• EEVblog #368 – EPIRB Teardown

    Teardown Tuesday
    What’s inside an analog 121.5MHz EPIRB/PLB Personal Locator Emergency Beacon, and how do they work?

    Be Sociable, Share!

      About EEVblog

      Check Also

      EEVblog #821 – Mailbag

      EEVblog #821 – Mailbag

      More Mailbag! Extended Teardown Video HERE Forum HERE SPOILERS: Military Mystery Item Teardown – UPDATE: ...

      • Joop

        What you can also do is put the SA on smallest BW in “max hold” mode and let it run for a while. You probably will see the sidebands of the AM signal 700 to 1400 Hz away from the carrier. Might give you a nice image to look at.

      • http://www.dgkelectronics.com Kalle Hyvönen

        Some good examples why regular oscilloscope probes with alligator clips are useless for a lot of stuff. Maybe make a 50ohm terminated coax probe (terminate the scope end to 50ohm, then use a piece of coax as a probe and put a 1k resistor in series to bring the loading down) and show the difference between that and the regular alligator clip arrangements?

      • Pingback: Demontage d’une balise EPIRB | Actu-Radioamateur Demontage d’une balise EPIRB | L'actualité radioamateur, et nouvelle technologie…()

      • f4eru

        Dave ! don’t transmit on emergency frequencies ! Just don’t do it !

        Removing the antenna means you send some 20 dB lower, but you still transmit, and your scope probe and it’s ground wire is an antenna…

      • Billy

        Dave, would of using a sampler type setup and a dummy load to get a direct connection into the SA give you a better signal to see?

      • Gert Y

        Nice one Dave!

        I am quite interested in the power supply stuff of the thing? How does it run (just straight at the battery) and how much mA does it draw?

        Thx and keep up the good work!

      • http://alternet.us.com hpux735

        The 121.5 emergency frequency is still very much in use. The “SARSAT” satellites are actually the US NOAA weather satellites, and are still in use. The only allowed intentional transmissions (not for lifesaving activities) are in the first 5 minutes after the hour, and even then only for three “chirps”.

        Please respect the time and resources of the search and rescue teams!

        • michael

          I have been waiting till the end of the vid for some ‘friends’ in full gear ‘rescuing’ you… too bad your lab is a 121.5 Mhz shield, too bad!


          – michael

      • D.Garlans

        I’m listening on headphones now, and I noticed that when you pressed the button and turned it on for real, I could hear the siren modulation tone very faintly in the background.

        Pretty neat..

      • Steve

        Hi Dave.
        Just wanted to let you know that your RSS feed has the wrong file size listed for the m4v file. It’s listing it at about 16MB instead of the full size. This broke my downloader…lol!

        • Steve

          Dave – best practice would be to do this inside a Faraday cage which an be a small box, like ones used for pager/two-way radio testing, and output the RF into a dummyload to minimize the radiation from being “leaked” out. 121Mhz (VHF) can make it out of buildings quite easily!

          And as previously pointed out, you disconnected the antenna and attached a probe, which can act just like another antenna. 1/4 wavelength at 121.5 Mhz is 0.59 meters, and I wonder if your probe is a multiple of that…


          • f4eru

            a cheap and good way to make a proper shielded box (with window!) is to get an old microwave oven, drill a hole on the side and install a passtrough like this one :

            a filtered sub-D can be good for DC signals, you can connect it to the power switch.

      • Graham

        Hi and a great demonstration in the use of test gear. However, I must concur with the other comments in the turning on and therefore transmitting on the 121 mhz band. The ideas of a faraday cage (microwave oven ect ) are good but surely do not go far enough. Simply putting the device in a cage and then connecting a cable (antenna?) to it would not shield it. The signal would surely go up the coax and out into the ether. Surely the test equipment etc also needs to be inside the faraday cage.

      • Richard

        Some people have hinted at this, but I’ll point out the history of this frequency. 121.5 is a frequency in the aviation band, surrounded by ordinary frequencies that are used for AM voice transmissions for air traffic control and similar aviation purposes. It’s reserved for mayday calls and emergency beacons. Every airplane with a radio is capable of monitoring and transmitting on 121.5, and most planes with multiple radios will routinely keep one receiver tuned to 121.5. 243 is in the military aviation band, and serves a similar function there. These frequencies are definitely monitored, but the extent of monitoring may depend on whether an airplane is nearby as you’re transmitting. It doesn’t take much to “hit” a low-flying plane who happens to be listening to the guard frequency.

