Need a 72 Mhz AM receiver

If you're very near to the transmitting antenna, I'd think that even a simple un-tuned crystal radio would be sufficient. A length of wire, a 1N34 diode, maybe a small capacitor, a crystal earphone, and a clip lead to attach to ground would oughta do it.

If you want to get fancy, add a series-resonant circuit (small air variable cap and a hand-wound inductor) to tune it to 72 MHz.

A 100 MHz (or faster) oscilloscope with a wire connected to the BNC jack would probably pick up enough to show you the modulation, if present.

I work on fire alarm systems in outlying areas that transmit a signal to the fire department via 72 Mhz AM. The signal period is very brief and the carrier is modulated by a series of tones which first pertain to the box ID and then successive tones corresponding to which zone or zones are in alarm. The transmit power is one watt and cannot be increased.

This particular site is about 9 miles from the receiver. The transmit site is low and the terrain is hilly.

** Sounds impossible.

Five years ago we installed a three element beam to replace the existing omni directional antenna they had. Communication with that omni was spotty at best. With the beam, although the received signal was low, test transmissions were consistent and reliable.

** I bet it was low and badly affected by rain too.

I'm concerned about the modulation factor because this box seems to have suffered some type of event to lock it up the way it was when I first found it. I also have no idea what caused the transmitter fuse to blow.

Other than my service monitor/spectrum analyzer which tunes .450Mhz to 1.0 Ghz FM, (only) I have no way to hear this AM signal. Does anyone have any ideas?

** Err - ever heard of a radio scanner ?

... Phil

Seems like you should be able to take an old FM tuner and modify it a bit, buit actually I think I might hav rean even better idea.

What about a cable ready TV tuner ? Not just the tuner, rip out the flyback and all that shit, they don't use keyed AGC anymore. I think channel 3 or

4 should tune that. then just figure out how to get a little more selectivi ty if you need it. Maybe shift it tuning a bit to put it near the sound car rier, but instead of feeding a limiter nad an FM detector, just feed an AM detector. Possibley you could use the bandpass filter for sound on the vide o.

Not sure, the split carrier sets might not work right because of the synchr onous detection. Know what ? Fukit, just use the video carrier and filter i t right, you are ooking for audio frequencies right ? Unless there is somet hing within 20 Khz of it you won't hear it anyway. Just feed the video dete ctor to a small amp and speaker.

You wil haqve to find a set that can tune it in. You will have to set the A FT to get where you want, masybe even modify that a bit. as long as it work s off the jungle IC with normal AFT you should be able to do it.

I would say use the stuff from an old VCR but it is likely to be too cramme d to do any modifications. What do you have laying around that's really old ?

(you know I used to pick up cellular on UHF before they removed the channel s above 69 :-) (that was a LONG time ago)

Anyway, yup, do it with an old TV. Don't use a black and white, they are to o rare. I wouldn't mind having one just for the hell of it. In fact there i s a guy on Craigslist wanting to buy them who posts from time to time.

For about $20 on eBay, you can buy a USB Dongle that allows you to watch European standard Digital TV (DVB-T). Some of them are really just receivers with wideband analong to digital converters, the signal decoding is done by your computer.

This makes them easily useable as wideband receivers, and there is a lot of software to support them.

For example, SDR# (s d r sharp) which runs on Windows, and there is an Android app too. I had to root my cheap Chinese tablet to get it to work.

Look for "RTL2832U" on eBay. Be careful because some of the programs only support the E4000 tuner chip (e.g. the Android App) and most of the USB sticks on the market today use the newer but incompatible R820T tuner chip.

Some of the vendors go as far as say "Perfect for E4000" and never mention that they have the R820T chip in them.

Current production ones use a Belling-Lee antenna, aka PAL (ouch), antenna connector. The R820T ones use an MCX connector (very small), look at the picture, if it has an MCX connector it is NOT an E4000.

Note that if you need a remote receiver, you can shoehorn the whole thing into a Raspberry Pi.

Geoff.

--
Geoffrey S. Mendelson,  N3OWJ/4X1GM/KBUH7245/KBUW5379

The crystal-set and old-TV-tuner ideas that have already been mentioned; of those two I like the crystal-set idea the best. (A super quick and dirty version of the crystal set would be to hook a piece of wire to one end of a diode and feed the other end of the diode to the receiver half of a phone line tone tracer.)

