I don't see fading as a big issue. With the 70 cm atenna mounted off-center, I was worried that induced phase modulation may be an issue. If the antenna is 30 cm from the spin axis, the path length will vary by cos(offpointing angle) * 60 cm. In the worst case (MA 32 and MA 224), the arrival angle will vary between 30 and 60 degrees, depending on the ground station's location on Earth. The distance then varies by 0.43-0.74 wavelengths, resulting in phase modulation at the spin rate. If the maximum spin rate is 0.17 Hz, and the carrier tracking loop can handle it, it's no longer an issue.
The remaining issue is then interference from adjacent uplink channels which will depend on uplink channel spacing.
73,
John KD6OZH
----- Original Message ----- From: "Robert McGwier" [email protected] To: "John B. Stephensen" [email protected] Cc: "Phil Karn" [email protected]; "Jim Sanford" [email protected]; "eagle list" [email protected] Sent: Sunday, November 05, 2006 18:15 UTC Subject: Re: [eagle] Re: [Fwd: Re: Re: Please check these calculations]
I am loathe to see a 10W PEP transmitter be needed to mitigate poor phase noise in the RX LO's or to mitigate spin modulation induced fades. If we have a spin rate of 6-10 RPM and we use low power and FEC providing time diversity by encoding in blocks, surely the better solution is improve the phase noise performance of the LO's in the receiver and require less peak power on the uplink?
John B. Stephensen wrote:
I was thinking of phase modulation due to the spinning off-center antenna. A wider bandwidth would reduce the effects. The idea has been to have a 90-120 degree beamwidth antenna at 70 cm so that it works well over 75% of the orbit. The last antenna proposal that I saw was a single patch so there shouldn't be much fading on the class 1 uplinks due to the antenna pattern.
Since the access method for class 1 hasn't been decided, another approach to widening the bandwidth would be to use TDMA on one or more uplinks and a higher data rate. Ground station costs proabably don't change much up to 10 W PEP. It would be nice to keep class 1 uplink and downlink bandwidths below 2.5 kHz so that a sound-card modem could be written that works with existing transceivers.
73,
John KD6OZH
-- AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair "You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat." - Einstein