A simple rectanna

Basically a rectifier converts AC power to DC, and a rectanna converts microwave power to DC. It’s like “rectifier” and “antenna” combined, hence the name “rectanna”.

I always wanted to have a dipole based rectanna on hand, when I explain circular polarisation to someone. And I have designed a very simple microwave rectifier with RF Schottky diodes bat62 from Infineon long ago, but hesitated to make the PCB. Untill recently JLCPCB offered $1 pcb prototyping for 2 layer board within 10cm by 10cm.

The Rectifier

Here is the schematic of the microwave rectifier. It is basically a voltage doubler, but use RF diode instead. The source will be replaced with a dipole to form a full rectenna.  The load resistance R1 mimics a LED.

Simulation shows that the input impedance does not change much if the capacitance C varies between 47pF and 100pF, and load resistance R1 varies between 30ohm and 1kohm.

The Antenna

The antenna is modified from a RFID antenna ( Alien 9662), and has been opted to match the rectifier. The loop like structure is a T-match trace to tune the input impedance of the antenna.

Antenna simulation shows good impedance match around 920MHz, which falls into North American UHF RFID band (905~928MHz).  In the above figure, S11C represent complex S-parameter, which is referenced at the rectifier input impedance instead of standard 50ohm.

Full Rectanna

Altium was used to build the full circuitry and antenna,  and below is the schematic. The antenna was represented with a shorting wire. Diode is bat64 with sot143 package from Infineon. The diode is just a normal 0805 LED, green works better if you ask (Green always looks brighter with same current).

Here is what the PCB looks like, it is 0.8mm FR-4 board. Since JLCPCB’s offer is within 10cm by 10cm, so I placed two in a row. And you can download the Gerber file here.

It works!

To test the rectenna, a Motorola RFID hand held reader was used to send the microwave signal (30dBm output with a 3dB circular polarized antenna). Although the diode rectifier is not very efficient the reader signal can still light up the LED even after 1 meter away.