Why does everyone run so much transmit power to AO-40 on 435 MHz?

A recent post to the AMSAT-BB prompted me to once again consider why excessive power use on the uplinks to AO-40 has become the norm.

There are several things which I believe contribute to this problem.

  1. In some cases it is an attempt to compensate for a simple lack of receive sensitivity. Stations hearing the excessivly strong L-band signals don't realize how marginal their receive set up is.
  2. The L-band rx AGC does not restrict users of this band to the recommended MB-10db signal level. As a result some stations attempt to achieve the same excessive levels using the 70cm uplink resulting in decreased U-Rx
  3. The U-rx AGC has been driven so high for so long that everyone now considers it normal to run 4 to 8 times the recommended 100w EIRP.
Prior to launch AO-40 articles were pubplished in the AMSAT Journal listing 100w EIRP as all that would be needed for a good SSB uplink. I've mentioned this before on the AMSAT-BB and been told by regular operators that "those numbers just are not realisic". It is on this point that I find I must strongly disagree.

Below is a chart showing both the L and U-Rx AGCs for a 5 consecutive orbit period.
From this chart you can see the following

The good news is that AO-40 does have extremely sensitive receivers. The bad news is that too few people ever get to experience just how good AO-40's receivers really are.

U-Receiver AGC
AGC (db) Percentage
16-18 0.6
13-15 3.3
10-12 19.6
7-9 27.1
4-6 28.8
1-3 20.5
So just how bad is it? The table to the right gives a break down of the numbers. As you can see, half the time the AGC is above 7 db, two-thirds of the time it is above 6 db. This means that most people are having to run four times as much power as should be required over two thirds of the time. Based on this experience it is not surprising that most people mistakenly believe that 400-500w EIRP is required for good signals into AO-40.

But that's not all. As much as one third of the operating time the AGC gets up to 9 db. Now the power requirements are up eight-fold. Finally, almost a full quarter of the available transponder time is consumed with stations running 10 times the required power or more. It becomes no great surprise that all too often we hear deep "fading" on the passband. I suspect that because fading is such a natural and frequent event on HF, it doesn't raise an immeadiate flag that someone is doing something wrong. One of the greatest benefits of satellite operation should be steady, predicatable and reliable communications.

The last two charts below are graphical versions of the table above. Click on the chart to get a full size image.
Conclusion: Until everyone recognizes that they must control their uplink power so that they are no more than 10 db BELOW the strength of the middle beacon, we will probably be doomed to this continuous and needless escalation of required uplink power. This also means that we must be willing to resist the temptation to immeadiatly reach for the power knob when someone starts hammering at the AGC.

I'd like to express a special thanks to:

Lee-KU4OS