Another idea on phased array configuration
Hi Tom,
Thanks for sharing your thoughts on the phased array approach.
I especially appreciated your block diagrams which really helped. I'm sorting through the approach and will likely have some questions.
Regarding amplifiers for 5.7 GHz, Sirenza has a 1/4 watt for $4.80. It draws 165mA @ 5V or 7 amps for 43 of them.
Now here's the real heartburn device. The Hittite HMC409LP4 (I think the one you referred to) provides just over 1 watt, costs $8.52 (not bad at all) but runs 615mA at 5V - or 26 Amps for 43 antennas. If that input power doesn't bother anyone - there's part of our design!
I've never heard any estimates of what DC power Eagle is planning to deliver. Is that figure available yet?
Thanks again Tom for your time and thoughts.
Regards...Bill - N6GHz
Tom Clark, K3IO wrote:
I have been concerned about the complexity of doing the phased array. Most of the discussion so far has been along the lines of this set of premises:
1. We assume an X-element array (where X is somewhere in the 30-45 range to get enough gain to get a good signal to the ground) 2. For a lot of reasons (minimizing losses, redundancy, generating RF power, etc) we like the idea of distributing the PA function to have one PA at each array element. This means that in addition to X antennas, we have X amplifiers. 3. There exist several nice PA chip amplifiers that can each make ~1 watt (I'll use that value because one watt's as good as another). All these breeds of amplifier have a gain in the 10-20 dB (i.e. the numeric gain G is in the [10-100] range, corresponding to a drive level D in the 10-100 mW range). 4. With a PA at each element requiring ~D watts of drive, then we need a driver capable of X*D watts, assuming there are no losses in the X-way power splitter and all the intervening coax cable. 5. It is not hard to imagine that the power splitter+cable losses will be at least 3 dB (it's C-band, remember) so the drive requirements rapidly grow to the 2X*D range. OUCH! -- the driver is now at least as big a deal as the PA at the antenna element! 6. We still need some way to generate the phase shift necessary to point the beam in the desired direction, and be able to update the pointing to compensate for the fact that the spacecraft spin axis doesn't point at the earth.Several recent additions to these thoughts have included:
7. From Franklin Antonio: How about distributing a lower frequency phase reference and add a PLL to generate the microwave frequency? 8. Also from Franklin: Put a programmable phase shifter along with the PLL at each antenna to obtain the necessary antenna phasing? 9. From me: The phasing can be made easier if the array consists of collinear elements. The required phasing over the whole array needed to keep the beam pointed to the earth is a simple, linear gradient. If the array consists of several linear arrays, then the projection of the needed phase on each of the linear arrays is also linear.Pardon me while I digress, but I think this story is relevant. When I did my thesis many eons ago, I built a large 10 MHz radio telescope made up of a lot of east-west dipoles. For the north-south arm of the telescope, I made a km-long 450 ohm terminated transmission line made from #12 copperweld supported between a horizontal 2x4 which was held up by fence posts. And I chose 0.5 wavelengths as the between element spacing. The elements were supported in the air by a 20' pressure treated 4x4s at the middle and both ends.
Each antenna element was 3 wavelengths long -- central full-wave dipole fed with a half-wave of tubular 300 ohm TV cable; then at the end a shorted quarter-wave stub, and another full wavelength of wire on each side (this is sometimes called a Franklin collinear). The central feed-point impedance was high (several kohms, as compared with the 450 ohm open wire t-line. The T-line was marked off in 0.1 wavelength units; since the elements were spaced a half-wave, every 5th mark was at an antenna pole. To phase the array to a given declination might have required (as an example) a phase increment of 0.15 wavelengths. The first element was tapped onto the line at position 0. the second (a half wave away) was at position 0.515 but with the connection block flipped to get a 180° phase reversal. then the 3rd element should have been at 1.30 with no reversal, but it was closer to tap at 0.80 with a reversal. And so forth thru all 64 elements. It took about an hour to go thru the array to re-phase it for a different declination. Because the taps ended up contributing a (nearly) randomized set of reflections, the inter-element interactions were quite small unless the array was phased to the zenith (when all the individual ~1.1:1 VSWR phasors added up).
Thinking back on these sins of my youth, I came up with a new idea based on points 7 & 9, which is seen in the first attached drawing "tapped delay line".
As a variation on Franklin's idea #7, my thoughts are to use the Nth sub-harmonic of the carrier. We might make N=8 so that we distribute 730 MHz (corresponding to 5840 MHz center downlink); at this frequency, one wavelength ~ 411 mm. For a 7-element array, I show a 6*L piece of coax, with taps uniformly space L and terminated in its characteristic impedance. There a 7 uniformly tapped steps and at each tap is a high impedance buffer amplifier (think coax Ethernet here). The buffer feeds a *N multiplier (I suggested N-8 because Hittite has some really nice active microwave doublers); alternatively, it could involve a PLL, perhaps with a DRO "puck" as the resonator. The microwave signal from the multiplier feeds a double-balanced mixer to generate BPSK, followed by a PA and the patch antenna element. Note that the BPSK modulation could be done at a lower frequency inside the multiplier, in which case the modulation phase shift is < 180° by some integer divisor.
