       6      FM
>47      47      43      43      12     FM
>60      65      49      54       6      AM

>AMSAT-DE pulished an article on HELAPS design many years ago and I beleive
>that the peak efficiency (fully occupied transponder) was 50% and the
>average was around 25%. Someone with a copy could check. The relative power
>required from solar panels is in the average columns. PEP would approximate
>peak current from the batteries.

Many numbers and data's. The really important is how much battery power is
needed to send information from A to B. For spoken words FM is the most
inefficient way, because it takes full-power all the time even if there are
no words spoken. AM is some better but SSB is the best.
When there are no words spoken there is no power taken from the battery.

The efficiency of the needed class-a amplifiers are not that good as the
class-c for FM, but for satellite use the HELAPS system is the solution.
Then has SSB onother advantage that it needs abt. 4x less PEP compaired to
FM to have an equal communication from A to B.

Even with the 25% efficiency of ssb it is still winning from FM, see below:
40W FM (80% eff) = 50W battery power
10W SSB (25% eff) = 40W battery power

With the HELAPS it will be much more difference.
Then about the 50% efficiency for fully occupied and 25% for average of the
HELAPS.
The numbers seems to be low for average, but take in mind that with "no
spoken words" the efficiency is lower, but is not relavant as the PEP is
lower. While speaking the efficiency is higher what is needed because the
PEP is higher.

But with many "no spoken words" FM will need much more duracels than SSB :-)
Maybe with duracel FM wins :-)

>73,

>John
>KD6OZH

73 de PE1RAH, William
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