[amsat-bb] Why too much uplink power is bad

Paul Stoetzer n8hm at arrl.net
Sun Jul 26 15:24:14 UTC 2015

Hi Skyler,

There are two issues:

The first issue is the automatic gain control in the satellite
passband. Linear transponders only have a limited amount of power
available to spread across the 20 kHz - 100 kHz passband, so the gain
needs to be limited to keep the transponder output linear. When a very
strong signal is in the passband, everyone else's signal is weaker.
For example, I can usually receive my own signal (using 5 watts from
an FT-817 to an Arrow) about S8 above 5-10 degrees on FO-29. On Field
Day, it was tough to get a decent signal even above 40 or 50 degrees
because of all the high power stations in the passband. Your signal
should be no stronger than the beacon.

The second issue is how it could damage the satellite. The issue is
when an aging satellite's power system (such as AO-7 or FO-29) can't
manage to provide the full power ordinarily needed to run the
satellite. In AO-7's case, too strong of a signal can bring the
voltages available to the transponder down to a point where it can
cause the satellite to switch modes. In FO-29's case, strong signals
in the passband while the satellite is in eclipse could cause the
battery voltages to drop to a point where the satellite would shut off
to preserve the batteries.

This doesn't apply to FM satellites, but using a lot of power to
dominate the satellite by using the FM capture effect to keep weaker
signals out is considered poor practice.

Remember that the FCC amateur service regulations require the use of
the minimum power necessary to conduct communications. The regulations
also require you to not cause harmful interference to other amateur
stations. Using excessive power on a linear transponder violates both
of those provisions.

Also remember that when we're talking about "excessive power" that's
ERP, not actual output. Even 500 mW could be excessive power if you're
using a huge antenna array and the satellite is right overhead. I have
heard stations use the excuse that they are only running 2 watts when
called out for pounding the transponder with an excessively strong
signal. Well that might be too much, turn it down more. If your radio
doesn't allow you to turn the power any lower, you may need to add an
attenuator pad to your station.

Also, a CW signal is much harder on a linear transponder than an SSB
signal because a CW signal at keydown is full power, where a SSB
signal only reaches full power on voice peaks.


Paul, N8HM

On Sun, Jul 26, 2015 at 11:03 AM, Skyler F <electricity440 at gmail.com> wrote:
> Can someone explain to me why it is a bad thing for the satellite to have
> too much uplink power to it?
> Even if you send a huge amount of power up, I don't see how it could damage
> the satellite, by the time it reaches the sat, the power would be so low.
> Is this only something for the linear birds, does it apply to FM satellites
> as well?
> Thanks
> Skyler KDØWHB
> --
> Skyler Fennell
> amsatnet.info
> electricity440 at gmail.com
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