In Ukraine's eastern Donetsk region, soldier Mykola shows off his drone signal jamming system: it may look like just a large white thermos, but it serves as a crucial shield for both sides in the war.

Scramblers have become a mainstay in recent months in Ukraine, used to neutralise the little killer drones that now saturate the sky over battlefields.

More than two years into Russia's invasion, the devices have become crucial for Ukrainian forces.

Mykola describes them as being "like a bullet-proof vest that adds a bid of protection, a bid of confidence."

"And so the chances of survival go up," the soldier, from the Achilles drone battalion of the 92nd brigade, told AFP.

The system weighs four kilogrammes (nine pounds) and fits into a backpack, making it "one of the first portable electronic protection devices" of the Ukrainian infantry, Mykola said.

"It protects our infantry from Russian FPV drones."

First Person View quadcopter drones are small, relatively cheap devices that come with a camera providing the pilot with live images from the field -- as if he were right there aboard the device.

Equipped with an explosive charge, they serve for direct, kamikaze strikes on enemy units within a radius of a couple of kilometres.

"Drones kill more soldiers on both sides than anything else at present," Ukraine's Ground Forces Commander Oleksandr Pavliuk recently told British newspaper The Times.

To neutralise them, the scrambler emits jamming signals on the same frequency as the drone control signal, thus cutting off the link between the device and its pilot.

- 'Loss of control' -

Mykola said his device is effective at a distance of 30 metres (100 feet) and leads to a "total loss of control" for the enemy FPV.

Designed by a Ukrainian company and funded with private donations, the scrambler has been available on the battlefield since this winter.

Other larger types of portable jammers also equip an increasing number of four-wheel-drive vehicles used by Ukrainian troops in critical areas.

And "large, remote-controlled electronic warfare stations" are also located behind the front, said Mykola.

The Russians use jammers too.

One Ukrainian drone pilot, a 22-year-old who goes by "Coyote", admits to losing up to 40 percent of his devices to enemy interference.

To defend against Russian scramblers, nothing matters more than experience, the 28th brigade soldier said.

"If you know which frequencies (the scramblers) operate on, because you've already flown there, you can change the frequencies" of the drone, he said.

"Otherwise you can try to fly around the scrambler, to fly over it."

- 'Giant battle' -

According to Mykola, Russia was one step ahead as it had been working on electronic warfare systems "for over 30 years now".

He said Ukraine only "began to develop equipment starting in 2014", when Russia annexed its Crimea peninsula.

Today, "everyone is on the lookout for free frequencies to produce FPVs and other drones operating on them. And everyone is mulling how to counter the frequencies".

Each side also comes up with sometimes surprising ways to counter drones.

Footage has shown Russian tanks entirely covered with welded sheet metal plates, with only the cannon protruding -- dubbed "turtle tanks" on social media.

And Ukrainian and Russian troops alike have filmed themselves placing large fishing nets over their trenches.

A senior officer specialising in electronic warfare said Ukraine is currently able to jam 60 to 70 percent of Russian FPV drones.

But "every three months, we need to think of new methods," the Ukrainian told AFP.

"It's a giant battle."