However, at this point we have not phased the array -- the interelement phase is determined by the tap interval L and the frequency f/N. If we could make the tapped delay line from rubber, then we could get an incremental change in the phase by simply mechanically stretching it. Since we can't change the physical length, can we tune the delay line electronically? I think that the scheme shown in the 2nd "rubber" drawing will work. At each tap point, we add a varactor and set the bias on all the varactors with a D/A. As we change the voltage across the diode string, each diode's capacity will be changed by the same amount, making the equivalent of computer-controlled "rubber". Voila -- we can generate the smooth phase gradient needed to point the antenna.
Note that the phase "swings" from the end where the f/N oscillator injects its signal. But IMHO, we want to "lock" the phase of the central element, half-way down the tapped line. I suggest that, in addition to programming the DC voltage necessary to point the antenna, the computer adds a "DC" constant to each string based on keeping the central element as the phase reference. Note that some of the delay line taps will not have an PA/antenna element attached to it; the degenerate case is the central element which can only be physically present in one of the intersecting arms.
The antenna geometry I tend to favor is the 43-element "12-spoke" ("/two bits/" * 43 = $10.75) version which has 3 9-element arms (with a single common central element) and 3 6-element arms (with the central 3 elements missing). All 6 would use an 8*L delay line, with the central phase of 5 of the lines slaved to the 6th.
As usual, please enter into a lively, feedback-provoking discussion -- 73, Tom
Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
For 43 of the Hittite devices it would take 132 w to deliver 43w of RF. This doesn't count the circuitry needed to drive the finals or other spacecraft requirements. This will also present a huge thermal problem with the need to dump around 100w of heat on the top of the spacecraft. At least 250w of solar capacity would be needed to support this based on current requirements.
On Apr 26, 2007, at 1:08 AM, Tom Clark, K3IO wrote:
Bill Ress wrote:
Hi Tom,
Thanks for sharing your thoughts on the phased array approach.
I especially appreciated your block diagrams which really helped. I'm sorting through the approach and will likely have some questions.
Regarding amplifiers for 5.7 GHz, Sirenza has a 1/4 watt for $4.80. It draws 165mA @ 5V or 7 amps for 43 of them.
Now here's the real heartburn device. The Hittite HMC409LP4 (I think the one you referred to) provides just over 1 watt, costs $8.52 (not bad at all) but runs 615mA at 5V - or 26 Amps for 43 antennas. If that input power doesn't bother anyone - there's part of our design!
I've never heard any estimates of what DC power Eagle is planning to deliver. Is that figure available yet?
Thanks again Tom for your time and thoughts.
Regards...Bill - N6GHz
Bill -- it was the Hittite HMC408 that we looked at. We (N4HY, W3GPS & I) got one of the HMC408 eval boards and tested it in Sep.'05. I presented a quick summary of our tests at the Eagle working group meeting in Pittsburgh in Oct.'05. I looked around for my PowerPoint presentation and found it; it can be gleaned on my "freebie" website at http://mysite.verizon.net/w3iwi/HMC408.ppt. The presentation, as posted, has one glaring error: the Hittite brick is missing from Slide #7. It's pretty obvious, bur it is a block between the DB6NT upconverter and the -16 dB directional coupler. I never had a reason to re-present the talk, so I never bothered to fix the drawing.
Anyway, the main conclusions from our "quickie" tests were It worked as advertised in the data sheet (within ~ 1 dB which was comparable to our measurement accuracy). We concluded that we could affect you could optimize efficiency at any given power output level by tweaking Vctl. The peak DC>RF efficiency we saw was ~ 22% for Vctl in the 3.5-4V range and Vcc1=Vcc2=5V. There is a possibility of doing even better by doing a more careful optimization of Vcc1 and Vctl. I suspect that Vcc2 needs to be the recommended 5V. This is especially true if we are going to be able to use constant envelope BPSK. (i.e. something approaching Class-C). Any amplifier can be easily shut down by setting Vctl=0. Let's hope we can find a more efficient amplifier chip!
73, Tom
Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
And thermally that all is pretty bad (being as civilized as I can say it)! We cannot allow ourselves be sucked into such dreadful thermal situations.
Dick Jansson, KD1K [email protected] [email protected] --------------------------- -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Louis McFadin Sent: Thursday, 26 April, 2007 1508 To: [email protected] Cc: AMSAT Eagle Subject: [eagle] Re: Another idea on phased array configuration
For 43 of the Hittite devices it would take 132 w to deliver 43w of RF. This doesn't count the circuitry needed to drive the finals or other spacecraft requirements. This will also present a huge thermal problem with the need to dump around 100w of heat on the top of the spacecraft. At least 250w of solar capacity would be needed to support this based on current requirements.
