Data over UHF - Uniden or similar

#1
Many of the comms threads try to use a cheap low power Tx/Rx for data transfer and it is always a problem. The market is coming flooded with 'cheap' handheld UHFs and I would like to build something with range. I would also like to transmit data across the link
Would it be possible to transmit from a picaxe to a micophone input and recieve through a speaker headphone connection? Could this be done by pulsing the 'ones' on say a 9600baud serial link?
 

MFB

Senior Member
#2
Ham radio operators have been transmitting digital data over voice grade links for many years. A common approach is to use the type of FSK modem chips that were originally designed for low speed telephone communications. There was an interesting article in the February 2007 issue of Elektor magazine that used the CML FX614 modem chip, with a PICAXE-18X, to send telemetry data over the audio channel of a 2.4GHz video transmitter at a rate of 1200bps.

Two potential limitations that you should consider are available bandwidth (4.8kbps would probably be the very highest rate possible) and legal restrictions on the use of unlicensed radio equipment. If you plan to operate in the UK, pay a visit to the Ofcom web site to check these out.
 

manuka

Senior Member
#3
I've had a lot to do with UHF data over the years,recently using ~470MHz UHF CB sets as "engines". These lend themselves to all sorts of easy links handling SSTV, Hellschreiber as well as classic TTY etc. However rigid regulations limit possibilities, as here in Oz/NZ just 3 seconds an HOUR of telemetry/telecommand is allowed... Dr_A & I periodically muse possibilities.

2.4GHz certainly is open, but it's a NOISY band & signals are further very subject to LOS propagation. For my money I'd look at some of the slots lurking in 30-300MHz VHF for longer links. Where are you ? What distances intended?
 
#4
As manuka says, we do muse possibilities from time to time. First up, get a couple of radios and see what you can do. You may be able to crack the problem. Personally, I think I've taken it as far as I think I can, with a hacked radio, picaxe controlled switches, injecting the audio into the microphone section etc. It does work if you are prepared to accept morse code speeds. Three problems though. One is that when you up the speed to picaxe baud speeds, the waveforms get very distorted. A sine wave in does not come out as a sinewave, and a square wave comes out nothing like a square wave. Second problem is the 3 second rule. I dismantled my perfectly working system because of this rule as it was taking 10 secs to send the data. Third problem - most of the handheld cheapies have the data channels 22 and 23 deliberately disabled.

Also, the range doesn't seem much better than dedicated modules. This setup http://www.instructables.com/id/Build-a-500-metre-radio-data-link-for-under-40./ actually goes further than a cheapie UHF radio, which is possibly not surprising given the modules used were 1W and the UHF handhelds are usually 500mW.

I'm working on a universal picaxe board with lots of RS232 inputs and outputs, relays, solar power and RF, with the capability of taking a RS232 signal from a PC and routing it through any number of modules, wires, rf, wires then rf then wires then rf etc to a final device. Just waiting on getting a proper license code from Cadsoft for Eagle PCB as the PCB is getting a bit large...
 

Tom2000

Senior Member
#5
I'm working on a universal picaxe board with lots of RS232 inputs and outputs, relays, solar power and RF, with the capability of taking a RS232 signal from a PC and routing it through any number of modules, wires, rf, wires then rf then wires then rf etc to a final device.
Dr, did you ever try one of those gain antennas? Any luck?

Tom
 

manuka

Senior Member
#6
Re. other freqs - assorted spots all over the 27-49MHz spectrum find "unrestricted" use for RC toys & model aircraft,cordless phones etc, & these lower freqs naturally have better "punch" thru' terrain & vegetation. Circuitry is pretty undemanding,although antenna will not be such high gain. A bonus with 27MHz is that old CB sets & maybe even kids 3-4 transistor super-regen. toy walkie talkies (selling here in NZ for ~US$5 a pair) could be hacked for parts- especially the crystals. Additionally most decent SW comms. radios cover that part of the spectrum, allowing easy listening to your outgoing signals. Yah!

