Wifi Ax Design ?? - Page 1
Forum section: electronics / rf-microwave

author: dazz1
author_rank: Frequent Contributor
timestamp: August 14, 2019, 08:42:49 am
content: HiI have just done some antenna testing on wifi 2.4GHz and 938MHz (UK license free freq).The results are somewhat disappointing, especially for the 938MHz Ax.I used a Siglent SA setup as detailed here:[url=https://www.eevblog.com/forum/testgear/test-results-for-the-e-bay-rf-bridge-in-an-enclosure/msg1627414/#msg1627414]https://www.eevblog.com/forum/testgear/test-results-for-the-e-bay-rf-bridge-in-an-enclosure/msg1627414/#msg1627414] https://www.eevblog.com/forum/testgear/test-results-for-the-e-bay-rf-bridge-in-an-enclosure/msg1627414/#msg1627414]https://www.eevblog.com/forum/testgear/test-results-for-the-e-bay-rf-bridge-in-an-enclosure/msg1627414/#msg1627414]https://www.eevblog.com/forum/testgear/test-results-for-the-e-bay-rf-bridge-in-an-enclosure/msg1627414/#msg1627414] https://www.eevblog.com/forum/testgear/test-results-for-the-e-bay-rf-bridge-in-an-enclosure/msg1627414/#msg1627414to do return loss testing. Note the peaks above 0dB in the attached charts are due to local area radio signals being detected by the antenna.I expected to see a high return loss at the design frequency.The first result (ax1d.png) is from an Aliexpress supplier of a 938MHz Ax. Looking at the test results, it performs much better at 1.5GHz than anywhere else. As a 968MHz antenna, the measurements indicate poor performance. Being Aliexpress, quality is not always something that can be expected.So I tested another Aliexpress wifi 2.4GHz antenna (AX2.png). This indicated a -26dB return loss @ 2.4GHz, which is better than I should get from the directional bridge I used.I also have a "quality" 968MHz antenna supplied with a development kit by Digi(AX3.png). As far as I can tell, it is this antenna here:https://nz.element14.com/abracon/aeacaq190012-s868/whip-antenna-lin-868mhz-3-5dbi/dp/2886242?st=antenna%20868mhzsourced from a USA based company. The results should have been good, but weren't. The return loss was good but not at 968MHz. The measurements show that the "high quality" antenna had 2db less return loss at 868MHz than the cheap Aliexpress antenna.These results show that price and country of origin is not correlated to performance.I suspect that regardless of where I buy these antenna, or what brand is on the bag, they likely come out of the same factory in China. If that is true then there is little point in trying different suppliers. That leaves me with modifying/tuning the antenna. I could do that by trial and error but I would much rather start from a mathematical approach.Does anyone have any design information on this style of antenna? When taken apart, I can see that they are based on a dipole but they include a coil in the top wire and various other features that could be important. I found this page:https://skootsone.yolasite.com/wifi-ant.phpbut lacks engineering analysis/modeling. I anticipate there are more than a few thesis or journals out there that have sufficient information for me to intelligently tune these antenna.Dazz

author: dazz1
author_rank: Frequent Contributor
timestamp: August 14, 2019, 09:13:21 am
content: HiFor reference, this is the plot of my bridge. It compares a short (full reflection, min return loss) with a 50ohm termination.This plot effectively defines the range of the bridge across the frequency range.The glitch at ~1.6GHz is consistent with resonance within the enclosure. Something I should do more to fix.Dazz

author: dazz1
author_rank: Frequent Contributor
timestamp: August 16, 2019, 10:04:31 pm
content: HiFor comparison, the two attached images are exactly the same except one makes use of the Siglent SA30xx return loss measurement feature.A common feature with these antennas seems to be dual resonance to widen the bandwidth, at the expense of lower gain.There is likely to be improvements gained by optimizing the design and tightening the manufacturing tolerances.Dazz

author: tautech
author_rank: Super Contributor
timestamp: August 16, 2019, 11:07:05 pm
content: dazz, some of the antennas out there are real borderline and do not meet spec.Even from reputable suppliers some are very suspect in even they offer a wideband performance their best Log Mag can be well outside your frequency of interest.IME select antennas with a narrow band spec at the frequency you need it to get reasonable results.Have you used these guys ?https://www.gowifi.co.nz/antennas.html

author: dazz1
author_rank: Frequent Contributor
timestamp: August 17, 2019, 04:40:32 am
content: HiI am building a system for the UK which uses 868MHz. Not legal in NZ so there are no local suppliers of suitable antenna.Further test results show that paying a high price from reputable dealers is no assurance of good performance.The mechanical tolerances required to achieve on-spec performance are likely to be much tighter than the actual manufacturing tolerances.My best option may be to rebuild cheap Chinese made (which is probably all of them) antenna.Dazz

author: tautech
author_rank: Super Contributor
timestamp: August 17, 2019, 05:10:27 am
content: Understood.As an example I swept one of these for a customer that being used at 925 MHz and it was so far off optimum it would be close to the 868 MHz you need.https://www.gowifi.co.nz/omniaaa/ant-53.htmlThey had erred on selection and gone for the cheaper wideband antenna instead of one of these:https://www.gowifi.co.nz/omniaaa/ant-35.html

author: dazz1
author_rank: Frequent Contributor
timestamp: August 17, 2019, 11:23:40 am
content: HiI think I need to get back up to speed with NEC to model antenna designs.A long time ago I did antenna design with a program that used the equivalent of chromosomes, random parameter changes and generational evolution to optimise antenna design. It worked well. I will have to search and see if it is still available.Dazz

author: dazz1
author_rank: Frequent Contributor
timestamp: August 17, 2019, 11:27:42 am
content: HiFound this NASA paper on evolutionary antenna design. I don't know if it the same software I used but the methodology is the same.https://pdfs.semanticscholar.org/9852/e0cedb68343a6445a764051001688a95dbac.pdfDazz