If you can parlay it into an 1/8 wavelength, 25cm is probably not impossible to fit into some monstrously meandered beast that could fit in a phone's footprint. We've come a long way in terms of miniaturizing antennas, though I'm no expert.
But I think your second point hits the nail on the head - cell phones have enough antennas in them, and asking to add another one at such a low frequency is a great way to get your antenna and QA teams to look for a more sane employer, and to put your EMC compliance partner's kids through college :)
Fun fact: it's very difficult to get rid of built-up charge in space. And guess what incident EM radiation does to your electronics: that's right, it can build up charge! To my understanding, this is especially a problem when charge is built up in specific dielectrics/insulators or the very delicate structures of modern transistors and other semiconductor devices. This so-called "total ionizing dose" [1] can lead to "leakage currents, degrade the gain of a device, affect timing characteristics, and, in some cases, result in complete functional failure" [2].
Any RF or EMC engineer will tell you that ground is a dirty word, especially in extreme conditions. There's no one-size-fits-all approach, since so much depends on the physical context.
Depends on the definition of "simple", imo. The first thing that comes to mind is research into materials with good (tens of dB), wideband absorption in the mmWave bands. It's an area of active research [1] [2] (just a couple articles from a quick google, so caveat emptor).
My two cents, though I'm no expert: I'd bet it's what the computational EM community (and other fields) calls a "multiscale" problem. EM solvers - that is, simulators that numerically solve Maxwell over some geometry-and-source boundary conditions - find E- and H-fields at certain "mesh points". In other words, they discretize 3-space into a grid and calculate solutions to Maxwell at those points.
In general, you'll want your mesh to have subwavelength distance between points, and perhaps even less in regions with complicated geometry or parts of your geometry you're particularly interested in. In the microwave regime, this means mesh points will typically have tens of centimeters or less between them. However, given that the receiving antennas in satellite-based solar power are orders of magnitude larger than that, trying to simulate such a large structure and still keep your mesh points relatively dense is just asking for the curse of dimensionality to bite you.
In other words, it's certainly possible with enough compute time, but we have better things to do with our GPU cores, especially since the whole point of antenna simulation is to assist with design by allowing you to run a bunch of simulations to tune your design without having to fabricate a bunch of DUTs. Again, I'm not really an expert, but my understanding is that this kind of multiscale problem is a hot research topic right now, not only in computational EM but in many other areas of physics simulations, especially those governed by nasty PDEs (e.g. fluid dynamics) or those which involve complicated structures at multiple scales (e.g. VLSI design).
But I think your second point hits the nail on the head - cell phones have enough antennas in them, and asking to add another one at such a low frequency is a great way to get your antenna and QA teams to look for a more sane employer, and to put your EMC compliance partner's kids through college :)
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