Which Er to use for pcb antenna simulation and design? - Page 1
Forum section: electronics / projects

author: Spikee
author_rank: Frequent Contributor
timestamp: March 31, 2017, 12:35:23 pm
content: Hello,I'm wondering which Er value for Fr4 I should use for a pcb antenna design. Just the Er value shown in the dielectric datasheet at my frequency or the so called Effective Er since the copper of the antenna is exposed to the air (ignoring the solder mask).I have done a random test antenna design on a pcb and the center frequency was off from the simulation. Wondering if it is caused by effective Er or just manufacturing tolerances in dielectric thickness / Er value.

author: whollender
author_rank: Regular Contributor
timestamp: March 31, 2017, 04:05:13 pm
content: The fast answer is that your center frequency issue is most likely due to the fact that FR4 doesn't have a well controlled dielectric constant. It can vary a lot from manufacturer to manufacturer and batch to batch. Your best bet with FR4 is to measure the E_r on a board from the board house you plan on using, and use that in your simulations.The question of whether the effective E_r is a problem depends on whether you took the air interface into account when you designed the antenna. It's been a while since I did that sort of thing, but I'm sure that you can go about it either way (ignoring the effective E_r or not).I would also suggest removing the solder mask from around your antenna, as it will tend to load the antenna with its own dielectric constant, which is also un-controlled.There are more expensive substrates available that have controlled RF properties if you need that kind of precision. Most board houses have stackups with these materials, but be prepared to pay for it if you need it.

author: Spikee
author_rank: Frequent Contributor
timestamp: April 01, 2017, 04:02:39 am
content: It is a conversion from a wire antenna to a pcb antenna. So the "bare" antenna calculation is based on the dielectric of air.The formula of calculating this specific antenna is not known so it basically is conversion -> input into simulation program -> keep adjusting until correct.Not the most efficient method...

author: Spikee
author_rank: Frequent Contributor
timestamp: April 01, 2017, 04:55:23 am
content: I'm using ADS , PCB only contains the antenna , nothing else is on there.Antenna is a moxon which is basically a dipole with a reflector / concentrator I'd guess.On the next board I'm going to get the thinnest board thickness that they have (to combat losses) and route out the inside (empty) pcb space. Maybe that helps.Idea would to have it be low cost. A (decent) RF substrate is most likely too expensive.This is just me trying some things out and trying to understand some things about RF (antenna/Microstrip Antenna) design.This seems to be an area where you need a lot of resources to be able to take part.

author: Neganur
author_rank: Unknown
timestamp: April 01, 2017, 10:19:13 am
content: The only way to know for sure is to ask the manufacturer that you intend to use, they should be able to provide you with the info you need if you send them your stackup requirements.EDIT: or if you're impatient, copper tape and whatever FR4 you used before - then change the Er in ADS to see when the frequency matches your real antenna.Er effective is mainly a function of substrate thickness and trace width (to a lesser degree frequency, it affects more at higher values),I don't know how tight or loose the tolerances are for dielectric thickness but a 1mm pcb typically has +/- 10% specification I think.

author: Spikee
author_rank: Frequent Contributor
timestamp: April 01, 2017, 11:06:10 am
content: It is a single sided pcb only the antenna tracks and the SMA connector are on it.I have the tinyVNA to do the testing, it is ok ish but all I have for now.The copper tape method is a good one , saves me from spending 5-10 usd on getting 10 pcb's made each time and it is much faster.Will try it.Also I will ask my PCB fab for the Er of their exact material.In ads I tested different trance widths but they do not seem to effect the design(at all)

author: Spikee
author_rank: Frequent Contributor
timestamp: April 03, 2017, 12:06:32 pm
content: Just got a quote for RO4053B material and it was 7.5 usd in 100's qty, CHINA, 1L, 1.6mm, 110x70 mm. This seems rather excessive right ?Other option is FR4 and opt for dielectic control (I assume this means they will check/measure that the dielectic thickness / Er value is within your specified tolereance) ?Oshpark uses FR408 on their 4L boards, which seems pretty good. But that seems hard to get in china.Isn't there another dielectric that has a better tolerance spec than FR4 but is still fairly affordable and commonly used?

author: Spikee
author_rank: Frequent Contributor
timestamp: April 04, 2017, 03:36:10 am
content: It seems that the trick would be to put a coplanar waveguide at some places around the pcb antenna (with mouse bites, will break off) and make that 50z.opt for impedance control and specify those transmission lines as for them to check.Specify that those lines are 50z +-5% for example; the idea is if those lines are within +-5% the rest of the dimensions / dielectric thickness will also be.Thus providing a reliable and constant(ish) result.Because specifying and getting a specific stackup or better pcb laminate like FR408 is really difficult here in china. But getting impedance control (on lines) is fairly affordable.And by making it a co-planar waveguide with via stitching around the side (gnd) forces them not to change the design in my head but actually control the impedance.If anybody has a recommendation for a good pcb fab in china that has experience with these things please let me know.

