AM Wireless Modules

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George Sephton

Senior Member
Hi All,
I've been very interested in wireless data, I stated with the irout etc in PICAXE and enjoyed the results. I then looked at the IR tranceiver made my Vishay that works with serial data, still impress, although ir only works when pointing at each so won't be helpful should I want to leave room. So I've wanted to look at RF but this can be expensive, and I've found a cheap alternative, AM transmitters and receivers:
http://uk.farnell.com/jsp/search/productdetail.jsp?sku=1304024&_requestid=76697
http://uk.farnell.com/quasar/qam-rx4/module-receiver-am-433mhz/dp/1304027

These are 433MHz modules, just out of interest what do I use as an antenna and where can I buy one as I've heard ppl have used like just lengths of metal.
Also am I right in thinking on the transmitter I just do a serout to DATA and on the other end, I can get a result with serin on pin 6,7 (Data Input) on the receiver?

Thanks,
George S.
 
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Dippy

Moderator
On the antenna question you'll get variations on a theme and many Data Sheets have subtle differences.
Check page 8 of this as an example:
http://www.radiometrix.co.uk/dsheets/tx2arx2a.pdf

1/4 wave is the calc for a bit of wire.

Note that any wiring/PCB tracks, unless designed with correct impedance etc. will 'add' to the antenna length.

Pins 6 and 7 look right - noting the typo in the Data Sheet.

Also check out 'ground planes'.

Preambles are usually needed to settle the Rx. Depends on device of course.

Also, as a slight aside, bookmark this. There is some good advice.
http://www.radiometrix.co.uk/apps/apnt000.htm
 
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Andrew Cowan

Senior Member
You also generally need qualifiers block out all of the background noise. Lower baudrate equals better signal. But esentially, it is just serin serout.

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hippy

hippy

Ex-Staff (retired)
George Sephton said:
These are 433MHz modules, just out of interest what do I use as an antenna and where can I buy one as I've heard ppl have used like just lengths of metal.
Also am I right in thinking on the transmitter I just do a serout to DATA and on the other end, I can get a result with serin on pin 6,7 (Data Input) on the receiver?
Click to expand...
Aerials can often be just a length of stiff wire cut to the right length.

Communications is a little more complicated. For XBee ( after configuration ) it is as simple as SerOut / SerIn but for radio modules you have to generate preambles and pauses, data qualifiers, checksums and then send the data. The receiver has to look for a valid qualifier, read the data and verify the checksum.

Transmitter and receiver modules have various operating voltage ranges which should be taken note of.

There's been quite a lot of previous discussion on all these issues.
 
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Dippy

Moderator
Uh oh, don't complicate things with the inner workings of XBee techniques :(

Checksums etc. make things more reliable George, but for your first experiments just try preambles with slow/medium Serout bauds. It's what most people use to start with and can give acceptable results for simple stuff.

As hippy says, there are many postings on this subject.

Also have a look at Data Sheets from other manufacturers of similar AM/FM products. Some give some sage advice and provide tips to make life easier. Many people have their own preferred methods and no doubt you will get a lot of help.

And like many other very popular subjects (MOSFETs, Automotive) it would be nice to see a page or two added to Manual 3. (Says he beating head against wall ;) )
 
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MartinM57

Moderator
So on a slight diversion, if I just wanted 2 remote push-switches in a small (EDIT: battery powered, no on-off switch so miniscule quiescent current*) handheld box to be able to send 'pushed' signals to a PICAXE project in a box within 2 metres but not in line of sight, what (simple and very cheap) comms would be suggested? (but not wire!)

EDIT2: * or use DP push switches so that one pole switches on the power and the other indicates which button pressed?
 
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Dippy

Moderator
Martin, do you want to make this a separate thread question.

I can see you getting more replies than a Blonde in a Lonely Hearts ad. and it will digress from the original question too much :)

PS. What have you thought of already? I'm sure others will simply repeat what you have thought of already.

PPS. Your Edit2 already digresses from your first question, so maybe you should start another thread... please.
 
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George Sephton

Senior Member
They are good and the antenna lengths are wierd. I test these a lot on breadboards so assume that the longer the length of antenna on the transmitter the further the distance upto a point then the distance gets further but the functionality nearer the transmitter decreases, ie a 1 inch wire (antenna) allows u to receive next to it but not lets say 10m out, but a 10 inch wire allows u to receive 10m out but not next to it (this isn't the exact figures but is the same concept).

Weirdly the transmitter needs and antenna but the receiver can be used without one. It's worth doing experiments in a bread board but really a piece of wire is fine.
 
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BeanieBots

Moderator
George, the antenna needs to be the CORRECT length on both Tx and Rx units for maximum range.
Any length of connection on your breadboard must be included in the total length of your antenna.
So, longer is NOT better. However, 1cm too long will not impede transmission as much as 1cm too short.
 
