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Plasma Resonance Could Overcome Radio Silence For Returning Spacecraft
Zothecula points out this article about a workaround for a long-standing problem with space-flight communications: some of the most cruicial time of a re-entry is also time when the craft cannot send data to or receive instructions from the ground controllers. From the article: Returning spacecraft hit the atmosphere at over five times the speed of sound, generating a sheath of superheated ionized plasma that blocks radio communications during the critical minutes of reentry. It's a problem that's vexed space agencies for decades, but researchers at China's Harbin Institute of Technology are developing a new method of piercing the plasma and maintaining communications.
This means coupling the craft's antenna to that plasma sheath, "[causing] the sheath to act as an inductor. Together, they create a resonant circuit."
I thought this was no longer an issue? I think continuous communication had been in use for over a decade with the space shuttle before the end of the program. The solution was to use satellites, being on the other side of the plasma sheath, as relays to communicate between a reentering craft and the ground..
The effect is the same. Usually the capacitance is adjusted to series resonate with the inductance from length of feedline going to the drive point tap. Whether it's a physical series capacitor, or the effective capacitance of a coupling sleeve doesn't change the theory. Obviously the feedpoint impedance will vary with the length of the plasma plume. The transmission line length will be relatively short, so mismatched impedance isn't a problem there. The interfacing electronics needs to tolerate wide variations. While dynamic matching could be used, it would be tempting to have the operating frequency follow the resonance of the plasma plume. Unfortunately nearby planets might gripe about interference to other spectrum users. But if they set of an EMP first...
There is prior art for that... (load conditions, through feedback, setting the frequency) 7 inch 1945 Motorola television sets had a sleeve around the glass of the high voltage rectifier, essentially a feedback capacitor. It fed the grid (input) of the tube driving the high voltage transformer. Although it looked odd, the spring around that tube was very much a required part in the circuit. They could have used a manufactured capacitor, but it would have had to handle about 7,000 volts. The inductance of the transformer secondary resonating with the total of its own capacitance and that of the wiring and rectifier, set the oscillation frequency. Unlike larger screen sets that used magnetic deflection of the c.r.t. beam, the radar/scope-like c.r.t of the 7 inch sets had electrostatic deflection. That meant the high voltage transformer didn't have to run at a precise frequency to drive scan coils on the c.r.t. from a portion of the transformer winding.
https://web.archive.org/web/20...
So the plasma antenna should work, and it's probably less complicated than a 1945 television.