How to connect RF transmitter directly to reciever? 1

94Camaro · Jun 9, 2006

Hi everyone, I've got a problem that I need some help with. I've got a wireless transmitter and receiver setup (315Mhz) that controls a hydraulic valve system. The problem is that sometimes we need a wired solution. We've got a wired system, but it is a bit old and cumbersome. I had the brilliant(?) idea of using the wireless transmitter as both the wireless and wired controller by running coax between them. Now, my question is, how can I go about hooking the transmitter's antenna output directly to the receiver's antenna input? I'm afraid that the TXs output is probably too large for the RXs input and it'll just fizzle. Am I right in thinking this? Can I avoid this problem by simply placing a resistor inline to attenuate the signal enough for the RX to receive without blowing up? I went ahead and tested it without any attenuation and nothing bad happened, but I need to be sure problems won't develop if I don't attenuate the signal.

The TX output power is typically around +2dBm and the RX sensitivity is -102 dBm.

These are the data sheets for the TX and RX we use.
TX

http://www.linxtechnologies.com/documents/TXE-xxx-KH_Data_Guide.pdf

RX

http://www.linxtechnologies.com/documents/RXD-xxx-KH_Data_Guide.pdf

Thanks for any help anyone out there can give me. I'm not particularly knowledgable about RF systems, but I know just enough to be dangerous.

VEBill · Jun 9, 2006

Basically, yes. Cable connections plus an attenuator. It's done all the time for various purposes such as testing, training, etc.

You'll want more than the minimum signal at the receiver, thus you don't want the full 104 dB attenuation as the simple math might indicate. More like 30 to 60 dB range (a guess). If the cables are very long, then also account for the cable loss; but that is probably a trivial detail for any reasonable cable lengths.

Since +2dBm is only a bit more the one milliwatt, it is low enough that it is not likely to cause any damage even if connected directly. Depending on the receiver, direct connection might be too much due to receiver performance issues, thus some reasonable attenuation would be a safer bet than none.

Any reasonable receiver will provide sufficient dynamic range that you don't need to over analyse it. It's a very wide target (like the side of a barn from 10 paces). And with +2dBm would don't need to concern yourself with power ratings.

Note - I didn't look at the reference data sheets...

94Camaro · Jun 14, 2006

Thanks for the reply. I've had the remote hooked up directly and didn't seem to suffer any negative side affects. The remote's antenna is still hooked up, so some of the signal probably gets radiated. Since I don't seem to have any problems for now, I'll just run it how it is and keep an eye on it. If problems do develop I can easily build a ~30db T-attenuator with some resistors and fit it into the application.
Thanks again.

Higgler · Jun 22, 2006

If you need an attenuator, just don't connect the end of the cable, make an antenna coupler out of it. have your coax. near the transmit antenna, strip off the outer conductor and leave about 10 inches (1/4 wavelength) maximum center conductor hanging out. 10" will couple about -10 dB from your transmitter if you put the cable near your tx antenna. Maybe cut that 10" down to less than 5" (1/8 wavelength) and you will lower the coupled energy by making it shorter, hence you have a variable attenuator. If you make it real short, then change position of this coupling antenna, your results might change quickly.

kch

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How to connect RF transmitter directly to reciever? 1

94Camaro

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