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Inner Workings of High-Gain Mars Rover Antennas?
cavac asks: "I've been searching for detailed info on how the high gain antennas on the Mars Rovers work, but did not find much useful information except that they DO work. I've been wondering: they are disc-shaped and are approximately the size of a CD. They somehow reassemble parabolic antennas but actually aren't, are they? Anyway, how much use would a parabolic antenna that size have? When I first saw them, they reminded me of the old antennas[*] (enclosed in plastic) used on vacuum tube based radio projects[*]. So, what's really inside the Mars Rovers high gain antennas? Note: Links marked with [*] are german language but the pictures should be self explaining."
The MERs use X-Band for high data rate communications back to earth-- which has a wavelength of 3cm, making high gain antennas considerably smaller and more practical.
It's my understanding that the high gain antenna on MER is a compact phased array design. Even parabolic antennas could be practical at the 3cm wavelength, though they wouldn't be flat (which was obviously preferable for footprint issues).
Before Harris sold it to JetBlue, they developed LiveTV, a system to bring DirecTV to airliners in-flight. The receiver includes a phased array antenna that scans in elevation while sitting on a gimble that allows the beam to be scanned in azimuth.
Phased arrays use lots of power, but that's because each antenna element in the array requires its own amplifier(s) and phase shifter (or time delay unit). Fortunately, those amplifiers cam be much smaller than the monolithic amplifier required to drive a dish (since the signals from each amplifier in the array are summed together).
The rover antenna appears to be an example of a flat-plate phased array antenna, which is a generalization of the "slot antenna". The basics are that you have a feedpoint where energy is coupled to/from a cable which goes to your transceiver. This feedpoint is coupled, either through transmission line divider/combiner networks of the appropriate impedance or the equivalent in waveguides, to each individual radiating element. In this case the radiating elements are segments of the surface of the disc, which happen to be connected electrically (which is not of great consequence). So long as each slot is at least a half-wavelength long, applying an RF voltage across its center lets it radiate just like a dipole perpendicular to the slot. Connecting a large number of slots via feedlines or waveguides so that they are all driven in phase gives you a nice, flat wavefront, which is also what you get from the reflection of a spherical wave off a parabola. The details differ, the result is more or less the same.
None of this would have been strange to a techno-geek of fifty years ago, because geeks of that time were into ham radio instead of computers.
Time is Nature's way of keeping everything from happening at once... the bitch.
As others have pointed out, it's most likely a flat phased array antenna.
There's a couple attributes that would make it attractive for a extraterrestrial application. They're very compact for the gain they provide, and within the limits of the design they can be electronically steered (that is, no moving parts). I would imagine they probably have a mechanical coarse steering mechanism and electronic fine steering.
Sadly I can't seem to find any confirmation of this, just a few mentions of other spacecraft such as MESSENGER using phased array antennas.
If you're really a radio newbie you should know that gain is how well the antenna concentrates the signal. An isotropic radiator basically receives/transmits signals in a perfectly spherical manner. By sacrificing the directional coverage you can increase the gain. A great example is a flashlight bulb -- uncovered it radiates almost everywhere; with a parabolic reflector it radiates a beam. When they talk about using the low gain and high gain antennas they're basically talking about the radiation pattern.
You generally use low gain antennas for signal acquisition when you don't have control over where the antennas are going to be pointed. Once you know where everything is, you can point the high-gain antenna at the target. With more gain you have a better signal-to-noise ratio and can then crank up the data rates.
Phased array antennas work essentially by combining a large number (an array) of simple low-gain antennas such that they add their signals together (in phase) in a particular direction. In other directions the signals don't add the same way and there's much less gain. At microwave frequencies like X-band (about 8 GHz), a simple dipole antenna is only about an inch long, so it's easy to put a bunch of dipole-equivalents in a small space to make an array.
One attains `high gain' by having a narrow beamwidth. That's all `gain' means when referring to an antenna -- the narrower the beam, the higher the gain.
The reason why almost all non-ham-radio antennas are specified in dBi's (decibels over isotropic) instead of dBd's (decibels over a dipole) is that you use dBi's when computing a link margin instead of dBd's. If you use dBd's you will be off by at least 2dB per end or 4dB on a total link - or over half (or double if you look at it the other way) of your power.
A isotropic antenna is basically a perfect omni. Imagine a perfectly round balloon (sphere shaped, not balloon shaped) A dipole (or any other antenna with gain) "squeezes" the balloon to make it "fatter" (higher gain) in the direction what the antenna is pointed. In the case of a dipole, the gain is increased by just over 2dB, making the "sphere" look like you had pushed in on opposite sides of the sphere (think sticking fingers in opposite sides of a balloon till they touch) causing the balloon/sphere to grow in diameter the other direction.