A UAV has many weaknesses:
1. It usually has no warning sensors of an incoming radar signal like RWR etc
2. It doesnt have CHAFF and Flares as standard counter measures.
3. They have straight wings for long loiter time like A-10 but at high altitudes and thus are not agile and manoeuvrable like fighter aircrafts. They fly slow and in straight line with slow turning.
4. Flying slow can also help a radar operator guess quickly that its a uav/ucav as it may have cross section of a stealth aircraft but speed of a very slow helicopter.
5. Most have big engines (e.g 900 hp) and turbo pop engines (Reaper MQ-9) that its hard to conceal heat signatures.
6. They may look small in sky but they are big in sizes usually.
IDEAS:
DETECTION
1.
Portable IR Sensor platforms optimized for UAV detection.
IRST not on an aircraft but on a ground vehicle operated by an AD battery.
2.
Acoustic sensors.
UAV's do have noise because of heavier bigger engines although newer UAV's may have electric propulsion systems to reduce noise signatures.
Also, UAV operator may also shut down engine and let the UAV glide in risky areas by pre defining path and trajectory for a steady glide.
3.
Most important. Targeting
UAV datalinks.
UAV data link is its main vulnerability and a good EW suite can be used to locate and track UAV. This is where jamming a UAV comes in and high powered radars can be brought to use here.
ENGAGING:
1.
Aerial Mines. They work just like anti-helicopter mines
https://en.wikipedia.org/wiki/Helkir_mine
https://en.wikipedia.org/wiki/Bulgarian_anti-helicopter_mines
2. Shoulder based / Man Portable
AD weapons
SAM's and AAA.
3. Using an
Attack helicopter armed with Air to Air missiles.
The slow speed of UAV makes a helicopter suitable for engaging it rather than an F-16 almost stalling in air.
You don't need to scramble your F-16s to take out a drone!! 
