Continuous outside 433mhz transmissions

lbenson

Senior Member
I had hoped to use 433mHz modules (from e-madeinchn) to monitor sensors in my house. For test purposes, I have a sender set up which I can activate by pressing a button. It sends a few characters. My receiver feeds into hserin on a 28X1. My problem is that someone somewhere seems to be transmitting continuously on the 433 frequency, in violation of the three-seconds-per-hour rule. How can I go about locating this transmitter? I am in a row house in an urban environment.

I have womai’s picaxe oscilloscope. When I set it up, with a one-millisecond per division display, and put the first probe on the input to the transmitter and the second on the output of the receiver, I can capture a snapshot of the serial transmissions, and the signals are essentially identical. This snapshot is triggered by a low-going transmission when I press the button on the transmitting module (which is a 14M which reads two DS18B20s and monitors its battery level). When the scope is free-running, my transmitter signal is flat, but there is continuous activity on the receiver—a little noise but mostly 3-volt transitions taking maybe an eighth of a millisecond or less, with the highs and lows held from a quarter of a millisecond to 4 milliseconds or more.

In other words, it looks to my inexperienced eye like a meaningful signal. I when look at my transmissions using sertxd I can see some of the characters which I am transmitting, but it is swamped by the garbage. I tried various settings with hsersetup to see if it was ASCII at another baud rate, but had no luck. It doesn’t look like the ASCII I am sending, even if stretched.

My ultimate use for this is in a rural location where there is not likely to be interference, but I won’t be able to test if I can’t get a good signal. What would I need to try to locate this apparently illegal transmitter. Of course, it could be multiple transmitters.

The attachments show the schematics of my transmitter and receiver modules, but the problem I have is continuous activity on the 433 frequency.
 

Attachments

lbenson

Senior Member
Attached are two scope views--the first of some of the random data appearing on the receiver output, and the second a snapshot of the transmitter input (red) and receiver output (blue). The receiver is I think slightly corrupted by the other transmission, but it is visually similar.
 

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moxhamj

New Member
How frustrating. The 3 second rule is there so everyone can share the airwaves. From a practical perspective, not everyone will need to open their garage door at the same time. But clearly someone doesn't know about the good neighbour rule when it comes to RF.

Two solutions:

One would be to go to 315Mhz - and the same modules are available at that frequency as well. So you could do the testing at 315 (hopefully that is clear) but drop in a 433 for the final project.

Or, you could build a directional antenna and triangulate this guy and have a chat. A yagi or a quagi will do the job. I built a quagi out of a broom handle and bits of wire. As you get close to the source you might need to turn down the amplitude of the input (? a resistor divider) so the receiver is not swamped. You might even be able to feed the output of a 433 module directly into a piezo transducer - or (heaven forbid - but it does work) into an 8 ohm speaker via a 1k dropping resistor.

But if you are doing scope tracings, does that mean you have access to a scanner?

Also (others might have some input on this) - you might even be able to do some directional work with a simple dipole. If you get a number of readings then work out the directions using a compass and draw them on a map it might be possible to find this person.
 
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leftyretro

New Member
I've always wondered why these modules for that frequency appear to be popular in the USA. If you look at the attached frequency allocation chart you will see that ham radio operators are authorized the 420 to 450Mhz band. They can operate up to 1,000 watts of power and that band is popular for club owned repeater sites that are on almost continously. So what you might be seeing is near by ham radio activity. Not their fault in that it is their band to use.

http://www.arrl.org/FandES/field/regulations/Hambands_color.pdf

PS: I'm unaware of any three-seconds-per-hour rule and I'm sure it doesn't apply to ham radio. Does someone have a source for that rule?
 
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Dave E

Senior Member
I have not heard of any 3 second rule either.

I use several 433MHz transmitters at home and at work and would be interested in any info on this "rule" also.

Dave E
 

lbenson

Senior Member
This document here: http://fjallfoss.fcc.gov/edocs_public/attachmatch/FCC-04-98A1.pdf along with much other information about an FCC change relating to RFID in the 433.5-434.5 mHz range, states in paragraph 25: "The 433.5-434.5 MHz RFID band we are adopting falls within the 433-435 MHz band that ARRL has designated for auxiliary and repeater links. Auxiliary stations are required by the Commission’s rules to operate on a point-to-point basis and are permitted to operate with a maximum power of 50 watts. Because point-to-point operations typically use directional antennas, there is less likelihood of interference from other sources."

