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# Antenna application theory

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#### DirtyLude

##### Well-Known Member
I just got a Radiotronix receiver module and a transmitter module from Mouser. I'm setting up a little test system to send serial data from one uC to another.

The datasheets talk about a 50ohm antenna and antenna tuning. I have no idea what this means. Anyone give me a simple breakdown on what this means and how I can make a somewhat optimally tuned antenna for this thing? I know there are 50ohm whip antennas sold seperately, but they cost 3x what each module costs, and I'm not looking to be that sophisticated. Is 50ohms the resistance of the antenna end to end?

I've googled some antenna theory pages on the net, but they are over my head and talk mostly about types, resonance....

Datasheets:

50Ohm antenna means that at the frequency the antenna is designed for the impedance is 50ohms at its input. This is the AC impedance of the antenna not the DC impedance. Any time you have a difference in impedance along a transmission line (wire) you get a reflection of the signal. Reflections decreace the power getting to the antenna and generally cause problems. You can see this effect with a long peice of coax cable (50 - 100 feet). you put a square wave in and measure the voltage at the input. You will see you square wave as it goes in and you will also see the square wave's reflection off the other end of the cable. Cable companies use this method to find illegal cable hookups.

You have a couple options: you can buy the antenna and make sure your PCB traces leading to the antenna are 50ohms (you can look up the equasions to get the right trace width). Or you can find a predesigned PCB antenna for your frequency. PCB antennas use traces on the board as the antenna. Designing an antenna isn't an option as it requires at best a lot of experience and at worst expensive test equipment and software.

Hope this helps
Brent

A simple antenna can be made if the frequency is VHF or higher, I got this design from the ARRL antenna Handbook, I have not built it:

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I have the 315mhz version of the transmitter and receiver. It says on the datasheet that the antenna can be made totally on the PCB.

Anyway, I'm just going to try a few things on the test setup and see if anything works. I'll buy a matched antenna if I put it into an actual design. In the meantime I'll try to read up more on what this all means.

Can the 50ohm cable be replaced with a 51ohm resistor?

Sure, you just have to somehow make the source = 50 ohms.

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