> Not surprisingly a very common failure mode is that if you induce currents in the coils of the brushless motors
No, that doesn't happen. Currents can be induced in the wires to the motors, but not in the motors themselves. For one thing, the outside surface of the motors is the aluminum rotor which is an extremely effective faraday cage. For another, coils don't act like antennas. Loops of wire in an electric field have the exact same voltage difference as a straight wire.
> Shielding helps of course, adds expense and adds weight, the two things that cut into how many you can make for $X and how far they can fly.
Shielding adds virtually zero weight; carrying a spool of fiber optic cable adds a lot of weight. All the drones in Ukraine right now are fiber optic but most of them are unshielded... the reason why is not that shielding is heavy, it's just that there are lots of jammers but very few truck-sized weapons intended to totally disable drones.
That's also assuming it would even work on a drone without an antenna. If these weapons are not relatively broad-spectrum then they will be very sensitive to the particulars of the circuitry, and they won't always work.
Currents can be perfectly well induced in the motors themselves, by the variable magnetic field of an electromagnetic wave.
Any electromagnetic wave has both an electric field and a magnetic field, hence its name.
An antenna can be made from either a straight wire sensitive to the electric field or from a loop of wire sensitive to the magnetic field.
The only reasons why a motor is usually a bad antenna is that it should have a case with good shielding properties (i.e. the magnetic circuits have only small gaps) and the high inductances of its windings act as low-pass filters for high-frequency induced currents, like those of a microwave transmitter.
There exist electric motors with very low inertia of the moving parts (to enable high accelerations), where the rotor does not have any ferromagnetic material and the stator has large gaps for the rotor. Such motors can be much more efficient antennas than standard motors, but such motors are not used in drones.
All the cheap radios for under 30 MHz signals used antennas made of a ferrite bar with a coil on it, very similar to a motor winding, except that the magnetic circuit had a much greater gap than in a motor, because they were more sensitive at small sizes than antennas sensitive to the electric field.
Moreover, brushless motors do not have an aluminum rotor. You are thinking about AC induction motors. Induction motors do not have brushes, but nobody calls them brushless, because they never had brushes. Only DC motors are called brushless, because their classic variant had brushes, which are replaced by power transistors in brushless motors.
The aluminum rotor of induction motors is normally inside, not outside. The inverted construction is rare.
Both induction motors and brushless motors have windings only on the stator, which is the external part in the normal motor structure, and those are equally susceptible to variable magnetic fields, except that they are usually bad antennas for the reasons mentioned above, especially at microwave frequencies.
In an ideal motor, the stator is not an electrical conductor (which is actually the case for ferrite stators), so it has no shielding properties for electric fields, but it has shielding properties for the magnetic field, if the gaps in its magnetic circuit are small.
Almost all quadcopter brushless motors are constructed as “outrunners,” with a fairly thick, sturdy aluminum motor bell (rotor!) with strong permanent magnets glued inside of it and the stator in the center. I agree that they are not immune to RF but at high frequencies it will require a really comical amount of power to do anything to one.
Oh it will absolutely work on a drone without antenna: in microwave range every PCB trace becomes an antenna. If the field is strong enough it'll just blow the gates on IC inputs. If it's far away it can still do soft upset of any periodic signal, e.g. one of the numerous clocks on digital circuits.
But typically with a much smaller number of turns. A motor coil should have a decently high inductance and thus act as an antenna only for pretty low frequencies.
It is not the number of turns that matters to distinguish coil antennas from motors, though indeed a high number of turns in both motors and antennas leads to a high inductance, which ensures that any resonance frequencies will be low, so a received radio signal of high frequency will not be amplified by a resonance.
The magnetic circuit of a coil antenna has a very big air gap, because its ferromagnetic core usually has the form of a cylinder or of a prism and the magnetic circuit closes through the air between the opposite ends of the core.
The magnetic circuit of a motor has only small air gaps between stator and rotor, which are required to allow the rotor movements. Because of the small air gaps, the inductance of a motor winding is much higher than the inductance of a coil antenna with the same number of turns and using a ferromagnetic core made of the same material.
No, that doesn't happen. Currents can be induced in the wires to the motors, but not in the motors themselves. For one thing, the outside surface of the motors is the aluminum rotor which is an extremely effective faraday cage. For another, coils don't act like antennas. Loops of wire in an electric field have the exact same voltage difference as a straight wire.
> Shielding helps of course, adds expense and adds weight, the two things that cut into how many you can make for $X and how far they can fly.
Shielding adds virtually zero weight; carrying a spool of fiber optic cable adds a lot of weight. All the drones in Ukraine right now are fiber optic but most of them are unshielded... the reason why is not that shielding is heavy, it's just that there are lots of jammers but very few truck-sized weapons intended to totally disable drones.
That's also assuming it would even work on a drone without an antenna. If these weapons are not relatively broad-spectrum then they will be very sensitive to the particulars of the circuitry, and they won't always work.