      • allan

        Dave, you have a spectrum analyzer! Awesome!!! Being a ham, I would absolutely love to have one.

        Time to build a Faraday cage!

      • Worf

        Are you sure it’s 121.5MHz? That’s the Aviation band emergency frequency (military is 243.0, double it – most ELTs will transmit 121.5/243.0 because a single antenna works and it’s trivial to build the transmitter to do both).

        Marine use would use marine VHF frequencies.

        Though, everyone’s moving to the general emergency frequency of 406MHz – aviation, marine, land, using coded transmitters with optional GPS.

        Oh, and 121.5/243.0 is monitored as it’s still the aviation band emergency frequency. It’s just that the COSPAS/SARSAT satellites in space do not receive on those anymore (they are now looking for a 406MHz signal). Before (approximately February 2010-ish?) the satellites would listen for a transmission on 121.5/243.0 and signal a SAR station that it received a beacon. Now, if you trigger a 121.5/243.0 beacon, you rely on planes and air traffic control facilities to do the alert.

        If you’re a pilot, there’s many ways to trigger a SAR response – if you’re close to civilization, then a local air traffic building will pick it up. Further out, most planes have a radio tuned it to to relay when they pick up a signal, and once SAR is called they will home in on a 121.5/243.0 signal.

        406 is better because the satellites receive a coded message containing the beacon ID (to identify the beacon so the owner can be contacted to see if it’s an accidental activation – pretty much 99% of all signals. But in addition, they can also send a GPS location for faster rescue – the old 121.5/243.0 doesn’t provide a location, just a beep, and SAR has to do the grid search and triangulation to figure out your position.

        Oh Dave – next time, UNPLUG THE BATTERY! It’s on a connector – just unplug the thing to eliminate any chance of accidental activation. Remember the fines are pretty steep (around $10K+ plus SAR crew expenses).

        These things can easily put out 1-2W will WILL get out of the office. Remember they were intended to trigger a signal to a satellite, and allow SAR to home onto the signal even in the worst conditions (say, in a valley).

        Also – get a dummy load and use it. And please, before you tear down more transmitting equipment, get a ham license to at least learn the basics of RF and all that – it’s really easy to do and teach you the necessary precautions and everything. I know you said you didn’t want to and consider it “dead”, but it’s still valuable knowledge about RF (and probably the cheapest way to experiment with RF stuff legally).

        Trust me when resources get expended trying to locate emergency beacons. All it takes is a little blip and people will start paying attention in case it repeats.

      • Benjamin Miller

        Ha, I knew everyone here would be talking about the frequencies used…

        I’m a pilot in the United States, and this frequency is very much still in use. As someone else said, 121.5 is the aviation emergency frequency (guard), and 243.0 is the military equivalent.

        They’re also making ELT devices in the 406mhz range now. As far as I know, all of these frequencies are universal worldwide, because airplanes can travel to other countries (obviously).

        Around here, if you turn one of these things on for more than a few seconds from xx:05-xx:00, all kinds of people will notice. If they can’t locate you in a short amount of time, the Civil Air Patrol will get up in the air and go looking for you.

      • idsawtooth

        I think you’re all overreacting a bit regarding Dave’s tests. The satellites are NOT monitoring 121.5 Mhz anymore. Look at the signal strength on the SA and that’s coupled right next to the board. The chance that anyone or anything heard the signal is next to nil…

      • David

        I think the RF detected signal you found that was fed back to the micro-controller is to verify the transmitter is transmitting.

        There should be a very similar detector circuit connected to the wire loop that inductively (not capacitively) couples to the antenna when it is folded. That second detector circuit also connects to a micro-controller input pin and inhibits transmit when the antenna is folded. I don’t think you found that part of the circutry.

        As you don’t have a proper RF probe, try putting some ferrites on the ground wire in-particular, and sometimes on the probe tip itself. This helps to block coupled RF when you want to see control signals.

        People have said it already, you are lucky you didn’t triggar some sort of response. At a minimum you should have replaced the antenna with a dummy load.