On Apr 26, 2007, at 1:08 AM, Tom Clark, K3IO wrote:
Bill Ress wrote:
Hi Tom,
Thanks for sharing your thoughts on the phased array approach.
I especially appreciated your block diagrams which really helped. I'm sorting through the approach and will likely have some questions.
Regarding amplifiers for 5.7 GHz, Sirenza has a 1/4 watt for $4.80. It draws 165mA @ 5V or 7 amps for 43 of them.
Now here's the real heartburn device. The Hittite HMC409LP4 (I think the one you referred to) provides just over 1 watt, costs $8.52 (not bad at all) but runs 615mA at 5V - or 26 Amps for 43 antennas. If that input power doesn't bother anyone - there's part of our design!
I've never heard any estimates of what DC power Eagle is planning to deliver. Is that figure available yet?
Thanks again Tom for your time and thoughts.
Regards...Bill - N6GHz
Bill -- it was the Hittite HMC408 that we looked at. We (N4HY, W3GPS & I) got one of the HMC408 eval boards and tested it in Sep.'05. I presented a quick summary of our tests at the Eagle working group meeting in Pittsburgh in Oct.'05. I looked around for my PowerPoint presentation and found it; it can be gleaned on my "freebie" website at http://mysite.verizon.net/w3iwi/HMC408.ppt. The presentation, as posted, has one glaring error: the Hittite brick is missing from Slide #7. It's pretty obvious, bur it is a block between the DB6NT upconverter and the -16 dB directional coupler. I never had a reason to re-present the talk, so I never bothered to fix the drawing.
Anyway, the main conclusions from our "quickie" tests were
1. It worked as advertised in the data sheet (within ~ 1 dB which was comparable to our measurement accuracy).
2. We concluded that we could affect you could optimize efficiency at any given power output level by tweaking Vctl.
3. The peak DC>RF efficiency we saw was ~ 22% for Vctl in the 3.5-4V range and Vcc1=Vcc2=5V.
4. There is a possibility of doing even better by doing a more careful optimization of Vcc1 and Vctl. I suspect that Vcc2 needs to be the recommended 5V. This is especially true if we are going to be able to use constant envelope BPSK. (i.e. something approaching Class-C).
5. Any amplifier can be easily shut down by setting Vctl=0.
Let's hope we can find a more efficient amplifier chip!
73, Tom
_______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
For the rest of the group I pose a question: If we assume that we are going to modulate the downlink with BPSK, can we get away with hard limiting (i.e. Class-C) amplifier? Do we really care about occupying a spectrum about twice as wide as "best engineering practice"?
This has long been decided -- we WILL be limiting, and everything will be constant envelope. Class C or even class E amplifiers should be used.
Matt
Tom Clark, K3IO wrote:
For the rest of the group I pose a question: If we assume that we are going to modulate the downlink with BPSK, can we get away with hard limiting (i.e. Class-C) amplifier? Do we really care about occupying a spectrum about twice as wide as "best engineering practice"?
73, Tom
We intend flying unfiltered BPSK and as efficient an amplifier as we can build. This will not be a problem as I see it.
Bob
The Hittites were interesting, they had potential, but they are not really efficient enough. We will find better parts.
Bob
Louis McFadin wrote:
For 43 of the Hittite devices it would take 132 w to deliver 43w of RF. This doesn't count the circuitry needed to drive the finals or other spacecraft requirements. This will also present a huge thermal problem with the need to dump around 100w of heat on the top of the spacecraft. At least 250w of solar capacity would be needed to support this based on current requirements.
Bob,
I don't think we'll find a device on the current marketplace that will suit our efficiency needs. But, if we're willing to develop (fund) a program to design and build a Class E, 1 watt, 5.7 GHz amplifier module, we might get to 70 and maybe 90% efficiency. I have seen Class E X Band amplifiers with 60 to 70%. The design, including the choice of active device(s) must start with the goal of being Class E since several interesting parameters are in play. You just can't take a Class C amplifier and "push" it into the switching mode. Perhaps a Class C amplifier might be enough.
But it's a very interesting design challenge - to be sure!! When do we start??
But, lets assume that we build amplifiers with 100% efficiency. 36 or 43 elements still requires 36 to 43 watts input. What has me concerned is that the current satellite structure design calls for 6 solar panels with, what I guess is about 25 watts per panel. That appears to be consistent with the 100 watts power generation stated in the current Eagle Functional Requirements.
Are we still working with 100 watts or did I miss something??
Regards...Bill - N6GHz
Robert McGwier wrote:
The Hittites were interesting, they had potential, but they are not really efficient enough. We will find better parts.
Bob
Louis McFadin wrote:
For 43 of the Hittite devices it would take 132 w to deliver 43w of RF. This doesn't count the circuitry needed to drive the finals or other spacecraft requirements. This will also present a huge thermal problem with the need to dump around 100w of heat on the top of the spacecraft. At least 250w of solar capacity would be needed to support this based on current requirements.
