Greetings
I personally prefer lower frequencies for satellite transmitters because of the lower path loss. Now, after viewing the power information on the eaglepedia meeting summary, I believe electrical efficiency is another good reason to support the lower frequencies.
From what I saw (probably estimates), the VHF transmitter will be about
60% efficient while the C band transmitter will be about 30% efficient.
I was very happy to see a respectable 20 watts planned for the VHF output and the same for the C band transmitter.
Thanks again to the design team. EEck out those watts and hope you have an alternative for power storage. Some bold new idea would be great.
Patrick, N2OEQ
----- Original Message ----- From: "McGrane" tmcgrane@suffolk.lib.ny.us To: "Amsat BB" amsat-bb@amsat.org Sent: Saturday, September 09, 2006 6:01 PM Subject: [amsat-bb] Eagle efficiencies
Greetings
I personally prefer lower frequencies for satellite transmitters because of the lower path loss.
Patrick, N2OEQ
Hi Pat, N2OEQ
At lower frequencies the path loss is lower but you requires bigger antennas.
The gain of a constant aperture antenna compensates for path loss changes with frequency.
In other words to get the same RF energy from the free space changing the frequency and path loss you need the same antenna aperture.
73" de
i8CVS Domenico
While lower frequencie do have better space loss, antennas have higher gain and the beamwidth is somewhat more narrow as frequency increases - . Steve NU5D - newbie.
Clarification, or more fuel to the fire... For a *fixed overall physical size (aperture)*, antenna gain is proportional to f^2 Pathloss is proportional to 1/f^2 So then the total link budget actually *increases* with increasing frequency (f^2*f^2/f^2=f^2), since you have an antenna at both ends, subject to the assumption stated initially. There are several 2nd-order terms to consider overall, such as decreasing transmitter power as frequency increases, thermal/galactic noise variations with freq, etc., but *for a given size* higher is better.
At some point (not that high in frequency) the beamwidths get so narrow the antennas are hard to point. Both on the satellite and on the ground. I don't think a spot beam would be popular with those living outside the spot. *for a given size* is a biased measurement.
73, Joe
----- Original Message ----- From: "Scott Townley" nx7u@arrl.net To: "Amsat BB" amsat-bb@amsat.org Sent: Saturday, September 09, 2006 2:04 PM Subject: [amsat-bb] Re: Eagle efficiencies
Clarification, or more fuel to the fire... For a *fixed overall physical size (aperture)*, antenna gain is proportional to f^2 Pathloss is proportional to 1/f^2 So then the total link budget actually *increases* with increasing frequency (f^2*f^2/f^2=f^2), since you have an antenna at both ends, subject to the assumption stated initially. There are several 2nd-order terms to consider overall, such as decreasing transmitter power as frequency increases, thermal/galactic noise variations with freq, etc., but *for a given size* higher is better.
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Certainly that's true. But I would say "for a given size" is a quite valid means of comparison. How many of us are truly unencumbered on antenna size? It all boils down to what you can squeeze out of your "given size".
At 21:25 2006-09-09, Gary "Joe" Mayfield wrote:
At some point (not that high in frequency) the beamwidths get so narrow the antennas are hard to point. Both on the satellite and on the ground. I don't think a spot beam would be popular with those living outside the spot. *for a given size* is a biased measurement.
73, Joe
----- Original Message ----- From: "Scott Townley" nx7u@arrl.net To: "Amsat BB" amsat-bb@amsat.org Sent: Saturday, September 09, 2006 2:04 PM Subject: [amsat-bb] Re: Eagle efficiencies
Clarification, or more fuel to the fire... For a *fixed overall physical size (aperture)*, antenna gain is proportional to f^2 Pathloss is proportional to 1/f^2 So then the total link budget actually *increases* with increasing frequency (f^2*f^2/f^2=f^2), since you have an antenna at both ends, subject to the assumption stated initially. There are several 2nd-order terms to consider overall, such as decreasing transmitter power as frequency increases, thermal/galactic noise variations with freq, etc., but *for a given size* higher is better.
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Clarification, or more fuel to the fire... For a *fixed overall physical size (aperture)*, antenna gain
is
proportional to f^2 Pathloss is proportional to 1/f^2 So then the total link budget actually *increases* with
increasing
frequency (f^2*f^2/f^2=f^2), since you have an antenna at both
ends,
True. But that assumes you can keep the antennas at both ends pointed at each other. Yes, this DOES give better links at higher frequencies but requires higher attitude control and pointing requirements on the spacecraft.
For LEO satelites that need OMNI antennas, then your have F^2/F^2 and these two effects cancel each other out.
Also, the key term is the "fixed size" comparison. Yes, phasing together FOUR 12 foot UHF yagi's will give the same overall "aperture" as a single 12 foot 2 meter yagi, but look how much easier it is to build a single 2m yagi. And then a 10 meter Dipole will also give about the same aperture too, and it is even simpler for the same aperture.
When you get to dishes at both ends, then I agree completely that all things are equal. Frequency doesn't matter, though pointing accuracy becomes harder and harder as one goes up. Bob
participants (6)
-
Gary "Joe" Mayfield -
i8cvs -
McGrane -
Robert Bruninga -
Scott Townley -
Steven Samuel Bosshard (NU5D)