Just Got My Omnidirectional UWB Antenna Design - Page 1
Forum section: electronics / rf-microwave

author: neilhao
author_rank: Regular Contributor
timestamp: August 22, 2021, 12:31:36 am
content: This is a Symmetric Antenna. It uses rectangular slot and attaching strip for size reduction. This antenna is printed on a 30mm*32mm FR4 PCB with desirable symmetrical radiation pattern. The bandwidth is expected to cover UWB Channels 2, 3,5 and 7.Project Wiki:https://uniteng.com/wiki/doku.php?id=uwb:evb#omnidirectional_uwb_antenna_designUWB Two Way Ranging Demo (Youtube ):https://youtu.be/uXin88bQMeg

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: August 22, 2021, 02:26:29 am
content: Looks like your cal is a little dodgy, as though you calibrated at the VNA port then measured with with a 1 meter cable to the antenna. The 125 MHz ripple can't be real for such a small antenna.

author: neilhao
author_rank: Regular Contributor
timestamp: August 22, 2021, 02:46:50 am
content: You are right, the result looks a little bit strange.. However, I still did not find out the exactly reason for this phenomenon. I calibrated the VNA several times, replaced the SMA cable, still got the similar results. I switched back to my previous version of UWB antenna, got the smooth results

author: Hamelec
author_rank: Regular Contributor
timestamp: August 22, 2021, 04:44:45 pm
content: You calibrated the VNA at the end of the cable or direct at the VNA?

author: neilhao
author_rank: Regular Contributor
timestamp: August 22, 2021, 04:56:57 pm
content: Quote from: Hamelec on August 22, 2021, 04:44:45 pmThe VNA should always be calibrated at the end of the cable, or the nearest point to the DUTHowever, for this time, calibration did not solve the problem

author: hagster
author_rank: Frequent Contributor
timestamp: August 22, 2021, 05:05:22 pm
content: What software did you use to simulate this? Looks like the measured lower frequency is a bit higher than the simulated.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: August 22, 2021, 05:42:37 pm
content: if you used the same VNA cal to measure your previous antenna and it was smooth, yet this one is not smooth with the same cal, just switching antennas real-time, then it's not the cal. But it must be something with about 4 nsecs delay as the ripple pattern shows. The antenna is way too small to support that ripple pattern (especially if the simulation does not show such resonances) so other things must be causing something like reflections from nearby (metal desk, metal wall), ground plane effects (if the ground of the coax is not isolated from the new antenna in the same way as the old one), or just a bad spot in the test-cable that comes and goes with flexing (move the cable around with the antenna on it, and see if the ripples come and go). If the ripples come and go with the antenna movement, replace the antenna with the load and move it around again. If the ripples show up in the load it implies a bad cable. If it only shows up with the antenna, then I think it must be some reflection from the environment I don't usually see such ripple with return loss measurements as the range loss is usually big enough to make this effect small; unless your antenna has a lot of gain; but this is wideband so I would expect it to not have much gain. Drop the antenna in a cardboard box lined with absorber and see if they ripples go away (attached to the end of the cable and VNA of course). Where the antenna has a return loss of -19 dB we see a 1 dB ripple, indicative of a secondary signal adding and subtracting with the antenna return-loss at a value of about -37 dBc, which seems quite broadband (the +-0.25 dB ripple near 2.5 GHz at -5 dBc indicates the same level). Is you cal-load maybe not quite 50 ohms (-37 dB ~= 51.5 ohms or 48.5 ohms). But this would only account for the ripple if you calibrated at the end the VNA and then added the test cable after cal.

author: neilhao
author_rank: Regular Contributor
timestamp: August 22, 2021, 06:05:48 pm
content: I thought about the ground plane, the ground plane should be one factor to count. Since there are strong Ground Plane Effect for this kind of 1/4 wavelength UWB antenna. There are a lot of research papers related to Reduced Ground Plane Effect for UWB antenna. E.g. "An RF cable from the Vector Network Analyzer is connected to the SMA to excite the antenna. In the measurements of small antennas, the RF cable usually affects the performance of antenna under test greatly" - Zhi Ning Chen,Terence S. P. See, and Xianming Qing in their paper "Small Printed Ultrawideband Antenna With Reduced Ground Plane Effect".As a verification, I tried to affect the measurement results by touching the SMA cable. Notable difference of results was observed, high quality cable was better but the affection was still there. General monopole antenna never acts as this.However I am not sure the ground plane is the only major factor or not.

author: neilhao
author_rank: Regular Contributor
timestamp: August 22, 2021, 06:10:44 pm
content: Quote from: hagster on August 22, 2021, 05:05:22 pmHFSS

author: Hamelec
author_rank: Regular Contributor
timestamp: August 22, 2021, 07:10:05 pm
content: more or less well defined FR4 maybe not the best choice for 4GHz..