I'm short of time to explore regs., which of course could be country dependent, but suggest some bright spark check further. AFAIK the 27 MHz band is legal in most, if not all, countries for use with all types of RCs, but may be subject to interference from the adjacent classic AM/SSB CB band. Cheap toys seem to usually use Channel 4 if there is no channel stated, & may use resistor style colour coding system.

* 26.995 MHz -- Channel 1 -- Brown
* 27.045 MHz -- Channel 2 -- Red
* 27.095 MHz -- Channel 3 -- Orange
* 27.145 MHz -- Channel 4 -- Yellow
* 27.195 MHz -- Channel 5 -- Green
* 27.255 MHz -- Channel 6 -- Blue

Mmm- just recalled this snippet- regs. for some freqs. here could be worth checking? The allocation 433.05-434.79 (centre 433.92MHz) has been an ISM (Industrial, Scientific, Medical) band for many years, 433.92 being harmonically related to 6.78, 13.56, 27.12, 40.68 MHz etc. Dividing by 64 we get the 6.765-6.795 band, and by 32 the 13.533-13.587 band.

Yet another approach may be via low power FM band transmitters- the MP3 "iTrip" units now abounding naturally tempt. They have only a few metres range as is of course, but are very stable & maybe be great "engines" when amplified. Stan
 
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#7
Tom2000, the antennas are on the list of things to do. Need to get the board designed, then build waterproof boxes etc, then the antenna. So it is on the list. Unfortunately, work is also on the list as well and is taking around 70 hours per week. One advantage of getting older is that one needs less sleep, which frees up spare time!
 
#8
What about using model airplane transmitter/recievers. I don't know two much about them but I believe the newer type are digital as opposed to older analogue. Have a good range as a visit to any model field shows. What frequency are they by the way?
 

MFB

Senior Member
#9
Don't even think about it.

It would be easier to interface with equipment that has an audio channel because you can connect to the mic input and speaker output, without even opening the box. In the UK RC equipment now operates on the 35Mhz and 2.4Ghz bands but can only be used legally for this application, and MUST not be used to transmit data. Again, check out the Ofcom web site.
 
#10
Seconded, don't even think about it!

Hefty fine and even possible imprisonment if you use RC frequencies in the UK.
If I catch you, the punishment would be FAR worse!
 
#15
You might get away with sending anything over the RC system. That is, until the RC chaps get together for a weekend meet and all their planes fall out of the sky. Then they might be a bit miffed, especially when some of those planes are the product of years (decades even) of hard work.

I suspect the rules in the UK are more stringent because of the higher population density.

I don't think there is a problem sticking within the rules, and I add another rule of my own which is to try to keep RF transmissions within my own property. This means using a larger number of repeater nodes with limited (and known) range, rather than just one high power unit that sends signals all over the place.

I have got a maximum range of about 500 metres with these handheld units, but then again, sitting on top of the hill one sometimes hears transmissions from a long way away - eg truckies who are calling out traffic reports. So tempting as it is to use these handheld units, I don't think they are the answer.
 
#16
Just like driving, the RF world has rules, regulations and guidelines for good and valid reasons. These are mainly for safety reasons and to ensure that everyone can work together harmoniously.
If you want to ride a 50cc lawnmower in your back yard, fine, just do it.
If you want to send 10mW RF signals within your boundry, then fine again, just do it.
If you want to drive down the motorway at 70mph, then you must have a liscence and you must do it on the correct side of the road.
If want to transmit long distance, then (in most cases) you must also demonstrate that you know what you are doing by passing an exam to get a liscence. If you breach the rules, as with driving, your liscence will be taken away.

Not all RC planes/helicopters are 100g balsa toys. Some are over 20Kg moving at over 150mph and flown at public shows. They kill a dozen at a time when landed in a crowd. Hence the possibility of a penal sentance for incorrect use of RC transmitters.
 
#17
Rod, i think there has been a few fundamental questions that have been missed,
1, where are you?
2, what are you trying to do? (be as specific as you can)
3, which picaxe are you using?
4, what is your current level of experience?
5, what age group do you fit into?
6, is this for any terrorist related activity? (just to be legally covered)
7, what equiptment do you have? eg powersupplies scope...etc
8, have you managed to get a picaxe to flash an LED yet?
9, please include anything else that maybe useful in your response
 
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