REQ Field Strenth Meter to detect 406 and 121.5 MHz freqency

Hi Folks..

I'm looking for a bit of info on a Field Strength meter for detecting EPIRB 406MHz and 121.5MHz signals.

My reason for asking is that I am part of our local coastguard unit which gets sent out from time to time after an EPIRB has been activated.

Out at sea, it is a bit easier.. not much .. but a bit easier than trying to detect it on land so what I'm after is some sort of directional Field Strength meter that will detect this frequency and allow me to 'home' in on it then deactivate it.

The last EPIRB we had to search for was behind a shed in a harbour full of trawlers. 6 team mebers, a lifeboat and a helicopter were tasked to this at 4 in the morning

Any info greatly appreciated

WiseOne

Reply to
WiseOne
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If you are part of a legitimate Coast Guard unit, your squadron has more information on Direction Finding than anybody else in the country.

If you are part of a legitimate Coast Guard unit, you would NEVER EVER spell it coastguard.

Buzz off, troll.

Jim USCG AUX Squadron Communications/Electronics Officer, Deactivated some years ago.

Reply to
RST Engineering (jw)

My error, flotilla not squadron. I'm mixing up my Air Force and Coast Guard nomenclature.

Jim

Reply to
RST Engineering (jw)

Reply to
Barry Lennox

You shouldn't be looking for a "field strength meter", you should be looking for a "radio direction finder", RDF.

Try a google search, like

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Good Luck! Rich

Reply to
Rich Grise

There has been a number of VHF DF receivers published over the years, "VHF Comms" is a good source, see

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for the complete index.

However, I have used an Icom R-1 with a double-switched dipole antenna to localise on radiating 121.5 MHz beacons with a fair degree of success. 406 is much tougher with low-cost equipment as they only radiate a short burst at intervals (IIRC its about 0.5 sec at random

48-52 second intervals) However, many 406 beacons also radiate on 121.5 as well.

Barry Lennox

Reply to
Barry Lennox

Since this is s.e.d., nothing is impossible. I would start with a CC1000 single chip RF transceiver (300MHz to 1GHz) with optional IF output (150KHz). Send this IF output to an ADS7816 (12 bits, 200KHz A2D), and read the results into a micro. 150KHz would be too high for low cost mcu, so the external A2D would be better. Costs for the 3 chips are:

CC1000 $4 ADS7816 $4

8051/PIC/AVR $5 Misc $4

We can build it for $20

Reply to
linnix

You should contact an amateur radio direction finding group, and look for equipment they use that can be modified. For example, the unit at the URL below is very popular worldwide, made by a friend of mine (tell Bryan I sent you), and able to be tuned to the 121.5MHz band. It's specially designed for DF competition.

.

I don't think Bryan makes a 70cm receiver, but others do - he'll point you in the right direction.

I hunt with one of the Melbourne ARDF teams, and we routinely hunt down a 2m transmitter smaller than a shoebox across 10Km of city within 10-20 minutes, in the dark. See for more info. The cars have rotating antennae on the roof, interchangeable for 2m (most common), 80m, 70cm, 23cm, and that gets us within 100-300m - usually the "fox" is hidden in some wasteland or in a tree in a bushland park where you can't drive, so one or more runners jump out, with hand-held "sniffers", which are an antenna with a receiver like the unit above, to race to the finish. Hide-and-seek for grown-up geeks... Great fun!

Clifford Heath.

Reply to
Clifford Heath

They have already spent the better part of a billion on an upgraded GMDSS and you're asking for homebrew????

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Reply to
Fred Bloggs

The 406 is quasi spread spectrum, and only on for short bursts. dont waste your time with it. The 121.5 , which is supposed to be phased out (sadly, big mistake) is AM and most of the ham radio double ducky type antenna switching direction finders will therefore not be able to home on it. Get a old aircraft radio, have a local ham disable the transmit. Tune it to121.5. pull out the "agc" ie automatic gain control signal, isolate it and tie it to a meter. Buy a yagi antenna for that band with at least 4 elements. Put the yagi on a stick to get it away from your body. Buy a couple of inline antennuators from digikey ie

30db, 20 db, 10 db, and 3 db. As you get closer, stick in the next biggest attenuator in the cable to knock the signal down. Hunt on 243 for non military epirbs, the harmonic will always be very weak. For very close in, hunt at 364.5 , this will be a very weak signal resulting from harmonic generation in teh output stage. You'll need a yagi with gain for that frequency.

If your really in trouble, tie a rope to your handhand 121 receiver and lower down a short piece of metal pipe, just a bit longer (say 4") then the radio plus antenna length this is the "waveguide beyond cutoff" technique and attenuates the signal nicely when your close in. IE the pipe shields the antenna, on a hadheld, the whole radio, body AND rubber ducky, is the antenna

I still have a hard time beliveing a USCG search team would not have a compentent Radio Officer around, and no access to regional or national consultant or local hams. If your really USCG, try talking to your local CAP squadron, they may bebetter able to teach you.

Steve Roberts

Reply to
osr

The word that I've heard (not from anyone in the Coast Guard but from old-timers at NRL) is that since they sunk so much of a billion into GMDSS that the high-ups in the Coast Guard refuse to spend a penny on any of that "old-time" direction-finding equipment. The story also includes the grunts having to homebrew their own DF equipment on their own dime, as they can't even requisition commercial gear to do DF using Coast Guard moneys.

Hard to believe either story, but my NRL sources here agree with the OP's story! And it's so easy to believe the "congressional billion-dollar purchasing bungle" has rendered a branch of the military improvising their way to functionality.

Tim.

Reply to
Tim Shoppa

In that case, a $20 solution is better than a billon bucks. You just need a single chip RF transceiver with RSSI (receiver signal strength indicator) and a directional antenna.

Reply to
linnix

I'm on the wrong side of the Pond!! or is that the right side??

I'm in the UK

Wise>

Reply to
WiseOne

That's governments for ya.

I'm on the other side of the pond. We usually take a few years to catch up with the US.. and still we don't learn!!

Wise>Fred Bloggs wrote:

Reply to
WiseOne

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