OBSERVATIONS FROM SCOTLAND 28 May 2008 GM1SXX
Here's a good question from a regular reader.
> Why is it
bad to use more power than absolutely necessary when transmitting to a
> satellite and how does this affect the battery capacity of the satellite?
There are two answers to this one.
1)
With an FM bird, power is not an issue.
FM is a continuous duty mode. When a signal is present on the transponder, no
matter whether you are whispering or yelling, the power consumed is, or should
be, exactly the same.
FM transponders generally operate in Class C. With Class C amplifiers, when a signal is
present, the transponder consumes what would be considered a 'normal' amount of
power.
With no signal present, the power amplifier (PA) consumes virtually no power,
and since the PA is *the* major power consumer on a satellite, this is
good. Only the receiver and IF strip are active with no signal present. Power
consumed from the battery therefore depends primarily on how much time the
transponder is active for during each orbit.
OK, I've neglected the other power consumers on the bird such as the command system etc, but these are not affected by the behaviour of users.
2A) With a LINEAR transponder bird,
running excess power on the uplink is a VERY serious issue.
On a LINEAR transponder, the transponder's bandwidth (typically 100Khz) is
'shared' between all of the users users in the transponder's 'passband'. The
transponder literally takes a chunk of spectrum, amplifies and re-transmits it's
entire contents on a different frequency.
The total transponder power available is therefore SHARED between the users. If one
user runs excessive power, the automatic gain control (AGC) in the satellite's
receiver reduces the overall passband gain to compensate. This reduces the
strength of the downlink signal for EVERYONE.
You should therefore realise that if everyone ran the MINIMUM
uplink power,
the satellite AGC would never be operating and therefore the DOWNLINK signals
(for everyone) would be stronger.
This is an important point.
If the power level of even ONE user is greater than the AGC range can cope with
(all too common I'm afraid), the transponder 'limits' (bottoms out), and in
extreme cases goes non-linear with the excessively strong signals 'FM
modulating' it... the complaint known as 'FM'ing' . In short, it consumes a great deal of power while NOT providing
any useful communications.
2B) Now for the serious stuff.
Linear transponders operate in Class AB. The power consumption of the
transponder is roughly proportional to the strength of ALL the signals in the passband. If one signal dominates, it will use most of the available downlink
power, If it's strong enough to trigger the AGC into action, this lowers the
downlink power available for everyone ... including the offending station.
So, with a linear bird, the amount of power consumed from the battery is heavily
dependent of the strength of the strongest signal.
You may be interested to hear that 5W is enough power (to a modest yagi) to work
CW on Amsat Oscar7. It was certainly also the case for the sadly now defunct
Russian radiosport satellites.
Satellite receivers are generally VERY sensitive indeed. The limiting factor
with Linear birds is the IQ of the most thoughtless user. Sad, but true. Many users are
sensible, but it only takes ONE user running excessive power into a linear
transponder bird to ruin the party for all.
Since the power consumption of a linear satellite bears an almost linear
relationship to the total power in the passband, the use of excessive power will
rapidly run down the battery. This compromises the expected lifespan of
the satellite by degrading the battery components.
Satellites in Low Earth Orbit have brutal demands made of their batteries. 14
orbits a day is typical for these birds, so the battery suffers 14
charge-discharge cycles each day. Try doing that to a set of Ni-Cd
batteries and see how long they last!
Because the operating regime is so harsh, the batteries need to be over-rated ,
in order to provide an acceptable battery life expectancy. Battery Charge
Regulators (BCR's) also help greatly to provide an acceptable working life for
amateur satellites.
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Put simply, here are some basic satellite facts
FM satellites
PRO's.
Can be used with cheap FM equipment.
CONS
Encourages bad operating practices thanks to FM Capture Effect.
Only supports one QSO.
Linear satellites
PRO's.
Can support many concurrent users AND modes.
CON's.
Easily 'clobbered' by FM type operating practices.
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So the operating practices for linear and FM satellites are very different. Unfortunately some users don't realise this.
73 Al.
GM1SXX
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