Saturday, March 30, 2013

Contemplating Privacy and Radio Communications 5

Something I neglected to cover yesterday - not deliberately; I just couldn't find reference details for it - is how resistant to interference or jamming these digital modes are.  I can tell you they won't be as resistant as a specially designed communications data link because they just don't have all the attributes of one, but I can't give you numbers for how resistant they are.  Unfortunately, I don't have a lot of operating experience with the various wider band/faster amateur digital modes.  Also, the exact effect of an interferer or jammer would depend on what sort of radio system we're using, FM or Single Side Band (SSB).  An SSB radio has the effect of taking the modulation band we produce at audio and converting it to a small band of radio frequencies, wherever we choose to place it.

Jamming is complex phenomenon, but it usually works by overloading the radio; exposing it to a signal so strong that some part of the radio can't respond properly.  Some systems are easier to jam than others, but basically older modulation forms (AM and FM) are easier.  Depending on the modulation mode and type of jammer, it can cause the detector to just respond to the jammer.  AM radios will receive everything on the channel they're tuned to, and make annoying squeals and squawks.  FM radios have a limiter that amplifies everything to high level, and in doing so, "captures" the stronger signal.  Even a slightly higher signal level can dominate the audio delivered to the speaker.  (In an engineering lab I worked in, ages ago, one of the guys hated the music another guy tuned in on his FM radio.  His answer?  Put a signal generator on that FM frequency and stick a small wire in it's output connector as an antenna.  The generator's unmodulated output made the FM receiver produce no audio output).  The weak point of jammers is that they need to either put lots of power everywhere, or they need to some power on frequencies they know an "enemy" is using.  The second one is much cheaper and easier. 

In the case of these complex MFSK modes, especially the ones with error coding like Olivia or MT63-2000, I imagine they'd be rather resistant to jamming, because they're designed to be resistant to interference.  Some jammer effects can probably be made up by error correction.  They'd need to wipe out the entire audio passband.  I believe it would be easier to find and jam a narrowband FM system running these modes than to find and jam an SSB system running the same modulation.  (This page has tabs for many of these modes that lead to descriptions of how they work).

True story about jammers.  I heard this as hall talk, I did not see it with my own eyes, but have no reason to doubt it.  Major Southeast Defense Contractor made an AJ (Anti-Jam) modem for the military.   The buyers specified a variety of jammers it would be tested against, with very specific parameters.  Late in development, after all these specified tests were all passed, someone happened to turn on a Dremel Moto-Tool next to the system while it was passing data.  The Dremel completely jammed this complex, multimillion dollar data link better than all of the jammers specified by the customer. 

I tell this to bring up a simple point: jamming can work both ways, just like tracers.  It's not just simple systems that can be jammed by simple methods.   I'm picturing a huge Dremel now... maybe 40 feet tall... just kidding.  Maybe a TIG or MIG welder.  But with online stores selling GPS jammers, movie theaters adding cellphone jammers, and other electronics stores selling jammers of all types, it is becoming a rather brave new world these days, isn't it?  I've heard of people going down the highway, with their in-dash or other GPS receiver telling them when the next turn is coming, suddenly losing their GPS link when a truck or panel van goes by. 
(an FM broadcast jammer schematic.  Takes out the entire FM band.  Quite illegal.  Not responsible for you.  Educational use only.  YMMV.  Do Not Remove This Mattress Tag.  Blah blah blah)

Probable conclusion tomorrow...


  1. GPS is an incredibly weak signal, so jamming it is very easy. LORAN and OMEGA are (were) much higher powered and in some ways a lot more difficult to jam. And when you start doing serious phase analysis and accounting for differences in day/night propagation they could be way more accurate than the book said. Just like the "fact" that it was impossible to jam more than 3Kbits/sec down a phone line.

    Uh, me, a geek?

  2. is worth a look at.

    1. Didn't look particularly useful as a receiver, but for its intended use, it might be good enough.