Bill Ress wrote:
Bob,
I don't think we'll find a device on the current marketplace that will suit our efficiency needs. But, if we're willing to develop (fund) a program to design and build a Class E, 1 watt, 5.7 GHz amplifier module, we might get to 70 and maybe 90% efficiency. I have seen Class E X Band amplifiers with 60 to 70%. The design, including the choice of active device(s) must start with the goal of being Class E since several interesting parameters are in play. You just can't take a Class C amplifier and "push" it into the switching mode. Perhaps a Class C amplifier might be enough.
I am listening and more than willing to consider anything that gets the job done with a reasonable budget.
But it's a very interesting design challenge - to be sure!! When do we start??
But, lets assume that we build amplifiers with 100% efficiency. 36 or 43 elements still requires 36 to 43 watts input. What has me concerned is that the current satellite structure design calls for 6 solar panels with, what I guess is about 25 watts per panel. That appears to be consistent with the 100 watts power generation stated in the current Eagle Functional Requirements.
Are we still working with 100 watts or did I miss something??
What we are working with is "whatever we can get away with" so long as it meets the communications systems goals as stated in our working document from the San Diego meeting of last summer. That is the system we want to design and the concept we wish to support. If this involves us working on our own amplifier design, and not using monolithic designs from Hittite, etc., so be it. The efficiency translates directly to multiple scenario big wins for us. Class E for this is completely acceptable.
Regards...Bill - N6GHz
Robert McGwier wrote:
The Hittites were interesting, they had potential, but they are not really efficient enough. We will find better parts.
Bob
73's Bob N4HY
Bob,
Thanks for your reply.
I'll get with John Stephensen and kick some "West Coast" ideas around. Didn't you have some amplifier guru's on the East Coast? Perhaps they can weigh in.
Since this widget will surely drive the satellite power consumption issue, perhaps it should be high up on this year's "action item" list.
Regarding satellite power generation, I used the Eaglepedia info indicating 6 solar panels and the stated goal of 100 watts (perhaps it is out of date??). Then I remembered your talk at the 2006 symposium which showed a 12 panel fold out design. So have we better refined what power the satellite will provide??
Robert McGwier wrote:
Bill Ress wrote:
Bob,
I don't think we'll find a device on the current marketplace that will suit our efficiency needs. But, if we're willing to develop (fund) a program to design and build a Class E, 1 watt, 5.7 GHz amplifier module, we might get to 70 and maybe 90% efficiency. I have seen Class E X Band amplifiers with 60 to 70%. The design, including the choice of active device(s) must start with the goal of being Class E since several interesting parameters are in play. You just can't take a Class C amplifier and "push" it into the switching mode. Perhaps a Class C amplifier might be enough.
I am listening and more than willing to consider anything that gets the job done with a reasonable budget.
But it's a very interesting design challenge - to be sure!! When do we start??
But, lets assume that we build amplifiers with 100% efficiency. 36 or 43 elements still requires 36 to 43 watts input. What has me concerned is that the current satellite structure design calls for 6 solar panels with, what I guess is about 25 watts per panel. That appears to be consistent with the 100 watts power generation stated in the current Eagle Functional Requirements.
Are we still working with 100 watts or did I miss something??
What we are working with is "whatever we can get away with" so long as it meets the communications systems goals as stated in our working document from the San Diego meeting of last summer. That is the system we want to design and the concept we wish to support. If this involves us working on our own amplifier design, and not using monolithic designs from Hittite, etc., so be it. The efficiency translates directly to multiple scenario big wins for us. Class E for this is completely acceptable.
Regards...Bill - N6GHz
Robert McGwier wrote:
The Hittites were interesting, they had potential, but they are not really efficient enough. We will find better parts.
Bob
73's Bob N4HY
Bob, I am very worried about this huge requirement for power. I am concerned that we are making a satellite that that AMSAT will not be able to afford.
We also need to resolve the question of how much power needs to be supplied during the worst case eclipse scenario. This has a major impact on the amount of energy storage is required.
Another question is whether we are going to stay with the requirement of being sun angle neutral. These are all drivers for the size of the spacecraft and the structure.
Lou McFadin W5DID [email protected]
On Apr 27, 2007, at 3:30 PM, Robert McGwier wrote:
Bill Ress wrote:
Bob,
I don't think we'll find a device on the current marketplace that will suit our efficiency needs. But, if we're willing to develop (fund) a program to design and build a Class E, 1 watt, 5.7 GHz amplifier module, we might get to 70 and maybe 90% efficiency. I have seen Class E X Band amplifiers with 60 to 70%. The design, including the choice of active device(s) must start with the goal of being Class E since several interesting parameters are in play. You just can't take a Class C amplifier and "push" it into the switching mode. Perhaps a Class C amplifier might be enough.
I am listening and more than willing to consider anything that gets the job done with a reasonable budget.
But it's a very interesting design challenge - to be sure!! When do we start??
But, lets assume that we build amplifiers with 100% efficiency. 36 or 43 elements still requires 36 to 43 watts input. What has me concerned is that the current satellite structure design calls for 6 solar panels with, what I guess is about 25 watts per panel. That appears to be consistent with the 100 watts power generation stated in the current Eagle Functional Requirements.
Are we still working with 100 watts or did I miss something??
What we are working with is "whatever we can get away with" so long as it meets the communications systems goals as stated in our working document from the San Diego meeting of last summer. That is the system we want to design and the concept we wish to support. If this involves us working on our own amplifier design, and not using monolithic designs from Hittite, etc., so be it. The efficiency translates directly to multiple scenario big wins for us. Class E for this is completely acceptable.
Regards...Bill - N6GHz
Robert McGwier wrote:
The Hittites were interesting, they had potential, but they are not really efficient enough. We will find better parts.
Bob
73's Bob N4HY
-- AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair "If you're going to be crazy, you have to get paid for it or else you're going to be locked up." Hunter S. Thompson _______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
Since the generated beam needs to cover the entire earth from a maxmum of 35,000 km in alttude, the beam can't be made very narrow. I don't think that all 36 or 43 elements ever need to be on at one time. Many could be held in reserve.
73,
John KD6OZH ----- Original Message ----- From: Louis McFadin To: Robert McGwier Cc: [email protected] ; AMSAT Eagle Sent: Saturday, April 28, 2007 14:36 UTC Subject: [eagle] Re: Another idea on phased array configuration
Bob, I am very worried about this huge requirement for power. I am concerned that we are making a satellite that that AMSAT will not be able to afford. We also need to resolve the question of how much power needs to be supplied during the worst case eclipse scenario. This has a major impact on the amount of energy storage is required.
Another question is whether we are going to stay with the requirement of being sun angle neutral. These are all drivers for the size of the spacecraft and the structure.
Lou McFadin W5DID [email protected]
On Apr 27, 2007, at 3:30 PM, Robert McGwier wrote:
Bill Ress wrote: Bob,
I don't think we'll find a device on the current marketplace that will suit our efficiency needs. But, if we're willing to develop (fund) a program to design and build a Class E, 1 watt, 5.7 GHz amplifier module, we might get to 70 and maybe 90% efficiency. I have seen Class E X Band amplifiers with 60 to 70%. The design, including the choice of active device(s) must start with the goal of being Class E since several interesting parameters are in play. You just can't take a Class C amplifier and "push" it into the switching mode. Perhaps a Class C amplifier might be enough.
I am listening and more than willing to consider anything that gets the job done with a reasonable budget.
But it's a very interesting design challenge - to be sure!! When do we start??
But, lets assume that we build amplifiers with 100% efficiency. 36 or 43 elements still requires 36 to 43 watts input. What has me concerned is that the current satellite structure design calls for 6 solar panels with, what I guess is about 25 watts per panel. That appears to be consistent with the 100 watts power generation stated in the current Eagle Functional Requirements.
Are we still working with 100 watts or did I miss something??
What we are working with is "whatever we can get away with" so long as it meets the communications systems goals as stated in our working document from the San Diego meeting of last summer. That is the system we want to design and the concept we wish to support. If this involves us working on our own amplifier design, and not using monolithic designs from Hittite, etc., so be it. The efficiency translates directly to multiple scenario big wins for us. Class E for this is completely acceptable.
Regards...Bill - N6GHz
Robert McGwier wrote: The Hittites were interesting, they had potential, but they are not really efficient enough. We will find better parts.
Bob
73's Bob N4HY
-- AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair "If you're going to be crazy, you have to get paid for it or else you're going to be locked up." Hunter S. Thompson _______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
------------------------------------------------------------------------------
_______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
John B. Stephensen wrote:
Since the generated beam needs to cover the entire earth from a maxmum of 35,000 km in alttude, the beam can't be made very narrow. I don't think that all 36 or 43 elements ever need to be on at one time. Many could be held in reserve.
Where do you come up with this? What sort of analysis?
Matt
The spreadsheet distributed early last year shows a 9 degree squint angle at the leading and trailing horizons at apogee. An antenna with an 18 degree half-power beamwidth has a gain of 41000/(HPBW^2) = 127 = 21 dBi. It would be best to illuminate the earth with a loss of only 1 dB at the edges and the -1 dB beamwidth is going to be 2/3 or less of the -3 dB beamwidth. The gain should then be adjusted by 4/9 or 3.5 dB less. This would be 17.5 dBi.
73,
John KD6OZH
----- Original Message ----- From: "Matt Ettus" [email protected] To: "John B. Stephensen" [email protected] Cc: "Louis McFadin" [email protected]; "Robert McGwier" [email protected]; "AMSAT Eagle" [email protected]; [email protected] Sent: Saturday, April 28, 2007 19:57 UTC Subject: Re: [eagle] Re: Another idea on phased array configuration
John B. Stephensen wrote:
Since the generated beam needs to cover the entire earth from a maxmum of 35,000 km in alttude, the beam can't be made very narrow. I don't think that all 36 or 43 elements ever need to be on at one time. Many could be held in reserve.
Where do you come up with this? What sort of analysis?
Matt
So what? We can have, and were planning on having, an 18dB gain array. That doesn't mean we need to turn any elements off.
Matt
John B. Stephensen wrote:
The spreadsheet distributed early last year shows a 9 degree squint angle at the leading and trailing horizons at apogee. An antenna with an 18 degree half-power beamwidth has a gain of 41000/(HPBW^2) = 127 = 21 dBi. It would be best to illuminate the earth with a loss of only 1 dB at the edges and the -1 dB beamwidth is going to be 2/3 or less of the -3 dB beamwidth. The gain should then be adjusted by 4/9 or 3.5 dB less. This would be 17.5 dBi.
73,
John KD6OZH
----- Original Message ----- From: "Matt Ettus" [email protected] To: "John B. Stephensen" [email protected] Cc: "Louis McFadin" [email protected]; "Robert McGwier" [email protected]; "AMSAT Eagle" [email protected]; [email protected] Sent: Saturday, April 28, 2007 19:57 UTC Subject: Re: [eagle] Re: Another idea on phased array configuration
John B. Stephensen wrote:
Since the generated beam needs to cover the entire earth from a maxmum of 35,000 km in alttude, the beam can't be made very narrow. I don't think that all 36 or 43 elements ever need to be on at one time. Many could be held in reserve.
Where do you come up with this? What sort of analysis?
Matt
It seems to me that it would be advantageous to design for operation with less than the maximum number of elements. If only 25 of 36 elements need to be used, 11 can fail and be replaced before the link budget degrades significantly. If more than 25 elements are required to get 18 dBic, then it would be advantageous to fly 40-50% more elements than that number.
73,
John KD6OZH
----- Original Message ----- From: "Matt Ettus" [email protected] To: "John B. Stephensen" [email protected] Cc: "Louis McFadin" [email protected]; "Robert McGwier" [email protected]; "AMSAT Eagle" [email protected]; [email protected] Sent: Sunday, April 29, 2007 06:57 UTC Subject: Re: [eagle] Re: Another idea on phased array configuration
So what? We can have, and were planning on having, an 18dB gain array. That doesn't mean we need to turn any elements off.
Matt
John B. Stephensen wrote:
The spreadsheet distributed early last year shows a 9 degree squint angle at the leading and trailing horizons at apogee. An antenna with an 18 degree half-power beamwidth has a gain of 41000/(HPBW^2) = 127 = 21 dBi. It would be best to illuminate the earth with a loss of only 1 dB at the edges and the -1 dB beamwidth is going to be 2/3 or less of the -3 dB beamwidth. The gain should then be adjusted by 4/9 or 3.5 dB less. This would be 17.5 dBi.
73,
John KD6OZH
----- Original Message ----- From: "Matt Ettus" [email protected] To: "John B. Stephensen" [email protected] Cc: "Louis McFadin" [email protected]; "Robert McGwier" [email protected]; "AMSAT Eagle" [email protected]; [email protected] Sent: Saturday, April 28, 2007 19:57 UTC Subject: Re: [eagle] Re: Another idea on phased array configuration
John B. Stephensen wrote:
Since the generated beam needs to cover the entire earth from a maxmum of 35,000 km in alttude, the beam can't be made very narrow. I don't think that all 36 or 43 elements ever need to be on at one time. Many could be held in reserve.
Where do you come up with this? What sort of analysis?
Matt
The length of each side of a rectangular broaside array with a given directivity is sqrt(directivity/12.6). For a 17.5 dBi gain this works out to sqrt(56/12.6) = 2.1 wavelegths. A perfect 16-element array with 0.5-wavelength spacing would be slighty too small. However, a lossy 25-element array should provide sufficient gain. 9 elements of a 36-element array or 14 elements of a 41-element array could be held in reserve.
73,
John KD6OZH
----- Original Message ----- From: "John B. Stephensen" [email protected] To: "Matt Ettus" [email protected] Cc: "AMSAT Eagle" [email protected]; [email protected]; "Louis McFadin" [email protected] Sent: Sunday, April 29, 2007 06:21 UTC Subject: [eagle] Re: Another idea on phased array configuration
The spreadsheet distributed early last year shows a 9 degree squint angle at the leading and trailing horizons at apogee. An antenna with an 18 degree half-power beamwidth has a gain of 41000/(HPBW^2) = 127 = 21 dBi. It would be best to illuminate the earth with a loss of only 1 dB at the edges and the -1 dB beamwidth is going to be 2/3 or less of the -3 dB beamwidth. The gain should then be adjusted by 4/9 or 3.5 dB less. This would be 17.5 dBi.
73,
John KD6OZH
----- Original Message ----- From: "Matt Ettus" [email protected] To: "John B. Stephensen" [email protected] Cc: "Louis McFadin" [email protected]; "Robert McGwier" [email protected]; "AMSAT Eagle" [email protected]; [email protected] Sent: Saturday, April 28, 2007 19:57 UTC Subject: Re: [eagle] Re: Another idea on phased array configuration
John B. Stephensen wrote:
Since the generated beam needs to cover the entire earth from a maxmum of 35,000 km in alttude, the beam can't be made very narrow. I don't think that all 36 or 43 elements ever need to be on at one time. Many could be held in reserve.
Where do you come up with this? What sort of analysis?
Matt
Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
I still think people are not reading the San Diego document. We posited a reasonable 60w DC for the C band amplifiers with an aggregate 20 -ish w output, split amongst the elements or something reasonable like this. That is 2-ish watts per antenna DC. Again, in the San Diego document, we guessed at the overall power budget and as before, we guessed at a number that is identical to the one you have been worried about our needing and further, we acknowledge in that document that is costs $1200/w. We are completely aware that the solar generator is MOST of the cost of the satellite and by a lot.
READ THE DOCUMENT.
Bob
Louis McFadin wrote:
Bob, I am very worried about this huge requirement for power. I am concerned that we are making a satellite that that AMSAT will not be able to afford.
We also need to resolve the question of how much power needs to be supplied during the worst case eclipse scenario. This has a major impact on the amount of energy storage is required.
Another question is whether we are going to stay with the requirement of being sun angle neutral. These are all drivers for the size of the spacecraft and the structure.
Lou McFadin W5DID [email protected] mailto:[email protected]
Louis McFadin wrote:
Bob, I am very worried about this huge requirement for power. I am concerned that we are making a satellite that that AMSAT will not be able to afford.
We also need to resolve the question of how much power needs to be supplied during the worst case eclipse scenario. This has a major impact on the amount of energy storage is required.
Another question is whether we are going to stay with the requirement of being sun angle neutral. These are all drivers for the size of the spacecraft and the structure.
What I'm most worried about is that no decision is ever final.
What I'm most worried about is that questions which were quite settled are now being reraised.
What I'm worried about is that nobody seems to remember what we concluded at past meetings.
What I'm worried about is that a simple question about the phasing system for the patch array has become an opening to question the shape, size, and power of the satellite.
What I'm worried about is that in late April 2007 we are repeating discussions from 2004.
The decision was 36 patches at 1/2 to 1 Watt apiece. The link budgets were done assuming just a little bit more than 1/2 watt apiece.
For 1/2 watt each, we get 18 W of RF, and assuming 40% efficiency, 45 Watts DC. Maximum heat load = 27W.
For 1 Watt each, we need 90 Watts DC input to generate 36 output. Maximum heat load = 54W.
Matt
When I was comparing C/X and S/C transponders, I put the efficiency in the file name for each data sheet that I downladed. Here are some with 30% or greater efficiency.
73,
John KD6OZH
Let's hope we can find a more efficient amplifier chip!
73, Tom
The multiple-tap phase shifter could also be built using a lumped-element version of a transmisson line. In either case, there will be an upper frequency limit to its operation because of the capacitance of the amplifiers at each tap. If we assume 3 pF at each amplifier input and that a reactance of 100 ohms is allowed, that limit would be about 500 MHz. Each multiplier/PA will have a slightly different phase shift so there would still have to be phase shifters associated with each antenna element.
73,
John KD6OZH ----- Original Message ----- From: Tom Clark, K3IO To: AMSAT Eagle Sent: Wednesday, April 25, 2007 21:56 UTC Subject: [eagle] Another idea on phased array configuration
I have been concerned about the complexity of doing the phased array. Most of the discussion so far has been along the lines of this set of premises:
1. We assume an X-element array (where X is somewhere in the 30-45 range to get enough gain to get a good signal to the ground) 2. For a lot of reasons (minimizing losses, redundancy, generating RF power, etc) we like the idea of distributing the PA function to have one PA at each array element. This means that in addition to X antennas, we have X amplifiers. 3. There exist several nice PA chip amplifiers that can each make ~1 watt (I'll use that value because one watt's as good as another). All these breeds of amplifier have a gain in the 10-20 dB (i.e. the numeric gain G is in the [10-100] range, corresponding to a drive level D in the 10-100 mW range). 4. With a PA at each element requiring ~D watts of drive, then we need a driver capable of X*D watts, assuming there are no losses in the X-way power splitter and all the intervening coax cable. 5. It is not hard to imagine that the power splitter+cable losses will be at least 3 dB (it's C-band, remember) so the drive requirements rapidly grow to the 2X*D range. OUCH! -- the driver is now at least as big a deal as the PA at the antenna element! 6. We still need some way to generate the phase shift necessary to point the beam in the desired direction, and be able to update the pointing to compensate for the fact that the spacecraft spin axis doesn't point at the earth.