This document states that unlicensed 270-460 MHz band must have at least 30 times the dead time as opposed to active, with 10 seconds minimum between transmissions: http://www.radiotronix.com/downloads/RCTcommunicationtomicrocontroller.pdf

This ARRL document states: "The current Section 15.231(e) provisions for periodic radiators, however, permit field strengths of less than 5,000 uV/m at that frequency (measured at 3 meters), with duty cycles of less than one second, assuming a silent period between transmissions of at least 30 times the duration of the transmission." http://isotc.iso.org/livelink/livelink/fetch/2000/2122/327993/327946/4495289/4495082/4495496/31N14883.pdf?nodeid=4499240&vernum=0

I had heard of the 3-second-per-hour rule applying in Australia and, I think, Europe, and had heard that a similar rule applied to unlicensed 433 mHz transmissions in the U.S. Apparently, and loosely, "similar" means a maximum of 1 second on and 30 seconds off--much more liberal than the Australian regulation. An expansion of that time specificaly for RFID in commercial and industrial areas was FCC-approved in 2004. ARRL objected to that but did not prevail. There appears to be some power restrictions to the licensed ham use at this frequency.

Disclaimer: I don't know anything definitive about this, and would be glad to hear from those who do.
 

leftyretro

New Member
This document here: http://fjallfoss.fcc.gov/edocs_public/attachmatch/FCC-04-98A1.pdf along with much other information about an FCC change relating to RFID in the 433.5-434.5 mHz range, states in paragraph 25: "The 433.5-434.5 MHz RFID band we are adopting falls within the 433-435 MHz band that ARRL has designated for auxiliary and repeater links. Auxiliary stations are required by the Commission’s rules to operate on a point-to-point basis and are permitted to operate with a maximum power of 50 watts. Because point-to-point operations typically use directional antennas, there is less likelihood of interference from other sources."

This document states that unlicensed 270-460 MHz band must have at least 30 times the dead time as opposed to active, with 10 seconds minimum between transmissions: http://www.radiotronix.com/downloads/RCTcommunicationtomicrocontroller.pdf

This ARRL document states: "The current Section 15.231(e) provisions for periodic radiators, however, permit field strengths of less than 5,000 uV/m at that frequency (measured at 3 meters), with duty cycles of less than one second, assuming a silent period between transmissions of at least 30 times the duration of the transmission." http://isotc.iso.org/livelink/livelink/fetch/2000/2122/327993/327946/4495289/4495082/4495496/31N14883.pdf?nodeid=4499240&vernum=0

I had heard of the 3-second-per-hour rule applying in Australia and, I think, Europe, and had heard that a similar rule applied to unlicensed 433 mHz transmissions in the U.S. Apparently, and loosely, "similar" means a maximum of 1 second on and 30 seconds off--much more liberal than the Australian regulation. An expansion of that time specificaly for RFID in commercial and industrial areas was FCC-approved in 2004. ARRL objected to that but did not prevail. There appears to be some power restrictions to the licensed ham use at this frequency.

Disclaimer: I don't know anything definitive about this, and would be glad to hear from those who do.
Yes, it seems (the time on/off rule) to apply to the unlicened users (part 15 users). As I read the document there is no power restrictions for the licened ham radio operators, they are as free as they have always been on this band. So all in all I would suggest that using 433mhz in the USA is a dumb idea as long as their are other frequency bands avalible.

Lefty (WA6TKD)
 

lbenson

Senior Member
Here is a reference to the governing regulation: http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=cb7ed85f6c396203994cd264f44120fe&rgn=div8&view=text&node=47:1.0.1.1.14.3.240.20&idno=47

This refers to: Title 47: Telecommunication
PART 15—RADIO FREQUENCY DEVICES
Subpart C—Intentional Radiators
Radiated Emission Limits, Additional Provisions

§ 15.231 Periodic operation in the band 40.66–40.70 MHz and above 70 MHz.

Tables refer to permitted field strengths.

Two seconds per hour under certain conditions: "(3) Periodic transmissions at regular predetermined intervals are not permitted. However, polling or supervision transmissions, including data, to determine system integrity of transmitters used in security or safety applications are allowed if the total duration of transmissions does not exceed more than two seconds per hour for each transmitter. There is no limit on the number of individual transmissions, provided the total transmission time does not exceed two seconds per hour."

