How to build a cheap power and effective

VSWR & POWER METER

for Wireless LAN.
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This project will work best if it is in a diecast aluminium box, but it should work ok in open air. You will also need double sided circuit board for this project. Cut the board to the correct size, the through RF track should be at least 90mm long. You may extend the conductive track if you require longer, but do not make it shorter. The coupler sections need to be 30mm long each, you may not change these as they are critical to the operation of the circuit. You must also keep the width of the conductive track and the coupler sections to 3mm wide. This determines the impedance of the circuit tracks, but it is also dependant on the type of circuit board you use. I have no control over that so I've simply used a value that closely fits most boards. The spacing between the two RF tracks should be exactly 3mm. Any closer will detune the main RF track and any further away it may not pick up enough signal.



The other tracks on the board are for passive components and it isn't critical what size they are. The reverse side of the board will be ground, none of it is etched, so cover it with a sheet of contact to stop the acid eating it when you etch it. When you have finished etching the board it is ready for components. I would expect you to use panel mount N-Type female coax connectors, SMA or even BNC/TNC connectors if your trying to keep it cheap. Solder the conductor to the RF track and the shield to the ground plane on the other side of the board.

Next you will require two hot carrier schotky signal diodes. Again Jaycar and DSE sell them. The guy behind the counter won't have a clue in DSE, so use the catalogue to find the correct diode. You will also need two 1nF capacitors, two 47 ohm resistors, two 10K resistors, a 1K ohm potentiometer, a switch and the most sensitive Ammeter that you can get hold of. A microammeter (uA) is what you should be looking for.

Solder the two schotky diodes in place, make sure the black band on them is on the correct end (away from the RF). Then beside the diodes, solder in the 1nF capacitors and beside them solder the 10K resistors. At the other end of the sensor tracks solder the 47 ohm resistors. Remember that whenever you are soldering on an RF track to keep the leads as short as possible or use surface mount components if you can. Finally you must connect the back of the board to the front earth tracks. Drill small holes (yellow) in the board and solder a piece of wire through the hole so that both sides are connected. Now connect up the switch to the output of each diode and the meter and the 1K potentiometer and your ready to test it.




HOW IT WORKS.

The circuit board you have made is what is commonly known as a directional coupler. You can buy these commercially made for big bucks, but they are basically the same thing just made a little better. Most of your RF energy travels through the VSWR meter, but a small amount of signal is induced onto the shorter strips running parallel. About 1/10th of the power is picked up by the second track. This is signal is also travelling in the same direction as the original signal. When it gets to the diode it is converted to a DC value which is then displayed on your meter. The more power, the more DC signal.

The diode is acting just like a crystal radio you may have built as a kid, only this one is working at 2.5GHz. The potentiometer adjusts the level to make it more useable. Since each side of the circuit only detects power traveling in one dirrection you need to have a switch to swap from the forward reference power to measure the reflected power on the other side of the circuit. Both sides are identical.

If you have ever used a CB SWR it's operation is exactly the same. Turn on your transmitter so that it produces a single carrier. (your LAN card may not be able to do this and you will need to find another signal source). Set the switch to forward power and adjust the level so that the meter is set to the highest number on the scale and not hard up against the right hand wall. Then set the switch to reverse while the transmitter is still running. The lower this reading, the better your antenna. I would expect that whatever your meter has for it's highest value, that a good reading was anything below a third of that number.
example: 50uA maximum on the meter, then any reverse power less than ~20uA would be a good reading.

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