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Power detector using Low power Rectenna

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masamune42

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Low power rectenna

Hi
I want to design a RF power detector at 900MHz. No power supply, portable.
I was thinking of a rectenna for -60dBm input rf power and output of few mV.
What kind of antenna can I use ?
Initially i thought of antenna + resonator + rectifier, but now I'm thinking that may be inefficient , instead maybe I can use high q antenna + rectifier.
Know any high q antenna for this ?
 

You can make a simple rectenna for 900 MHz by installing a Schottky mixer diode into a half-wave dipole. The DC current to load can be taken using two small chokes connected to diode terminals.
Your idea of rectifying -60 dBm is not correct. Sensitive detectors usually start to generate several mV from a RF input > -20 dBm. To obtain some usable power, the RF power level to your dipole-detector rectenna will have to be > 10 dBm; then you can measure 1...3 V DC on a DVM with 10 Megaohms input resistance.
Generating a power to be used e.g. to light a LED would need > 10 dBm of RF power incident to your rectenna.
Typically, similar rectennas are combined in a matrix and irradiated by RF so that all operate optimally. Individual DC outputs are connected in parallel or in series, to deliver a desired voltage and current.
So far the best solution is being developed in "energy harvesters", to charge a capacitor over time ( the rectified current is lower than 1 mA), then, to transmit a short burst by a connected transmitter.
Rectenna like the described can be used to detect an operating transmitter but not more, I think. Many experimenters have tried to generate more power...
 

    masamune42

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Thanks for the matrix and charging capacitor methods.
I don't want to power LEDs just want to detect few mV , to see if the RF power in the area is above certain level. A power detector. I may require a high gain, narrow bandwidth (high q) antenna.
 

Dear friend:

With a simple detector what I wrote is valid. If you need to indicate a lower-power but narrow-band signal, you can use a suitable preamplifier with > 40 dB gain which can , with the diode detector, achieve the lower limit of -60 dBm.
You can also try AD8313 , a logarithmic detector IC from Analog Devices (www.analog.com), they have a number of such devices usable from ~DC up to ~ 8 GHz. I think the AD8313 can detect -50 dBm over a wide band.
Also, for a certain frequency of interest, you can use a directional antenna; its gain will also help you to lower the detectable power limit.
 
Hi
I want a passive circuit not active, so I cannot use those ICs. I want it to work as both passive power detector and rf energy harvester.
Using directional antenna is a good idea, that will help to increase gain.
Thanks
 

When omni directional behavior is not required, first start with dipole/patch over a ground plane to get some gain. When using a real microwave diode (BAT15, or equivalent), you can get significant increase of sensitivity by matching the diode to your source. In a parallel equivalent circuit, the parallel resistor will be in the kOhm range.

An end-fed half wave dipole has high impedance already. When placed above (parallel to) a ground plane it will increase to kOhms also, so you only need to compensate the diode capacitance with an inductor. The transmitting band for GSM is not very wide, so you can accept high Q-factor.

I would recommend the dual diode voltage doubling approach, as you can use a diode at the rectified output to prevent reverse breakdown. Reverse voltage rating of microwave diodes is about 4V. to get full DC EMF of the diodes, you need a voltmeter with > 1MOhm input resistance.

10mVp RF across the junctions of a voltage doubler, results in about 2mV DC out. 10mV into about 2 kOhm equals about -46 dBm. Every time you halve the RF input voltage, the DC output drops with factor 4. Also note that GSM uses 1/8 duty cycle, so the actual output voltage will be less.
 

    masamune42

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