Last week whilst taking some images I encountered a signal block and my drone subsequently disconnected from the controller for about 5 seconds.
I wasn’t that far at all from a vessel traffic service radar. Can a radar have an effect on a drone signal or could it have been pure coincidence?
In the past, to test, I’ve taken my MP up right close and personal to that particular radar scanner without any issues, so that far away I’d not expect it to.
It has been replaced and upgraded of late but just found it odd. Maybe it was something else, who knows. 
An “exploratory” flight may be needed, then. 
During my PfCO course we were told radar and antennae can sever the link between a controller and an aircraft as the signal is very weak. Radar etc being much more powerful.
The main concern though was if the compass would be affected which could cause a flyaway.
I’s be fascinated to know if you do ever find out what it was.
It will depend on the type of radar and the frequency of operation.
I expect the most problematic systems will be those that operate in the S-Band (2.3Ghz to 2.7GHz) and C-Band (5.25GHz to 5.975GHz). These are the ITU designated bands so will be mainly Civil Marine and Civil Air systems.
I haven’t worked on radar systems since the 1990’s and then it was mainly 10cm (2.7GHz to 3.7GHz), which may cause desensitisation of the drone’s receiver if the signal is strong enough and the drone close enough to the antenna. The other band was the 3cm band or X-Band (8.5GHz to 10.7GHz),which shouldn’t cause any issues.
There are other bands used for Radar, they’re even found on shortwave, but again would more than likely be of little concern to those flying DJI Drones.
As for interfering with the compass you’d need to be very close to the antenna, within a few feet for that to happen.
