Benefits of transforming 1/4 wave into 1/2 wave PCB Trace Antenna (AN043/SWRA11) - Page 1
Forum section: electronics / rf-microwave

author: okw
author_rank: Frequent Contributor
timestamp: January 07, 2025, 03:13:27 am
content: I've duplicated the 2.45GHz antenna (1/4 wave) from application note AN043 (document SWRA117D), and it works OK. But I have a lot more space to spare.Are there any benefits of making it into a 1/2 wave?And any pitfalls going with a 1/2 wave?If beneficial, should I double the length measuring from the tip of the feed point, excluding the ground stub. Pluss a little extra, so I can manually cut and adjust my design after experiments. Or how to I calculate?I added all feed segments (ex ground stub) in the original 1/4 wave, and found it to be 31.5mm (1/4 wave 2.45GHz should be 30.6mm). I then extended the antenna, and ended up with 61.44mm (1/2 wave: 61.2mm).I also have a nylon spacer mounted in the hole as seen on the image (outer ring symbolise the outer dimension of the spacer). Will the spacer degrade the performance significantly?If there are benefits with the 1/2 wave, I will have to rotate the design, so it will not run under any spacers. Only problem is, on that side of the board, there is a 6 wire usb cable, that might tangle up close, or might not, depending on how the user assembles the box after every battery change.Also, am I supposed to copper fill at the distance shown as "Ground layer 2" in the image (from AN043)? It's quite close to the antenna. Is this a distance they carefully measured in AN043 to be optimum, or is this the minimum, and that it would be beneficial if I give the antenna more space before the copper fill? And should the copper fill start on the same line on both layers? (I have 2 layers, yes, terrible.Would the ground vias I placed do any good? In a straight line, zig zag as I did, closer together? And should I place them closer to the antenna?Many questions, but I'm just learning RF.

author: tchicago
author_rank: Regular Contributor
timestamp: January 07, 2025, 06:22:44 pm
content: My personal somewhat-educated opinion on the first part: won't work. You can increase the gain of antenna by making it collinear, i.e. having two pieces of quarter-wave antenna connected by a delay line. But increasing gain means adding more directionality to the antenna (radiation pattern will become much less omni-directional), which is probably not what you want for your device. Also, the calculations and modeling of this meandered f-type antenna shape are very non-trivial and are omitted in TI's appnote. They should be re-done when converting it into a collinear type.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 07, 2025, 11:42:13 pm
content: IF the antenna is 1/4 wave, the other half of the antenna is the ground , which makes a 1/2 wave antenna.If you make one part of the antenna 1/2 wavelength, you get a 1 wavelength antenna which does not work.

author: rsjsouza
author_rank: Super Contributor
timestamp: January 08, 2025, 12:45:27 am
content: I didn't do any calculations, but I don't think that simply doubling the dimensions on the meandering antenna will have the same irradiation pattern.A bit of additional information about the subject are in the e2e post below and the AN058 (SWRA161) app note.https://e2e.ti.com/support/wireless-connectivity/other-wireless-group/other-wireless/f/other-wireless-technologies-forum/114288/small-size-2-4-ghz-pcb-ifa-antennahttps://www.ti.com/lit/an/swra161b/swra161b.pdf

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: January 08, 2025, 02:59:11 am
content: For a dipole antenna, making the total size 1 wavelength completely destroys the performance of your antenna (see the chart below). The impedance of the antenna will be infinity which means it will basically be an open and you won't be able to get any power into it.Luckily your antenna isn't really the same as the classic dipole antenna. There is still literally zero benefit to doing this. You don't want extra directivity because then you'll have nulls, which means that your performance will drop significantly if you point the device in the wrong direction. Without EM simulation software it would also be difficult to figure out what the radiation pattern actually is. It's better to stick with the proven design from the appnote.

author: okw
author_rank: Frequent Contributor
timestamp: January 26, 2025, 05:43:47 pm
content: Thanks for all the help and input.I guess most of these zig zag antennas was made to save space(?).Would I benefit any from stretching the zig zag part out in a long line?

author: tchicago
author_rank: Regular Contributor
timestamp: January 27, 2025, 06:23:28 pm
content: Quote from: okw on January 26, 2025, 05:43:47 pmThat would be a non-meancered inverted F antenna. That one on your screenshot likely needs the additional impedance matching elements for connecting to a 50-Ohm radio.For a proven and thoroughly measured design for PCB use with 50 Ohm impedance, you may want to look here:https://www.ti.com/lit/an/swru120d/swru120d.pdf

author: iMo
author_rank: Super Contributor
timestamp: January 27, 2025, 06:30:59 pm
content: The benefit of 1/2wave is a bit better efficiency, but its feed impedance is high compared to 1/4 (low feed impedance), so you have to match it differently, imho.