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@NgPh's answer to How will the LICIACube cubesat transmit DART impact images back to Earth? What kind of antenna and radio will it use? quotes LICIACube on DART mission: an asteroid impact captured by Italian small satellite technology (SSC20-WKIII-06 from the 34th Annual Small Satellite Conference):

A paper authored by Simonetti: LICIACube on Dart Mission ... provides corroboratively the following information:

The [communication] subsystem includes an X-Band transponder that manages downlink and uplink communications and is connected to four X-Band patch antennas.

Hence, the X-band is confirmed. Further,

The set of antennas consists of:

  • The main pair composed by a transmitting antenna, with 22 dBi gain, and a receiving antenna, with 6 dBi gain, both placed in the solar side structural panel;
  • The secondary pair composed by a transmitting antenna, with 12 dBi gain, and a receiving antenna, with 6 dBi gain, both placed at the opposite side of the main pair.

In this answer I find that an X-band downlink (space to Earth) frequency may be about 8.4 GHz. For a circular aperture which approximates a flat patch antenna array as well as a sphere approximates a cow, a wavelength of 3.56 cm would need a "dish" diameter of

$$G \approx \left( \frac{\pi D}{\lambda} \right)^2$$

$$D \approx \frac{G^{1/2}}{\pi} \lambda \approx 4 \lambda \approx 14 \text{cm}.$$

Dart is a 6U cubesat so one 10 x 30 cm side could easily support a patch antenna of a similar area, so the numbers work.

But the radiation pattern of the patch array could be complicated and lumpy and offer maximum gain in an awkwardly shaped pattern constraining the spacecraft attitude during transmit modes.

Recall that a patch array will likely be some planar arrangement of individual flat radiators in such a way that their amplitudes add in-phase in a given direction, mostly likely normal to the surface. Such a planar, passive like the single user Starling ground terminals. where the elements have different and variable phases so as to steer the beam.

That's opposed to a phased array where the elements are phased in groups or individually in order to steer the beam electronically.

So I'd like to ask:

Question: How does a 22 dBi X-band "patch" antenna get so much gain and how well behaved is its high-gain radiation pattern?

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  • $\begingroup$ According to Wiki, "phased array" implicitly implies "electronically steerable". This is to contrast with passive phased arrays, in which the "look angle" is set with fixed (passive) phase shifters. $\endgroup$
    – Ng Ph
    2 days ago
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    $\begingroup$ According to this other Wiki, a patch antenna is low-profile so that it can be mounted on a surface. Further, Multiple patch antennas on the same substrate (see image) called microstrip antennas, can be used to make high gain array antennas, and phased arrays in which the beam can be electronically steered. $\endgroup$
    – Ng Ph
    2 days ago
  • $\begingroup$ I'm not sure what the question is here. Even a "patch antenna" can be designed with different size patches so as to do the desired beamforming. Further, it's not unusual for an antenna array to consist of several linear patch arrays, each line addressable with a different phase. $\endgroup$ 2 days ago
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    $\begingroup$ No issue. Just for everybody to be on the same page (about terminology). Yes, Wiki is not systematically the reference. But for these terms, its definitions are correct. $\endgroup$
    – Ng Ph
    2 days ago
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Being non-expert in patch array antennas, I can't answer the "how".

Nevertheless, X-band antennas in this range of gain and compatible in form factor to 6U Cubesats are commercially available.

Here is an example of an 8x4. The datasheet says it has 20.7-21 dBi and it measures 20 cm x 10 cm x 0.3 cm., weight 90 g.

Of course, the beam pattern cannot be symmetrical, but it doesn't look "ill-behaved" (whatever this means). This simply implies that LICIACube must maintain a better pointing in one direction ("X-cut") than in the orthogonal one ("Y-cut"). The Half-Power Beamwidth is 10° for X-cut and 18° for Y-cut (see datasheet). I can't see any big constraint from this.


From the above linked "High-Gain X-Band Microstrip Patch Array for CubeSat Systems" example and PDF datasheet:

High-Gain X-Band Microstrip Patch Array for CubeSat Systems

High-Gain X-Band Microstrip Patch Array for CubeSat Systems

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