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Dippy

Moderator
George, go onto Raiometrix or RF Solutions website. Find a product using the same frequency. Grab the DATA SHEET and read up about antenna lengths - it is fairly important.

But, if you only want short range and an aesthetically pleasing antenna then carry on as you are.... i.e. guessing ;)
 
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manuka

Senior Member
Practical antenna design is an art and science, & the field continues to both fascinate & taunt many -including myself.

Classic vertical "whip" types are ¼ of the signals wavelength long. As vel. = freq x wavelength hence a 300 MHz signal would have wavelength 3 x10^8/3 x10^8 =1m, with antenna a 250mm vertical. Signals slow down somewhat in conductors ( known as Velocity Factor = VF), so it'd be slightly shorter (by ~90%) in practice. A ¼ wavelength whip for 433 MHz should be 3x10^8 /4.33 x 10^8 = ~173mm, but (once the VF is considered) this may work best at 155-160mm. Start longer & progressively trim to suit. Don't become picky however -my experiences with VHF/UHF over the decades indicate a clear line of sight (LOS) signal path is FAR more often the crucial factor.


For simple needs length is NOT usually critical, & almost any radiator can do in a pinch at low power. During the WW2 Stalingrad nightmare, barbed wire around trenches apparently served the German Hellschreiber system "admirably".

N.B. Most 433 MHz TX/RX units become overloaded when very close & often perform better a good 10 metres appart.
 
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russbow

Senior Member
George, it is easy to get frustrated with antenna design. Resonance and impedence matching are important but at 433Mhz these are difficult to achieve without proper test gear.

Google "j-pole" and "slim jim" for ideas or useful homebrew jobs. My favourite is a slim jim built out of a length of 00 scale model rail track.

For these modules however I always go back to a 1/4 wave whip. 3 radials give a good ground plane & these can be bent down to get a good impedence match - back to test gear.

Have a look at this little PDF. Might clarify a little.

Manuka is right, close proximity of TX will swamp the RX. If you need to test them that close, replace the antenna with a 47 ohm resistor to earth.
 

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moxhamj

New Member
Re "These are 433MHz modules, just out of interest what do I use as an antenna and where can I buy one as I've heard ppl have used like just lengths of metal."

The answer is a piece of wire 17.3cm long (as mentioned in the middle of Stan's post). Or 16.4cm. Or several numbers in between and it probably isn't that critical given the comments in post#9 (which indicates a far more important problem of overload from modules being too close to each other)

It can be any sort of wire really. Probably better if it is stiff though, as it can then stand straight up. You need the antenna in the air, not running along a bench etc. Single core copper wire works fine - eg the wire out of cat6 cable. (that sort of wire is also perfect for wiring up breadboards)

Those am modules will work fine with picaxe. Just solder your 17.3cm bits of wire for the antennas and start doing some code http://picaxe.orconhosting.net.nz/433rpt.jpg

Note that in that picture the antenna has been wound into a bit of a spiral. That is ok - it will decrease the output slightly but only by an insignificant amount, and a spiral antenna is much more convenient.

If you want to increase range, get the antenna up higher. It makes a huge difference moving from 30cm from the ground to 2 metres from the ground. In practice, that means getting the module plus antenna up higher. You can put the module plus antenna inside something that is transparent to radio waves, and a convenient way to do that is to put it inside PVC pipe - eg 25mm pipe will work, with caps on the end for waterproofing.

Re "I test these a lot on breadboards so assume that the longer the length of antenna on the transmitter the further the distance upto a point then the distance gets further but the functionality nearer the transmitter decreases, ie a 1 inch wire (antenna) allows u to receive next to it but not lets say 10m out, but a 10 inch wire allows u to receive 10m out but not next to it (this isn't the exact figures but is the same concept)."

Yes those experiment results are true, but the reason is that nothing is reliable at short distances. All the modules really need the standard antenna length and also to be at least 5 metres away from each other.

What you are finding is that with modules close to each other, they are overloading, and by making the antenna shorter you are decreasing the RF output and so they don't overload. The random length antennas will then work very close to each other, but won't go much further than from one side of the room to the other.
 
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hippy

hippy

Ex-Staff (retired)
I've only done a little 433MHz work and came at it not knowing what antenna to use. I adopted the 'about 17cm' approach and found it to work and also experimented by taking longer bits of wire and triming the length to see what happens. It's a destructive test ( because you need to go past peak signal strength ) but then you make antennas of the found ideal length and you're done.

I didn't find the length to be that critical but it probably wasn't a very efficient setup with maximal range.

If you want to do better than that there's no option I can see than reading as much as possible on antenna design and theory, ground planes etc and learning the subject. A bit like tuning a car, you can do some things with low precision and see a performance boost but if you want to get the maximum performance you have to know exactly what works and why and how changes are affected by other factors.
 
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