eli5: NASA mostly uses UHF antennas for interspace communication, why?

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eli5: NASA mostly uses UHF antennas for interspace communication, why?

In: Physics
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Ultra high frequency radio waves travel mostly in a straight line and are blocked by solid objects, but do not interact with the ionosphere (the electrically charged layer of the atmosphere energized by the sun). Lower frequency radio waves can bounce off the ionosphere, which is a bad thing when trying to communicate with space.

Plus, the higher the frequency, the more data you can shove into the signal. That’s why wi-fi is also UHF signals rather than operating in the AM/FM bands.

You are off by about an order of magnitude as most communications is in the SHF range. Just looking at the current Deep Space Network data and all of its links are in the 7-8GHz range while UHF is only between 300MHz to 3GHz. The reason for these high frequencies is because of the small size of the equipment and the large bandwidth it is capable of. Radio equipment is generally pretty similar and have roughly the same size measured in wavelength and roughly the same bandwidth measured in percentage of carrier frequency. This means that by increasing the frequency you are also reducing the size of the components including the antenna and also increasing the bandwidth. The bandwidth is important not necessarily in order to transfer data faster but it allows you to add error correcting code which allows you to correct any errors in the data stream which again means you can reduce transmission power. The reason they are not using even higher frequencies is that the atmosphere is absorbing much more of these high frequencies. And while there is not much atmosphere between a ground based antenna and a space probe it is still significant enough to matter quite a bit. NASA is actually testing out communications over frequencies that is even higher where the atmosphere lets it through again. And although we have used it to test communication with some space probes the technology still have some way to go before it can be used reliably.

Lower frequencies wouldn’t be able to penetrate the earth’s atmosphere while higher frequencies would be be attenuated by it (lose a lot of power)

Low frequency radio waves are like a shotgun, they spread out and stick to the ground, AM radio is a good use, you want to hit as much targets as possible but it’s low and slow. UHF signals are like a sniper rifle you only want to hit 1 target but it’s further away. Higher frequencies provide more directional control but are more concentrated.
Airplanes use an in-between frequency: VHF it gives a strong-ish signal over a reasonable range

SHF, actually. There are a couple of reasons for this. One, lower frequencies (pretty much anything under 20 MHz) rarely, if ever, make it past the ionosphere (and it takes a ridiculously large amount of power for those low frequencies to do so). The higher the frequency, the of a chance that the ionosphere reflects anything, and the less attenuation (lowering of power) from the atmosphere, overall. Also, the higher the frequency, the smaller the antenna.

There are some downsides, of course, to using higher frequencies. The higher your frequency, the smaller your footprint, or range. UHF and SHF antennas counter this by being directional antennas (usually, depends on the application) that focus all of their energy in a much tighter beam (like an open bulb versus a flashlight). UHF and SHF also don’t like stuff in the way; they don’t diffract or refract very well, which is why your wifi doesn’t like structural walls.