The United States is already trialing the technology against drones (Picture: US Army).
By David Hambling, technology expert
The Ministry of Defence recently announced plans to put £130 million into Directed Energy Weapons which strike at the speed of light.
Two types of device are mentioned: lasers, universally familiar from science fiction, and radio frequency weapons.
While lasers get most of the glamour, radio frequency weapons may turn out to be more important in the long run because of their ability to target electronics.
What are radio frequency weapons and how do they work?
A radio frequency weapon uses radio waves at power levels high enough to cause electrical disruption.
Microwaves (high-frequency radio waves) are usually preferred, so this type of device is sometimes called a High-Powered Microwave, or HPM weapon.
Radio frequency weapons also overlap with Electromagnetic Pulse, or EMP weapons, which emit a powerful burst of radio waves in all directions.
Unlike jammers, which just interfere with the signal received by radios or radar, radio frequency weapons do actual damage.
They do this by two methods: 'front door coupling', where radio waves are picked by the antennas and aerials normally used for receiving, and in 'back door coupling', where wires inside the electronics act as receivers.
Either way, the effect of an extremely powerful pulse is to put far more current through a component than it can handle.
With front door coupling, this can be strong enough to permanently burn out components, while back door coupling is generally much weaker but will disrupt systems and cause them to malfunction.
This may only be a temporary irritation if it puts a radio briefly out of action, but if flight controls are knocked out then an aircraft is likely to crash.
The technology for radio frequency weapons has been around for decades - the Ministry of Defence has been researching this area for at least 30 years and tested a prototype weapon in 2002.
There was little motivation to develop them into end products at the time, though, because they were competing with highly effective existing weapons - missiles and guns.
What issues do these weapons pose to the military forces using them?
Radio frequency weapons also have several drawbacks.
One problem is that, unlike a missile, the effects of a hit are highly unpredictable.
What happens depends not just on the exact electronics on board the target, but also on which way it is facing – how strong a signal an antenna receives depends on which way it is pointing.
In addition, radio frequency weapons do no external damage.
Therefore, if an enemy tank has been hit with such a weapon, you cannot tell whether it has been effectively knocked out with the ignition, sensors, fire control and communications all out of action, or whether it is completely undamaged.
Commanders prefer to see a burning wreck to confirm the kill.
Another issue is friendly fire - electronics vary hugely in how vulnerable they are.
A radio blast powerful enough to guarantee destruction of a particular target may affect more sensitive electronics some distance away, including friendly forces.
The US Air Force has ruled that such weapons should only be mounted on unmanned aircraft, suggesting considerable danger from radio reflections and other unforeseen effects.
However, a new threat has emerged in recent years which has proven highly challenging for existing weapons: small drones.
These are too numerous to be tackled by surface-to-air missiles, and too small and agile to be engaged with guns.
Radio frequency weapons look like the ideal answer.
They have an endless supply of ammunition as long as the power supply lasts.
They move at the speed of light so aiming is easy and targets cannot evade and they can produce a fan of energy, not just a narrow beam, taking out an entire swarm in one go.
The UK has invested £130m into Directed Energy Weapons, but who else is interested?
Other nations are already developing their own versions.
The US military recently unveiled its Tactical High-power-microwave Operational Responder (THOR).
It looks like a shipping container with a giant satellite dish on top with 360-degree coverage.
It produces rapid pulses of energy which can reportedly disable small drones at ranges of up to 2,000 metres and is due to be tested in September.
Work is currently focusing on how to integrate it with air defence radar systems.
A second American radio frequency weapon known as PHASER has been developed by Raytheon – the radar company that invented the microwave oven.
In a demonstration last year, PHASER knocked a series of drones out of the sky, sometimes two at a time.
"You can simultaneously track and immediately move to the next target to address not just a swarm, but multiple swarms," according to Raytheon’s Don Sullivan.
A third US system, known as 'Counter-Electronic High-Power Microwave Extended-Range Air Base Air Defense' (CHIMERA) project, will start testing in 2020.
The German company Diehl have been working in a microwave weapon specifically for the counter-drone mission. This uses a sophisticated focusing method called time reversal to concentrate the microwave beam on the target.
No details have been released so far, but it is at the stage of testing against airborne targets.
Russia and China are also known to be working on competing, high-power radio frequency weapons, but the public statements on them have been vague and neither seem to be close to fielding a working system.
However, while radio frequency weapons may look like a magic wand to make drone swarms disappear, as usual every new weapon prompts the other side to develop countermeasures.
In this case, protection is likely to take the form of a ‘Faraday Cage’ - a mesh of conductive material which can block even high-power radio waves.
Many military systems already have extensive 'hardening' of this sort against the damaging electromagnetic pulses produced by nuclear weapons which will also stop radio-frequency weapons. Soon small drones may gain similar protection.
Such protection may reduce the effectiveness of radio-frequency weapons, but it will add to the cost of drones and the point of a swarm is that the individual members must be cheap.
Any unprotected drones are likely to be zapped extremely easily by the new generation of radio frequency weapons.