Circuit : John Samin VK1EME 
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What can you use to test how effective your antennas are for 2.4 Ghz? Which antenna has the best gain or, how do you know that there is any 2.4Ghz RF transmitted? Here are the details on how to build a general purpose 2.4Ghz Radio Frequency Field Strength Meter. This one was built using the microwave rated diode from a MICROTEK solid state microwave leakage detector (purchased from Dick Smith Electronics for around $24) these diodes can be more expensive than that if purchased in single units from electronics suppliers. There may be other suitable diodes available. Electronics stores also sell Schottky Hot Carrier Diodes that will probably also be suitable for this application.
The antenna is a 2 element quad. I've orientated it in the diamond configuration so it should be effective for both horizontal and vertically polarised signals. You could build the antenna in the vertical or horizontal sense if you like. The antenna was constructed on a right angled BNC connector, however I'm sure you could come up with a different sort of plug setup that would still provide good results. Just keep the lead lengths to a minimum to reduce losses. I have used an attachment that allows the BNC connector to be inserted into my Voltmeter. I switch the Voltmeter to Millivolts, point it at the 2.4Ghz RF and read the result. The yellow plastic cylinder is used to keep the antenna separation at 10mm. I cut a channel into the plastic to allow the wire to sit tight, and pushed some liquid nails into the hole to hold it. The bottom of the reflector loop is held to the BNC connector with another dollop of glue.
The detail of the antenna plugged into my Voltmeter.
Above is the antenna plugged into the Volt meter. It works pretty well, pointing it at the SUN also gets a reading! Point it at the microwave oven and it will exceed the Millivolt scale! With a little work I'm sure you could build a radar detector... I tuned the capacitor with a plasitc screwdriver to get maximum reading from a 2.4Ghz RF source. You should use a Wireless LAN card as the source.
Here is the schematic detail (not to scale), you should make the elements of the antenna as close to the correct size as possible. This will ensure maximum energy is absorbed at 2.4Ghz. The elements should be spaced around 10mm apart. The antenna will display some gain and uni-directionality, so point the smaller antenna loop (driven element) towards the RF source you wish to measure. I tried connecting the antenna directly to a microamp moving coil meter, however there was very little meter deflection from a Wireless LAN card. The electronic voltmeter is far superior.
DIODE Update!The original diode in the Microwave detector has been hard to find. I have found a supplier for the diodes.... Purchased here:
http://www.xs4all.nl/~barendh/Cateng/Cateng_diode.htm
Site Main Page : http://www.xs4all.nl/~barendh/Indexeng.htm
This site has many GHZ rated Diodes you may want to check out... Here is a quote from the website:
"Following point contact diode for Ghz usage are originally marked units. Being detectors for frequencies up to 12GHz depending upon type numbers these are also excellent noise sources, because of the extremely high cutoff frequency. Technical details are available on ordering. Stocked: 1N21B 1N21D 1N23ER 1N416B 1N416E from $3.58"
Questions?
Email jwsamin@mrx.com.au