Exemption for weaker transmissions: "e) Intentional radiators may operate at a periodic rate exceeding that specified in paragraph (a) of this section and may be employed for any type of operation, including operation prohibited in paragraph (a) of this section, provided the intentional radiator complies with the provisions of paragraphs (b) through (d) of this section, except the field strength table in paragraph (b) of this section is replaced by the following:" [1,500 to 5,000 microvolts/meter Field strength of fundamental for 433mHz transmissions]

Exactly what this means for my particular 433 mHz modules from e-madeinchn, I confess I don't know--but I will have no trouble limiting transmissions to less than 2 seconds an hour for each transmitter, and I believe to less than 2 seconds per hour for the entire system.
 

manuka

Senior Member
That "3 seconds an hour rule" only applies to the 2 data channels on Aus/NZ ~470MHZ 40 ch.UHF CB!

Sets on these PRS freqs can run as high as 5 Watts, although most are only ~½W. AFAIK no such power ruling applies to much lower power ( ~ 25mW) 433MHz ISM. In my experiences (here in NZ anyway),all manner of commercial telemetry gear (& perhaps nearby 70cm hams) can be found transmitting on & around 433 MHz near continuously. Tuning this band with a UHF scanner yields at times almost an African dawn chorus

Suggest a walk/drive into some rural quiet RF region, & determine that it's NOT your end that's causing the interference - it happens! Next use a simple UHF antenna to track down the offending signal. Suprise suprise- it may be your own smart utility meter,backyard wireless weather station or even a jammed garage door opener someone sat on.
 
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demonicpicaxeguy

Senior Member
i had a similar problem with those cheap $7 433mhz modules from jaycar,

with the 433 modules noise is an easy one to get around, the initial protocol i used for my remote meter reading box consisted a simple transmission method:
a logic high longer than 1 ms but less than 2ms is a "1" and a pulse longer than 2ms and less than 3 was a "0",

when i moved back in with the inlaws though their garage door stopped opening and closing, so i had to change how ofter the meter reading box sent data,
i've since had no other problems

i'm looking at using another module the MRF24J40MA by microchip, it's got an spi interface
i've ordered some and are waiting for delivery, it'll be interesting to see the range
 
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lbenson

Senior Member
I have some 315mHz modules. There appears to be no noise at that frequency. Using the scope I can see that the receiver tracks the sender--but at a low voltage--only 2 volts peak to trough. This doesn't appear to be enough for the hserin background to recognize it as serial input. Am I understanding this correctly? Is there a convenient way to amplify this signal? The receiving program hasn't changed from the one which was receiving and printing out garbage, along with snippets of my transmissions, at the 433mHz level.
 

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lbenson

Senior Member
False alarm. In trying to eliminate any minor noise in the 433mHz input, I managed to create a voltage divider--removing that got the output back into the 0-4V range, which background hserin picked up.

But out of curiosity, what transistor circuit would have increased my 0-2V to the full 0 to about 4.5V range? I tried the attached inverter circuit from http://www.kpsec.freeuk.com/trancirc.htm and it inverted very prettily, but still in a 2-volt range from about 2.5V to 4.5V. I suspect that one of the other circuits on that site will do what I wanted, but I haven't puzzled out which.
 

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moxhamj

New Member
Something doesn't sound right with the measurements. The transistor circuit above should go pretty much rail to rail. If you are measuring 2.5 to 4.5, I wonder if the output from the 315 module might in fact be correctly going from 0-5V.

What is the source impedence of the scope you are using - is it loading the line in some way?

I almost wonder if it is worth just building a quick serout to serin test over 315 and see if it gets there. Do it with a wire first to prove the code is correct, then change to wireless.
 

Tom2000

Senior Member
But out of curiosity, what transistor circuit would have increased my 0-2V to the full 0 to about 4.5V range? I tried the attached inverter circuit from http://www.kpsec.freeuk.com/trancirc.htm and it inverted very prettily, but still in a 2-volt range from about 2.5V to 4.5V. I suspect that one of the other circuits on that site will do what I wanted, but I haven't puzzled out which.
Try an emitter follower unless you absolutely need inversion.

Good luck!

Tom
 

lbenson

Senior Member
My apologies to those who responded. Tho it will only take a short while to set up, I haven't been able to get back to this.

Tom2000--thanks for giving me the term to search on--"emitter follower". I have what looks to be a good circuit for that, and will try it as soon as I can.

Dr_A--I'll also retry the inverter, putting a 10K pot in for RB so I can watch what happens as I change the values--I think I didn't originally had the resistors right. So many projects, so little time.
 
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