An unmanned aerial vehicle (UAV) was shot down by Turkish aircraft, which made me wonder, whether the Turkish pilots faced any defending moves from the UAV, or once they found it, taking down was a piece of cake?
The existing answers are, of course, correct that most current UAVs do not have much in the way of options for defending themselves against attacks by fighters. This, however, is due primarily to the design missions of most current UAVs, which are reconnaissance and attacking lightly-defended ground targets. Their mission is literally to be an expendable recon or attack aircraft, therefore it doesn't make a lot of sense to invest millions upon millions of dollars in giving them defensive capabilities. Even most normal manned recon aircraft will be short work for a modern jet fighter, with the typical use case being using actual fighters to keep enemies away from them. With a UAV, there's much less incentive to invest heavily in defending it, since they're generally much cheaper aircraft and losing them doesn't mean losing personnel.
Having said all of this, there's nothing inherent about being unmanned that makes an aircraft unable to defend itself. It's just a matter of most current UAVs being designed for use cases where they don't care all that much about defending it, so it's not worth equipping them with lots of excess available lift and thrust, extra sensors, weapons, etc.
There's nothing to say you can't design an unmanned fighter aircraft. For example, if the drone you're trying to shoot down happens to be a QF-16, then you're probably going to have a significantly more challenging mission ahead of you. The QF-16 is exactly what it sounds like - an F-16 that has been modified to act as a UAV. As such, it has the full range of evasive options available to it that you would expect from a normal 4th generation fighter aircraft, since it is one.
The U.S. armed forces use the 'Q' designator to mean that an aircraft is a UAV. This is a 'vehicle type' designator in the 1962 United States Tri-Service aircraft designation system. It is added to the F-16 designation for the unmanned F-16s, but it's also used for all military UAVs, such as the MQ-1 Predator, the MQ-9 Reaper, or the RQ-4 Global Hawk.
QF-16 flying without a pilot:
Can a UAV defend itself?
There were attempts to install defense systems on drones but such weapons have not proved effective, at least up to Nov. 2012 (see the article below).
"In 2002 the Air Force fitted some of its early-model Predator drones with short-range Stinger air-to-air missiles. But even with the right weaponry the robots were likely a poor match for enemy fighters, and the Air Force ultimately stripped them of the missiles. ... The one instance of an air-to-air-armed Predator fighting back against an attacking aircraft resulted in the drone’s destruction.", Source: Predator Drones Once Shot Back at Jets…, authour: DAVID AXE, 11.09.12
Short answer: No.
The drone needs sensors, and if the designers were preoccupied with outfitting it for its main mission and did not include anything to detect an incoming threat, a drone is a sitting duck. But even with complete situational awareness it still is close to a sitting duck.
Even with sensors onboard, the options are limited. If you have IR sensors which can spot the cooled IR seeker of an incoming missile, the drone can throw out flares and hope to confuse the missile seeker. However, modern missiles don't go for the hottest target in the sky, but can use the flares to better find their source. This is called "climbing up the stairs" in the vernacular of missile engineers.
The propeller driven drones are a difficult target for IR seekers which are optimized for the hot section of a jet exhaust, so the chances of being hit by an IR guided missile are low. Therefore, it makes sense not to include anti-IR measures - they cost weight and money.
If the threat is another aircraft, radar helps. But putting a radar transmitter on a drone will make it advertise its presence to all suitably tuned receivers. Better to use passive detection, then.
This would be optical or IR sensors which scan the sky and look for rapidly moving patterns. Once found, the drone can fly evasive maneuvers to get away from the threat. But since it is optimized to loiter in a target area, its lift and thrust reserves are most likely much smaller than those of a fighter aircraft. Consequently, maneuvering will only delay the inevitable contact with the enemy. And since the enemy can maneuver faster, he will be first to be able to shoot a missile or a salvo from his gun. Therefore, adding a missile to the drone will also not help. Without onboard radar, this can only be a short-range IR missile, and this needs to be pointed in the right direction before launch.
In the end, the drone will not benefit much from detecting a threat. Yes, it can transmit footage of the enemy closing in, which might help in identifying what happened and could be used in a political dispute, but then drones are built for flying where you don't want to send manned aircraft, so the value of that footage is most likely limited.
This is the price you pay for using low-cost drones instead of fully developed combat aircraft. Once the drone is detected, a determined adversary has little problem taking it out.
A UAV or UCAV can potentially defend itself from attack by other aircraft using ECM, evasive manouvering, the use of weaponry, or a combination of these depending on what is most effective and appropriate in a specific tactical situation. The development of low cost, solid state, light weight infrared sensors is an important recent advance, which makes it possible to equip an autonomous UCAV with the means to detect threats from missiles and manned aircraft, and react by taking evasive action or, potentially, by counter-attacking the threat.
Because it does not need to incorporate the cockpit structure, pilot and canopy, a UCAV can be smaller and have superior stealth characteristics compared with a manned fighter, other things being equal. In practice this would probably be more significant in air to air combat than superior high g manoeuvering capability, given the ranges at which air to air combat typically takes place, ie 10 to 40 km. The same rules apply to manned and unmanned aircraft as far as the physics involved in high g manoeuvering are concerned, which is to say that such capability can only be obtained at the expense of range and payload so that in the end a trade-off has to be made. That said, the ability of a UCAV to turn at 18 g could potentially decide the outcome of a dogfight with a manned fighter.