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Is it practical for HF skywave propagation to be utilized for space exploration robot telecommand when outside of microwave/mm wave range? Currently it appears that only the C band and UHF is used in general (on Mars). After all, some other planets (incl. Mars) do have an ionosphere, which can potentially be utilized for low-throughput intra-planetary communications, just like it has been traditionally done here on Earth.

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  • $\begingroup$ What is outside of microwave/mm wave range? Wavelengths below 1 mm? $\endgroup$ – Uwe Jul 27 '18 at 10:43
  • $\begingroup$ @Uwe Any line-of-sight propagation mode (f > 30 MHz/lambda < 10 m) is obviously limited by the radio horizon, unless there exist terrestrial repeaters, communications satellites, natural reflective objects (e.g., moons), etc. $\endgroup$ – Ruslan Nabioullin Jul 27 '18 at 11:17
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Assuming that you have a "good enough" ionosphere it is possible to use HF for telecommand, however microwaves will be more efficient.

HF has quite long wavelengths (meters, tens of meters). Antenna for such frequency is a fraction of the wavelength (eg. quarter or half wave), ie. an efficient antenna will be physically large compared to a microwave antenna (there are ways to make smaller HF antennas at the cost of efficiency). Large = heavy.

HF antennas have very poor bandwidth compared to microwave antennas. Again wideband antennas are even larger. Small bandwidth = small throughput.

Next issue - gain. It is much easier to focus microwaves than HF. Every dB of antenna gain can be spent on smaller transmit power (ie. power efficiency). HF directional antennas are large. Large = heavy and difficult to turn around.

I did not mention yet the atmospheric effects, noise floor, thunderstorms, space weather etc. (they affect microwaves less than HF).

You may try to get around the above limitations by more signal processing, but nevertheless the end result will not be compact, nor fast, nor power-efficient. That is why terrestrial HF today is mostly limited to specialized communication (ships, aircraft - and only on specific routes), some broadcasting, amateur radio and high-frequency trading.

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  • $\begingroup$ It is not the antenna that have a very poor modulation bandwidth, it is the frequency itself. The bandwidth is also small when the frequency is used over a cable without an antenna. $\endgroup$ – Uwe Jul 27 '18 at 14:10
  • $\begingroup$ @uhoh Well that's the whole point...imagine a base on the surface of Mars engaged in telecommand via HF skywave (utilizing the Martian ionosphere) to rover(s) under moderate AI control far away, but on the surface of Mars, beyond the radio horizon. $\endgroup$ – Ruslan Nabioullin Jul 28 '18 at 6:27
  • $\begingroup$ @RuslanNabioullin whoa I completely misread the question, my mistake! Yes I was thinking of 1. robotic satellites, not robotic surface vehicles, and 2.only Earth's ionosphere. This is a really, really interesting question btw! QRP on Mars! There is no or negligible lightning artificial sources of RF noise. Time to go read up on the Martian ionosphere! $\endgroup$ – uhoh Jul 28 '18 at 7:48

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