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Re: Beginner birds

RF path loss in free space is proportional to frequency.  Mode A has a low downlink frequency (29.4 MHz), which makes it possible for an omnidirectional antenna to hear a satellite's fractional-watt signal at 3000 km.

Path loss is greater at 2m, but it's easy to generate more power here on earth.  25W to a 2m ground plane gave a reliable uplink on RS-10 Mode A.  That is not a problem for the ANSI maximum permissible exposure limits.  For most people, 25W to a ground plane is easier than 3W to a yagi.  In that range, 2m power is cheap and easy.  Pointing a gain antenna is always more difficult than using an omni antenna.

In general, it is desirable to have the downlink at the lower frequency and the uplink at the higher frequency.  The reason is that the satellite's power is limited, so it's best to use the lowest-loss path (lower frequency) for the downlink.  Power is easier to come by here on earth, so we can use the higher-loss path (higher frequency) for the uplink.  This means that Mode A and Mode B have paths well suited to the available power at each end.  Mode J (FO29, UO14) has severely mismatched uplink and downlink paths.  The satellite has low power, but it is transmitting on the path with the highest loss.  We have lots of power available on earth, but we are transmitting on the path with less loss.  The result is that many people have strong uplinks, but can hardly hear a thing on the downlink.  AO40 has a similar mismatch.  You can generate the required uplink EIRP with an FT-847 and 6 dB of antenna gain.  But it takes 24 dB of gain and a low noise preamp to hear the noise flo!
 or of the transponder under ideal conditions.  Many other factors matter when choosing uplink and downlink bands: antenna size, transmitter cost/watt, allocated bandwidth, sky noise, interfering signals, tree absorption, etc.  But path loss vs. available uplink/downlink EIRP is a dominant engineering consideration.

You are correct that a 10m satellite downlink doesn't work very well when the 10m band is "open" for ionospheric propagation.  Fortunately (for mode A satellites), the 10m band is dead for more than 90% of each 11-year solar cycle.  There is a couple years at the peak of the solar cycle where a 10m satellite downlink is poor even late in summer evenings.  One interesting thing is that mode K (15m up, 10m down) permits over-the-horizon satellite contacts by using ionospheric propagation.  A couple people even got Satellite DXCC by working only RS-12.

Wayne Estes W9AE
Mundelein, IL, USA
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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