PCB omnidirectional antenna design - Page 1
Forum section: electronics / beginners

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: November 27, 2014, 07:25:15 pm
content: Hello, is anyone able to point me towards some good resources on PCB antenna design?Im currently working on a project which i will need an antenna to work on 2.4GHz-2.5GHz (ISM band).The chip i am using has a balanced antenna output (ant1 and ant2) in which its matched impedance is 15+j88Ohms (im saying this so you can help point me to the right resourcesThank you

author: Howardlong
author_rank: Super Contributor
timestamp: November 27, 2014, 07:49:25 pm
content: Does the device not have a reference design, or is this a wideband device that you're using at 2.4GHz?Typically, manufacturers of devices designed specifically for a particular band such as 2.4GHz will provide a balun filter for balanced RF connections out of a bunch of lumped inductors and capacitors to provide an unbalanced 50 ohm feedpoint, and possibly some PCB jiggery pokery in addition to Ls and Cs, which will specify a board stack up as well as a layout.If the antenna is to be fabricated on the same PCB, then the antenna match can be combined into the balun filter to minimise losses.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: November 27, 2014, 08:16:01 pm
content: Its an RF24L01+ and it does have a circuit design in the data sheet which has a 50ohm network however for it to be able to achieve the max power i need to make the antenna matched for the output of the chip (well thats my understanding anywayIt is a wide band when running at 2Mbps data rate

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 27, 2014, 08:24:04 pm
content: If it has a 50 ohm matching network, it already takes care of the 15 + j88 ohms for you. Of course, you have to make sure the antenna is 50 ohms where it connects to the transmission line.Tim

author: Howardlong
author_rank: Super Contributor
timestamp: November 27, 2014, 08:37:41 pm
content: In appendix D of the data sheet, there's a 50 ohm balun filter right there for you.The next step is to choose a PCB antenna, and add any matching, usually an L and a C, and as I mentioned earlier you can often combine that matching into the existing balun filter design to reduce losses and simplify the design to some extent.I did do quite a bit of work on patch antennas for 2.4GHz about fifteen years ago, both on PCB and with air dielectric, but patches are not omnidirectional.The other problem is that your average FR4 is a bit lossy at 2.4GHz, having an antenna in free space is always going to be better than having it shrunk down onto a PCB due to the permittivity of the substrate, plus usually that antenna these days is hidden away inside an enclosure, another battle for the RF to get in and out.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: November 27, 2014, 09:53:06 pm
content: Could i not design an antenna which directly matches the output instead of changing it to 50Ohm?

author: Howardlong
author_rank: Super Contributor
timestamp: November 27, 2014, 10:36:14 pm
content: The best way to get 50 ohms across the full 2.4GHz ISM band is to use a 50 ohm resistor, but it won't radiate very well, at least not at 2.4GHz. That's why it's called a dummy load. But it is broadband.Another way to get close is to use a 1/4 wave ground plane in free air, characteristic impedance not a million miles away from 50 ohms.Designing antennas is one of those wonderful areas of engineering where there are a number of parameters that are not mutually exclusive, they interact, and you have to come up with a compromise.Here's a list off the top of my head...a) Impedanceb) Radiation patternc) Bandwidthd) Efficiencye) SizeYou simply won't get an antenna that's 50 ohm resistive across the band, unless it's a resistor. The most efficient antennas are resonant, and so have a limited bandwidth. You can increase bandwidth, but it'll be at the expense of efficiency and/or size.So you have to choose the order of your priorities are design around that.By far my best recommendation from what you've said so far is to find a generally acceptable proven PCB design, and go with that. You can model antennas, but again, it's like the squidgy balloon metaphor, you fix one parameter and that messes up with one or two other ones. Far easier, unless you want to spend months if not years of your life learning about the various intricacies and voodoo of antenna design.However, if you do, Antennas by John Kraus is the generally accepted seminal work to start with. More applicable to what you're doing is the Microstrip Antenna Design Handbook or Microstrip and Printed Antennas: New Trends, Techniques and Applications, I have the former, it's a very dull read, you've really got to be desperate, I guess it was written by a few guys who were after their PhD.As I mentioned earlier, matching your antenna reasonably well to 50 ohms is usually a matter of an L plus a C for the 2.4GHz band, $0.10 of parts. If efficiency is important to you, your design may benefit from wirewound inductors which are less lossy than multilayer, but they're also higher Q, so you may find you suffer reduced bandwidth in your matching network. Standard C0G/NP0 ceramics are usually fine for the caps. There are a few online impedance matching calculators designed just for the job of figuring out the LC network. You enter your centre frequency with source and load complex impedances and they figure out the values for you. Usually there are two options, one high pass and the other low pass. Which one you choose is up to you, but as I mentioned you might end up with one that, when added to your balun filter, ends up with two Cs or Ls either in parallel or series, and so you can merger them into a single part. Equally, sometimes, one of the options might present a part that is simply silly, like 0.01pF or 100pH, in which case choose the other option.

author: KJDS
author_rank: Super Contributor
timestamp: November 27, 2014, 10:42:43 pm
content: ...and once you've followed Howard's advice, you're then left with the realization.Incorporating the strays into the design is where the difficult bit lies.

author: hagster
author_rank: Frequent Contributor
timestamp: November 27, 2014, 10:53:10 pm
content: Everything Howard says is true.Kraus is the bible, but it's a bit heavy going.http://www.antenna-theory.comis a good online sourc of knowledge.I would suggest a PIFA antenna as a fairly common solution to this problem that is fairly easy to tune to your needs.I'd also say that without the right test kit you will be just guessing and unlikely to achieve good results.There are chip antennas that you can use that will give ok and reliable results provided you follow the datasheets.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: December 01, 2014, 12:17:32 am
content: Thank you for the informationIl definatly be having a look into this, test equipment wise i should be fine its just getting the actual design of the antenna done