author: Spikee
author_rank: Frequent Contributor
timestamp: April 04, 2017, 05:39:49 am
content: Thank you for all the info.I have reached out to MyRo and PCBcart. Waiting for their reply.I actually stumbled upon this yesterday:https://www.jlab.org/eng/eecad/pdf/053designop.pdfI have also reached out to people involved in the moxon antenna development / calculator program, to get access to the formula itselfso I can transform it to a specific Er (since in my testing that is not a linear factor). But have not had any reply after a week.This one was centered on 846 MHz in simulation @ 4.6 Er (soldermask cover over whole pcb, 1.6mm PCB):See attachment(I did not check / was not aware of the specific Er / Dk /stackup being used at the time)

author: Spikee
author_rank: Frequent Contributor
timestamp: April 04, 2017, 06:12:53 am
content: Quote from: evb149 on April 04, 2017, 05:50:21 amI have done BGA board 6L with DDR3 and the likes on it. Had to point out the lines that needed pcb fab impedance control and which error range and they did it with no issue.But for antenna design (especially when you are converting it from an air dielectric model) this makes it hard. The main issue at the moment is getting the simulation / the actual board to match.Since ~30MHz in center frequency difference is too much in my taste.Will also compare ADS vs sonnet simulation to see if the results are similar.

author: Spikee
author_rank: Frequent Contributor
timestamp: April 04, 2017, 07:01:06 am
content: Nothing special just 868 and 915 MHz.PCBcart can supply this:https://erp.suntechcircuits.com/reffile/FEAD4081-C117-4BC9-B5FA-E974E62CC5FD.pdfI'm no RF expert nor do I have much experience with programs like ADS just trying to figure it out myself.Have to compare my antenna in various programs too see if there is much difference there.

author: Koen
author_rank: Frequent Contributor
timestamp: April 04, 2017, 07:24:03 am
content: You* don't need all this. Keep it simple. Order from a good-enough-for-you supplier and optimize your antenna for them. Control each batch. They don't change their suppliers or stacks every week. The variation between PCBs produced on the same batch will be negligible.* : emphasize mine for the guy who will later share his work story at Lockheed working on 12GHz antennas with the budget of Angola.

author: janekm
author_rank: Unknown
timestamp: April 05, 2017, 04:16:19 am
content: JDB-PCB (seemingly the same factory as PCBWay) say they use Kingboard KB6160A FR-4 material:http://eurotronics.be/documents/Kingboard_KB_6160A_data_sheet.pdfSo that might be a convenient datasheet to get "close enough"?

author: Spikee
author_rank: Frequent Contributor
timestamp: April 11, 2017, 05:09:51 am
content: I got all the information I need now. Going to do a new simulation and run and will let you know.

author: Spikee
author_rank: Frequent Contributor
timestamp: April 12, 2017, 03:35:34 pm
content: I just did a ADS simulation.(loss tangent 0.018)4.61 Er:857.8MHz(4.61 * 0.9) = 4.149 Er:874.5MHz(4.61 * 1.1) = 5.071 Er:842.7MHz

author: Spikee
author_rank: Frequent Contributor
timestamp: April 13, 2017, 10:09:56 am
content: IFA generated with Antenna Magus and simulated in CST:http://image.prntscr.com/image/106f0ceca7ac4c6fb7842d6e6f403c8f.pngHow the feed port is automatically generated by Antenna Magus:http://image.prntscr.com/image/7a56335fbcd041d8a03cd1a3e7e7df8a.pngSame antenna imported in ADS and simulation is run:http://image.prntscr.com/image/442faeb9ff2e4c9988670de16bcca645.pngManually put in feed port:http://image.prntscr.com/image/19e5355645f74414b3e63f4c8e31d763.pngWhich one is right ? :O

author: Spikee
author_rank: Frequent Contributor
timestamp: April 15, 2017, 03:52:22 am
content: Does anyone have experience with either of these programs and can show some insight about accuracy or if i'm doing it properly.

author: Spikee
author_rank: Frequent Contributor
timestamp: April 23, 2017, 07:23:58 am
content: Quote from: evb149 on April 13, 2017, 05:23:11 amFirst I trimmed my antenna since center frequency was 878MHz instead of 868. So I just made it for 915MHz by trimming off 1.5-2mm from the "folded dipole".915MHz trimmed model:Than I thought about your idea and I basically stacked two together (note bottom one is not electrically connected other than the ground trace).Result: