[eagle] Re: Another idea on phased array configuration

Bill Ress bill at hsmicrowave.com
Wed Apr 25 17:58:00 PDT 2007


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
>
>
>
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