Sender and receiver have synchronized clocks and spreading keys so know what frequency to use when. Hop time is short enough jammer can’t respond in time.
Fundamentals of Jamming radio signals (doesn’t favor jamming)
Jammer fills big chunk of radio spectrum with some amount of watts/MHz of noise
more than enough to drown out civvy comms like wifi that use <<1W signal spanning 10-100MHz of bandwidth even with short link far away from jammer.
Comms designed to resist jamming can use 10W+ and reduce bandwidth of transmission as much as needed at cost of less bits/second.
low bandwidth link (100kb/s) with reasonable power budget is impossible to jam practically until jammer is much much closer to receiver than transmitter.
GPS and satcom signals easy to jam because of large distance to satellite and power limits.
Jamming increases required power density to get signal through intelligibly. Transmitter has to increase power or use narrower transmit spectrum. Fundamentally signal to noise ratio decreases and Joules/bit increases.
Communication Stealth
Jammer + phased array antennas + very powerful computer gives ability to locate transmitters
Jammer forces transmitters to use more power
Phased array antennas + supercomputer:
computer calculates/subtracts reflected jamming signal
Phased array antenna+computer acts like telescope to find “dimmer” signals in background noise lowering detection threshold
EMP mostly affects power grid because power lines act like big antennas. Small digital devices are built to avoid internal RF like signals leaking out (thanks again FCC) so EMP doesn’t leak in very well. DIY crud can be done badly enough to be vulnerable but basically run wires together in bundles out from the middle with no loops and there’s no problems.
Only semi-vulnerable point is communications because radios are connected to antennas.
Best option for frying radios isn’t EMP, but rather sending high power radio signal at whatever frequency antenna best receives.
RF receiver can be damaged by high power input but circuitry can be added to block/shunt high power signals. Antennas that do both receive and transmit (especially high power transmit) may already be protected by the “switch” that connects rx and tx paths for free. Parts cost would be pretty minimal to retrofit though. Very high frequency or tight integration makes retrofitting impractical. Can’t add extra protection to a phased array antenna like starlink dish but it can definitely be built in.
Also front-line units whose radios facing the enemy are being fried are likely soon to be scrap (hopefully along with the thing doing the frying).
RF jamming, communication and other concerns
TLDR: Jamming is hard when comms system is designed to resist it. Civilian stuff isn’t but military is and can be quite resistant. Frequency hopping makes jamming ineffective if you don’t care about stealth. Phased array antennas are getting cheaper and make things stealthier by increasing directivity.(starlink terminal costs $1300 and has 40dbi gain). Very expensive comms systems on fighter jets using mm-wave comms and phased array antennas can do gigabit+ links in presence of jamming undetected.
civilian stuff is trivial to jam
EG:sending disconnection messages to disconnect wifi devices requires very little power
most civvy stuff sends long messages, if you see the start of a message you can “scream” very loudly to disrupt part of it and it gets dropped.
Civvy stuff like WIFI BT and cellular has strict transmit power limits typically <1W of transmit power.
TLDR: jamming civvy stuff requires less power than transmitting it. Still, amplifiers and directional antennas can help in the short term.
military stuff hops from one frequency to another using a keyed unpredictable algorithm.
Sender and receiver have synchronized clocks and spreading keys so know what frequency to use when. Hop time is short enough jammer can’t respond in time.
Fundamentals of Jamming radio signals (doesn’t favor jamming)
Jammer fills big chunk of radio spectrum with some amount of watts/MHz of noise
EG:Russian R-330ZH puts out 10KW from 100MHz to 2GHz (approx 5KW/GHz or 5W/MHz)
more than enough to drown out civvy comms like wifi that use <<1W signal spanning 10-100MHz of bandwidth even with short link far away from jammer.
Comms designed to resist jamming can use 10W+ and reduce bandwidth of transmission as much as needed at cost of less bits/second.
low bandwidth link (100kb/s) with reasonable power budget is impossible to jam practically until jammer is much much closer to receiver than transmitter.
GPS and satcom signals easy to jam because of large distance to satellite and power limits.
Jamming increases required power density to get signal through intelligibly. Transmitter has to increase power or use narrower transmit spectrum. Fundamentally signal to noise ratio decreases and Joules/bit increases.
Communication Stealth
Jammer + phased array antennas + very powerful computer gives ability to locate transmitters
Jammer forces transmitters to use more power
Phased array antennas + supercomputer:
computer calculates/subtracts reflected jamming signal
Phased array antenna+computer acts like telescope to find “dimmer” signals in background noise lowering detection threshold
Fundamental tradeoff for transmitter
Shannon channel capacity
Even with perfect spreading of signal across available spectrum, there’s fundamental capacity limits to avoid detection.
directional antennas/phased arrays
military planes use this to communicate stealthily
increases power sent/received to/from particular direction
bigger antenna with more sub-elements increases directionality/gain
Starlink terminals are big phased array antennas
this quora answer gives some good numbers on performance
Starlink terminal gives approx 3000x (35dbi) more power in chosen direction vs omnidirectional antenna
Nessesary to communicate with satellite 500+km away
Starlink terminals are pretty cheap
smaller phased arrays for drone-drone comms should be cheaper.
drone that is just a big Yagi antenna also possible and ludicrously cheap.
stealthy/jam immune comms for line of sight data links at km ranges seem quite practical.
development pressure for jam resistant comms and associated tech
little development pressure on civvy side B/C FCC and similar govt. orgs abroad shut down jammers
military and satcom will drive development more slowly
FCC limits on transmit power can also help
Phased array transmit/receive improves signal/noise
This is partly driving wifi to use more antennas to improve bandwidth/reliability
hobbyist drone scene could also help (directional antennas for ground to drone comms without requiring more power or gimbals)
Thanks for the info. What about RF weapons that is a focused short or EMP pulse against a drone. What range and countermeasures?
EMP mostly affects power grid because power lines act like big antennas. Small digital devices are built to avoid internal RF like signals leaking out (thanks again FCC) so EMP doesn’t leak in very well. DIY crud can be done badly enough to be vulnerable but basically run wires together in bundles out from the middle with no loops and there’s no problems.
Only semi-vulnerable point is communications because radios are connected to antennas.
Best option for frying radios isn’t EMP, but rather sending high power radio signal at whatever frequency antenna best receives.
RF receiver can be damaged by high power input but circuitry can be added to block/shunt high power signals. Antennas that do both receive and transmit (especially high power transmit) may already be protected by the “switch” that connects rx and tx paths for free. Parts cost would be pretty minimal to retrofit though. Very high frequency or tight integration makes retrofitting impractical. Can’t add extra protection to a phased array antenna like starlink dish but it can definitely be built in.
Also front-line units whose radios facing the enemy are being fried are likely soon to be scrap (hopefully along with the thing doing the frying).