An extension of the old-TV-tuner idea is to get one of the portable radios that had TV-band audio (try the thrift store) and see if you can hear it. Yes, TV audio was FM, so this might not work.

There is probably a scanner that can do it, but a lot of scanners have a hole between 54 and 108 Mhz, to avoid VHF analog TV channels 2-6 and the FM broadcast band. Also, some scanners don't allow you to pick the modulation independent of the frequency; they might be "hard wired" to AM for air band (108-137 MHz) and FM for everything else. I have one that is hard-wired like that, and another one which lets you choose.

If you happen to have a junk Cessna or Piper up on blocks in your yard, the marker beacon receiver in it is 75 MHz AM. The audio (Morse code between 400 and 3000 Hz) is decoded and sent through some filters to light up one of three lamps. Usually there is a way to access the audio before the filters; you'd just have to tune it down 3 MHz somehow.

Matt Roberds

snipped-for-privacy@gmail.com sendte dette med sin computer:

I just wonder: Is it not possible to put in a spare transmitter while you work on this? I'd be rather unhappy if the building burned dowm while waiting to get the alarm repaired.

Methinks somebody should look into their procedures...

When you do the "divide and conquer (sp)", you could work the other way. Can you hear/see the tones going into the modulator?

Leif

--

beslutning at undlade det.

TV set between VHF channels 4 and 5?

Yer average Bearcat won't tune TV bands (or slightly big guardbands). You need a wideband comms receiver like an ICOM or an AOR.

(...)

Well, probably not sensitive enough.

If you have an aircraft receiver, that goes from 108 to 138Mhz, it will demodulate AM. A scanner should suffice. Find a high level mixer (such as one from MiniCiruit Labs) throw in a local oscillator at: 120 + 72 = 192 MHz add some front end filtering and maybe some RF gain, and you have a 72 MHz receiver. Mix well, and serve. Be sure to do some intermod calculations to make sure you don't have any inband spurs.

Kinda sounds like you're going to be needing more than one of these receivers. It should be easy enough to build from scratch using an aircraft band AM receiver as the starting point, and just tweak the frequency sensitive components.

Maybe a modified runway lights control receiver which are often crystal controlled.

Maybe a modified 121.5 ELT receiver recrystalled and retuned to 72 Mhz.

Maybe a cheap shortware receiver with an added mixer and LO for the front end to receive 72 MHz.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

It depends on where you want to check the signal. On the transmitting end, with 1W power, a simple diode into a resistive divider and speaker will do. Totally frequency-insensitive, but you can probably check the carrier with a frequency counter separately.

On the receiving end, you'll have to look for some rather sensitive wideband receivers. This one will tune from 100 kHz to 1.3 GHz in 1 kHz steps:

Yupiteru does not directly sell their products overseas, but several Japanese online shops do export.

As Dave has noted, ICOM and AOR (both also from Japan) make several other receivers. These tend to be even more pricey but also have more selectable filter bandwidths and demodulation options. The Yupi above has AM, narrowband FM and wideband FM. Sensitivity ca. -6 dBu at your frequency and modulation type.

Regards, Dimitrij

** Some of the cheaper scanners do seem to miss 72Mhz. ** My 20 year old AOR 1000xlt ( hand held ) scanner covers the whole range from 100kHz to 1.3 GHz.

Demodulates AM, FM narrow and FM wide at any frequency too.

As do many others.

They go for under $100 on Ebay.

Thousands of uses ....

.... Phil

I have the 8200b3. I had to get one for work. I could never justify the price just for the hobby, as the house was already full of "scanners". It also has NAM (narrow AM), helpful for listening to HFBC, helpful for Air Band. I rarely listen to either on the beast. The "b" means "blocked", unfortunately.

Don't you have an oscilloscope? Loosely coupling the scope input to the transmitter, (laying a wire nearby), you should be able to see if the signal is being amplitude modulated.

You can get an HDTV signal finder for $30 at MCM. It'll need a preselector of some kind. Also, an old turret TV tuner will tune 72 Mhz.

According to the original posting, he's 9 miles from the transmitter. A 100MHz scope will work, if he can find a 9 mile long scope probe. Also, if the modulation is FM, one can't easily see it with a scope. However, the easy way is to read the FCC ID off the transmitter or a receiver, look it up on the FCC ID data dumpster, and reverse engineer the specifications.

72 MHz is the band assigned for RC (radio control) aircraft only (no model cars allowed): I would not be surprised if this system is nothing more than a repackaged RC device.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558