John, et.al:
I have sinned against you, by means of my erroneous calculations by an a darned decimal point! I apparently used an area of 100cm^2 whereas it should have been 1000cm^2. Sigh! I apologize for this misstep. Thanks for the correction.
The adjusted numbers are: An area of 31.6x31.6 cm is 1000cm^2 and with a dissipation of 42W, and emittance of 0.81, a power dissipation of 42W, an added solar input of ~14W, would create a device temperature of: Ta = 332K or 59°C With no transfer to the spaceframe. Presuming that the spaceframe would take perhaps 20W of the 42W, add the solar input of 14W, then: Ta = 298K or 24°C. These start to become manageable conditions, but achieving such results will still require some VERY careful thermal design of the devices and arrays, it is not a piece of cake.
Dick Jansson --------------------------- [email protected] ---------------------------
-----Original Message----- From: John B. Stephensen [mailto:[email protected]] Sent: Monday, 18 September, 2006 0854 To: Dick Jansson-rr; 'Robert McGwier'; 'EAGLE' Subject: Re: [eagle] Re: C-C Rider Band Plan Follow-up
If the X-band PAs occupy a 30 x 30 cm area, isn't the dissipation 42/900 or 47 mW/cm^2? This would result in 1/10 the temperature rise.
73,
John KD6OZH
----- Original Message ----- From: "Dick Jansson-rr" [email protected] To: "'John B. Stephensen'" [email protected]; "'Robert McGwier'" [email protected]; "'EAGLE'" [email protected] Sent: Sunday, September 17, 2006 18:35 UTC Subject: RE: [eagle] Re: C-C Rider Band Plan Follow-up
The consideration that John has made for an X band downlink is fraught by practical issues. The specific one that I address is that of the transmitter power dissipation. He proposes a 60W DC input to 36 amplifiers with an 18W RF output. Leaving 42W to be dissipated in a rather small area, which looked like an area of no more than 0.10m^2 (~>300mm each side of a square).
To grasp the magnitude of such a thermal issue that John's plan presents, let us examine the results of his plan. The proposed dissipation amounts to a heat flux of 0.42W/cm^2. If all of this had to be radiated to space on only one side, a ridiculous solution, the temperatures would achieve levels in the range of: Ta = 534K or 261°C. Presume then that only half of the 42W is directly radiated to space, the other half removed to the spaceframe, the temperature of the array would then come down to only: Ta = 449K or 176°C.
It is clear that such a plan as John proposes cannot be achieved without a pretty involved heat pipe cooling system, not just in a line but over an area. To me such a system is a bit mind boggling. I am not rejecting heat pipes, I worked them for AO-40, but such a system is very rapidly running away from one of the primary tenets of Eagle, that of KISS.
It is easy for you to toss around antenna gains, noise factors, path losses, and the like, but sometimes consideration of these factors seem to loose sight of a practical satellite system where it actually has to be constructed in real hardware and suffer the ramifications of real hardware.
Dick Jansson --------------------------- [email protected] ---------------------------
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of John B. Stephensen Sent: Sunday, 17 September, 2006 1629 To: Robert McGwier; 'EAGLE' Subject: [eagle] Re: C-C Rider Band Plan Follow-up
I agree that a 2-foot dish is useless at 1.26 GHz as it's slightly more than 2-wavelengths in diameter. S-band barely works at 4.5-wavelengths.
A C/X transponder seems feasible as there are efficient 500 mW X-band amplifier MMICs available. See the attached document.
73,
John KD6OZH