Several recent additions to these thoughts have included:
7. From Franklin Antonio: How about distributing a lower frequency phase reference and add a PLL to generate the microwave frequency? 8. Also from Franklin: Put a programmable phase shifter along with the PLL at each antenna to obtain the necessary antenna phasing? 9. From me: The phasing can be made easier if the array consists of collinear elements. The required phasing over the whole array needed to keep the beam pointed to the earth is a simple, linear gradient. If the array consists of several linear arrays, then the projection of the needed phase on each of the linear arrays is also linear.
Pardon me while I digress, but I think this story is relevant. When I did my thesis many eons ago, I built a large 10 MHz radio telescope made up of a lot of east-west dipoles. For the north-south arm of the telescope, I made a km-long 450 ohm terminated transmission line made from #12 copperweld supported between a horizontal 2x4 which was held up by fence posts. And I chose 0.5 wavelengths as the between element spacing. The elements were supported in the air by a 20' pressure treated 4x4s at the middle and both ends.
Each antenna element was 3 wavelengths long -- central full-wave dipole fed with a half-wave of tubular 300 ohm TV cable; then at the end a shorted quarter-wave stub, and another full wavelength of wire on each side (this is sometimes called a Franklin collinear). The central feed-point impedance was high (several kohms, as compared with the 450 ohm open wire t-line. The T-line was marked off in 0.1 wavelength units; since the elements were spaced a half-wave, every 5th mark was at an antenna pole. To phase the array to a given declination might have required (as an example) a phase increment of 0.15 wavelengths. The first element was tapped onto the line at position 0. the second (a half wave away) was at position 0.515 but with the connection block flipped to get a 180° phase reversal. then the 3rd element should have been at 1.30 with no reversal, but it was closer to tap at 0.80 with a reversal. And so forth thru all 64 elements. It took about an hour to go thru the array to re-phase it for a different declination. Because the taps ended up contributing a (nearly) randomized set of reflections, the inter-element interactions were quite small unless the array was phased to the zenith (when all the individual ~1.1:1 VSWR phasors added up).
Thinking back on these sins of my youth, I came up with a new idea based on points 7 & 9, which is seen in the first attached drawing "tapped delay line".
As a variation on Franklin's idea #7, my thoughts are to use the Nth sub-harmonic of the carrier. We might make N=8 so that we distribute 730 MHz (corresponding to 5840 MHz center downlink); at this frequency, one wavelength ~ 411 mm. For a 7-element array, I show a 6*L piece of coax, with taps uniformly space L and terminated in its characteristic impedance. There a 7 uniformly tapped steps and at each tap is a high impedance buffer amplifier (think coax Ethernet here). The buffer feeds a *N multiplier (I suggested N-8 because Hittite has some really nice active microwave doublers); alternatively, it could involve a PLL, perhaps with a DRO "puck" as the resonator. The microwave signal from the multiplier feeds a double-balanced mixer to generate BPSK, followed by a PA and the patch antenna element. Note that the BPSK modulation could be done at a lower frequency inside the multiplier, in which case the modulation phase shift is < 180° by some integer divisor.
However, at this point we have not phased the array -- the interelement phase is determined by the tap interval L and the frequency f/N. If we could make the tapped delay line from rubber, then we could get an incremental change in the phase by simply mechanically stretching it. Since we can't change the physical length, can we tune the delay line electronically? I think that the scheme shown in the 2nd "rubber" drawing will work. At each tap point, we add a varactor and set the bias on all the varactors with a D/A. As we change the voltage across the diode string, each diode's capacity will be changed by the same amount, making the equivalent of computer-controlled "rubber". Voila -- we can generate the smooth phase gradient needed to point the antenna.
Note that the phase "swings" from the end where the f/N oscillator injects its signal. But IMHO, we want to "lock" the phase of the central element, half-way down the tapped line. I suggest that, in addition to programming the DC voltage necessary to point the antenna, the computer adds a "DC" constant to each string based on keeping the central element as the phase reference. Note that some of the delay line taps will not have an PA/antenna element attached to it; the degenerate case is the central element which can only be physically present in one of the intersecting arms.
The antenna geometry I tend to favor is the 43-element "12-spoke" ("two bits" * 43 = $10.75) version which has 3 9-element arms (with a single common central element) and 3 6-element arms (with the central 3 elements missing). All 6 would use an 8*L delay line, with the central phase of 5 of the lines slaved to the 6th.
As usual, please enter into a lively, feedback-provoking discussion -- 73, Tom
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_______________________________________________ Via the Eagle mailing list courtesy of AMSAT-NA [email protected] http://amsat.org/mailman/listinfo/eagle
participants (7)
-
Bill Ress -
Dick Jansson-rr -
John B. Stephensen -
Louis McFadin -
Matt Ettus -
Robert McGwier -
Tom Clark, K3IO