Beginner Question: Rectennas - Page 1
Forum section: electronics / rf-microwave

author: norand
author_rank: Contributor
timestamp: February 28, 2020, 05:54:47 pm
content: Hi All,I have some very broad questions regarding Rectenna use for RF energy harvesting.For my final year project, I am attempting to design a low power monitoring system with the use of a Rectenna (most likely to recharge a battery to extend battery life - although before I start researching had some false hope on running the whole thing without batteries!). I am not looking for full solutions just pointers on where I should take the project.Now, I have spent the majority of my time designing the sensor system, and neglecting the Rectenna side. The original plan was just to design a basic rectangular air-spaced patch for simplicity (Considering GSM-900) with co-axial feed with rectifying element on the back of the patch, and hence the point of co-axial feed would be the point of Impedance match.Now there is a couple of issues with this that I can see:Only one rectifying element, and hence efficiency is poor, especially at light loadsThe patch for GSM-900 is narrowband and thus only harvest limited powerIt is very difficult to impedance match the diode without a known loadMy Supervisor is suggesting that I cut back the project and continue with idea above, or use an existing paper/design.I obviously would like my design to work as best as possible, but RF is a bit of an art and I don't have the complete knowledge or time to design a complete Rectenna from scratch (I've only studied the RF side of things for ~4 months).There are many suggestions which I could use to improve the existing design such as:Using a multiplier network on the output (two or more diodes)Attempting to designing a multi/dual-band rectennaUse another configuration of Antenna (such as cross-dipole)Use an array of Antennas, and combine them at the rectifying stageAs I approximately have around 10 weeks left to deliver the project, I don't want to add terrible amounts of complexity, but I would like the design to work well enough. A back of the envelope calculation suggests to me I need to harvest >150uW to stay above sleep current of my system, and account for leakage, rectenna efficiency (typ 40%) and regulator efficiency (10% at light loads). I think this is a achievable using the papers I have read as an examples, however, I know most of the papers use known RF sources, and they will also be a lot better than what I can design.I guess my real question is... Rectennas is a very broad sector, what is worth me learning/implementing into my deisgn within the time frame to get my best attempt out of this project I can? I know it's a very broad question it would just be useful to have some pointers. I feel my supervisor knows the result is going to be great so he is keen on getting me to a point that I have something (even if it isn't great) just so I have something to write about.I'm happy to provide more information if possible as I appreciate all the help on this forum. Thanks in advance from me, a worrying student!

author: Gribo
author_rank: Frequent Contributor
timestamp: March 02, 2020, 06:32:42 pm
content: TI has some reference designs (WiSP) that harvest RF energy, They use 2 stage multiplier. Which diode are you going to use? Analog devices has some parts that start working at 0.2V.

author: norand
author_rank: Contributor
timestamp: March 03, 2020, 07:59:15 pm
content: Was looking at using the following part as it looked the best from what I looked at:https://www.analog.com/media/en/technical-documentation/data-sheets/ADP5091-5092.pdfDiodes wise, I was looking at using HSMS-28XX series - but I haven't designed the rectenna stage as of yet. Using the AD part looks like I will need a voltage multiplier, haven't looked at how many stages I will need yet - so TI references are a good shout to look at.My tutor did suggest perhaps simulating the Antenna with tracking generator on a Spectrum Analyser, as I have ones available that can provide up to 0 to -50dBm.