433MHz ASK receiver questions

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#1
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Hi guys,

I am trying to use one of these 433MHz RF receiver modules to intercept transmissions from a weather stations outdoor sensor.

http://jaycar.com.au/productView...

While I have the reciever in line of sight of the transmitter (only about 15m away) I can receive the signals quite ok, but as soon as I move the receiver to another room no more than 3-4m further away I am having all sorts of reception problems.

My first question is what sort of antenna works best with these modules? I am currently using a 31cm piece of wire (which seems to give the best results), but I have also tried a 433Mhz stub antenna from another reciever I have and it didn't seem to work well at all. I probably should mention that the module at present is plugged into a breadboard, which could be part of the issue.

I also came across this forum post that suggested that the modules are prone to ESD damage, is this true? The module I am using also uses the same IC as is mentioned in the post.

http://forum.sparkfun.com/viewto...

I would much appreciate anyones thoughts and/or experience with these type of modules.

Cheers
Simon

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433Mhz is in the 70cm HAM radio band, there are plenty of antenna designs available. Just google for '70cm antenna'.

One of the classic designs for a directional antenne is the yagi:
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=13234

Good 70cm antennas are quite large, physically. All good designs are in the same order of size as the wavelength, your 31cm wire is half the wavelength, that is why it works well.

Markus

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A couple of thoughts for you.

Did you build the weather station? Do you have the ability to put a good antenna on it? If it has a small antenna inside the case that is rather inefficient at getting the signal out, then adding a good antenna to it would be the first step.

If that isn't possible, then perhaps a solution to consider is building a "Repeater". My house has aluminum siding, and hence I live inside a pretty good Faraday cage. I ended up using a repeater for a similar weather project.

You put a receiver inside the house, but where it gets good signal reception from the outside unit. A micro reads the data and then re-transmits the data using a matching transmitter module.

Lots of variations on the theme. You can use the same, low power, low cost, modules, on the same frequency, for the system, but one can also switch to XBee modules for the house interior communications.

If you are right on the margin for the receiver's ability to capture the signal you might have the option of relocating the transmitter or receiver to a different location. Sometimes even a small change in position will result in a significant difference in the signal level.

Again, if it is your weather station, switching to XBee's instead of the low power, low cost modules will give you a range of power options, modules with antenna connectors for an external antenna, built-in error correcting and re-transmission protocols, etc. The up side is better system performance, the down side is higher cost.

You also might be able to hack into the weather station to grab the digital signal feeding the transmitter and feed it to your own micro, and transmitter. You can then do simple things like transmit the data three times each time you send the packet, send several junk characters as a packet preamble to allow the receiver to synch in on the signal, etc.

If none of that works you could resort to Plan B:

Look out the window to see the current weather status.

JC

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Quote:
If you are right on the margin for the receiver's ability to capture the signal you might have the option of relocating the transmitter or receiver to a different location. Sometimes even a small change in position will result in a significant difference in the signal level.

This small change in position that works is because the position that doesn't work is where you have cancelation between waves from multipath.. This is a good point that JC metion, did you try it?
And yet, on the line of sight between Tx and Rx, did you have some metalic object? If yes you will have 100% reflection of the wave.. Think about it...

If this didn't work I'll go for the option that Markus said, got a better antenna, you could use a directional one, like yagui, but as Markus said it will be large because of your working frequency...

Regards,

Bruno Muswieck

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Markus,

Thanks for the response, but I am trying to keep this simple and compact.

Doc,

It is a commercel weather station I am trying to listen in on. It is made by Lacrosse and I am intercepting the data it is sending to its base station inside the house. The location of my receiver is not optimal (between a PC, 30" LCD monitor, laser printer, wifi access point. That has got to generating a lot of noise.

The interesting thing though is that yesterday it was mostly working, until we had 2 major weather events that both involved much lightning. As of today my receiver is no longer receiving any data (the Lacrosse base station is still receiving), so it has to do with the receiver module I am using.

I will get a new module tomorrow to see if a new module fixes the problem.

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Which Lacrosse wx station?

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It is a WS-3600 although it appears that all of the outdoor sensors communicate to the base stations using the same packet format. I based my code on something written for one of the 23xx series stations.