Need a 72 Mhz AM receiver
 

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. 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.

The fire department has recently reported that they have not received a test transmission for some time now. Oddly no one seems to know how long this has been going on but it is at least for the past month or so.

When I arrived at the site initially I found all the lights on. the B+ fuse for the final blown and the unit totally non responsive. I powered down, AC and battery, replaced the fuse, and then powered back up again. The unit seemed to power up normally.

The antenna looks fine on SWR and I ran the self test function and activated a couple of alarms, however the fire department did not get a signal. And although I can see a 1 watt carrier on my meter I cannot tell if it's actually being modulated. So although I know that I have an RF output my problem is that if the unit is only putting out a dead carrier the receiver will ignore it. These Id and zone modulation tones are all in the 500HZ and lower spectrum so they're all audible. So if I had an AM receiver that could tune the 72MZ band and allow me to hear these tones I would then eliminate the transmitter/modulator as being to blame. At that point I would then think about trying to turn the antenna, which is big and heavy, mounted to the side of the house, and not an easy thing to do, especially in the snow.

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? Thanks, Lenny

Need a 72 Mhz AM receiver
 

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? Thanks, Lenny

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.

Need a 72 Mhz AM receiver
 

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

Need a 72 Mhz AM receiver
 

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 ****, 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 selectivity if you need it. Maybe shift it tuning a bit to put it near the sound carrier, 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 video.

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

You wil haqve to find a set that can tune it in. You will have to set the AFT to get where you want, masybe even modify that a bit. as long as it works 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 crammed 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 channels 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 too rare. I wouldn't mind having one just for the hell of it. In fact there is a guy on Craigslist wanting to buy them who posts from time to time.

Need a 72 Mhz AM receiver
 

Need a 72 Mhz AM receiver
 

Need a 72 Mhz AM receiver
 

Need a 72 Mhz AM receiver
 

On 03/04/2014 05:20 PM, Phil Allison wrote:


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

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.

Need a 72 Mhz AM receiver
 

On Tue, 4 Mar 2014 16:12:04 -0800 (PST),
wrote:

(...)

http://www.ebay.com/bhp/75mhz-am-receiver
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
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Need a 72 Mhz AM receiver
 

wrote in message
...
....
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? Thanks, Lenny


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:

http://www.yupiteru.co.jp/products/r...500/index.html

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

Need a 72 Mhz AM receiver
 

"dave"
Phil Allison wrote:


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 ?


Yer average Bearcat won't tune TV bands (or slightly big guardbands).

** Some of the cheaper scanners do seem to miss 72Mhz.


You need a wideband comms receiver like an ICOM or an AOR.


** 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

Need a 72 Mhz AM receiver
 

On 03/05/2014 07:23 PM, Phil Allison wrote:
"dave"
Phil Allison wrote:


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 ?


Yer average Bearcat won't tune TV bands (or slightly big guardbands).

** Some of the cheaper scanners do seem to miss 72Mhz.


You need a wideband comms receiver like an ICOM or an AOR.


** 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.

Need a 72 Mhz AM receiver
 

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.

Need a 72 Mhz AM receiver
 

On 03/07/2014 07:21 PM, wrote:
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.

Need a 72 Mhz AM receiver
 

On Fri, 7 Mar 2014 19:21:59 -0800 (PST), "
wrote:

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.

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):
http://radiocontrol.wikia.com/wiki/72_MHz
http://www.modelaircraft.org/events/frequencies.aspx
I would not be surprised if this system is nothing more than a
repackaged RC device.



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