AMSAT News Service Bulletin 214.08 From AMSAT HQ
SILVER SPRING, MD. Aug 2, 2004
To All RADIO AMATEURS
BID: $ANS-214.08
The 67 Hz PL tone must be transmitted continuously for a station to be heard through the FM repeater.
Unlike previous similar satellites the Echo PL tone decoder both turns on the downlink transmitter and gates the audio path. The tone must be present on a signal for the audio to get through. If a signal without a tone captures the receiver no audio will get through the decoder chip and the satellite will be transmitting a
signal without modulation. If that condition persists for 10 seconds the transmitter will go off until it again hears a signal with PL.
Multiple uplink signals can mix and cause no clear PL tone to be present at the decoder. During this time no audio is being repeated. What has been reported as a dead carrier is usually a result of this condition. It s not a carrier with no modulation on the uplink, but no valid PL on the
uplink because of mixing, resulting in no audio passing through the satellite. May we again note this is just like a terrestrial FM PL controlled repeater?
The PL decoder chip easily detects PL tone of 1 Khz deviation and is quite forgiving. It is neither necessary or desirable to increase the deviation of the PL tone you transmit above the nominal values. It will not work any better.
It is possible to turn off the PL function. But there is no COR (carrier operated relay) in the satellite. So without the PL function to control when the transmitter is on it would have to be on all the time. In the first few orbits in this mode, the B transmitter has been on about half the time. If it was on all the time it would have to be operated at a lower power level. There is only so much DC power available in the satellite and it is a zero sum game one orbit at a time. One of the major advantages
of the PL control mode is it keeps the transmitter off when not in use and therefore allows it to be at higher power when in use. Of course another is the satellite will not re-transmit signals not intended for it. It is
not possible for the PL tone to turn on the transmitter but not gate the audio.
We are watching the power usage closely and considering how to modify the software to allow higher transmitter powers when adequate DC power is available, and not pull the battery down to far for Echo to make it through the next eclipse, and not go to deep into the battery capacity,
etc. This three week (aprox.) trial period is for the purpose of measuring these performance parameters (and of course having some fun with this cool bird).
The trial move of the voice downlink frequency to 435.300 MHz does not limit Echo in any way or change the number of options available for various modes. All three Echo transmitters are frequency agile in their bands. 435.225 MHz was initially chosen and coordinated with the IARU on
the assumption GO-32 was not active. The reactivation of that satellite required Echo to move. 435.300 MHz was suggested by the IARU frequency coordination group. If feedback from users around the world indicates this is a usable frequency we will complete the coordination process and continue to use it.
The power level of the B transmitter on 435.300 MHz has been at about .5 Watts for a couple of days now. This is as high as we can go in the current mode without over stressing the batteries near the end of eclipse.
There is no beep generator in Echo so it is not possible to send a courtesy tone type beep when a valid PL goes away. We put the idea of sending a short 9k6 digital packet from time to time on the voice downlink on our todo list. It would have to interrupt any voice repeat going on at the time for about half a second due to the switching that it would entail, but at first glance seems doable.
There is nothing wrong with the sensitivity of the uplink receiver. In fact if anything it is too sensitive. In any event it cannot be changed. There is nothing about the orientation that is causing a reduction of sensitivity on the uplink. The uplink antenna is for all practical purposes omni-directional.
Jim White, WD0E
Klla: ANS
