Thick FR4 with photoresist - Page 1
Forum section: electronics / rf-microwave

author: ilektrik
author_rank: Newbie
timestamp: March 17, 2019, 09:37:12 pm
content: Hello guys!Me and my team are trying to construct a microstrip patch antenna for a university project and would really appreciate your advice. We have simulated a design with electromagnetic analysis software, which consists of a substrate 5-6 mm thick and has a dielectric constant with value er=3.5 with very promising results. We would like to make a prototype in order to validate the analysis performed and thus we have been looking for photoresist FR4 boards with the aforementioned characteristics. Unfortunately, we have only come across boards with 0.8 or 1.6 mm thickness and er=4.5 in most casesDo you happen to know any companies that would produce thicker pcbs with photoresist?We have also considered multilayer pcbs but we are afraid that the cost will be prohibitive for us if we order a substrate specifically designed for our application. Do you have any thoughts or suggestions regarding this matter?Thank you!

author: E Kafeman
author_rank: Regular Contributor
timestamp: March 18, 2019, 03:31:32 pm
content: For cheap prototyping split antenna in several layers and order PCB from JLCPCB or PCBWAY. 0.8mm FR4 will cost almost nothing.Glue each layer together for needed thickness. Interconnect each layer with vias as needed.Dielectric constant affects mainly as a scaling factor for antenna element size. Can be adjusted to fit your needs.It is else easy to add air between each layer or by adding holes so that average dielectric constant is close to what you want.It happens I do like this. At same PCB do I design several versions of same antenna PCB surface.Each version have slightly modified scaling as it can be hard to exactly predict how a printed PCB behaves relative a simulation in ideal environment.That makes it fast to find a working antenna for actual need.Assume dielectric constant from data sheet is as a rough estimation, which affect by many factors. Effective value depends also on your design, such as if a lot of holes, vias, layers.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 18, 2019, 03:49:09 pm
content: Quote from: ilektrik on March 17, 2019, 09:37:12 pmI worry that you have started at the wrong end. Unless you are in research and your goal is to figure out what could be possible, you have to always start with what is available. Unless you have a big budget, you are almost certainly going to have to go for FR4, which tends to have a relative permittivity of 4.2 - 4.5 . Most PCBs are made to be 1.6 or 0.8 mm thick, though you can look around on the manufacturers you can work with (I don't know if your university requires you to stick to a certain manufacturer - I know back in my bachelors we had to use a specific vendor with a specific stackup). From there, you can try and design a antenna that meets your specs (important here is to also sweep over some variations in permittivity, to see how sensitive your design is. Usually, your professors will also like hearing you did such a thing in your report\$^1\$).If you can't meet your specs, you can look at using two very thin PCBs - one with a patch and the other as a ground plane. Then have four holes in the corners. Use 4 plastic/nylon screws and some nylon nuts to place the two PCBs above each other. Using the distance you can often even tune your antenna a bit to make up for variability. Make sure to include the bit of wire connecting the ground plane to your coax connector!I found an image of this concept on google, seems to be from a report or paper but cannot find original source:\$^1\$ As someone who teaches at a university, I would actually prefer students give me a shitty antenna, but looked at things like variability, robustness, understanding what is going on, instead of an antenna that works great because they had the EM optimizer work on it for 3 days, and the students then have no clue what is going on or why it stops working when I put my hand near it

author: geggi1
author_rank: Frequent Contributor
timestamp: July 07, 2020, 11:27:34 pm
content: I know this is a very late reply!How about using a thin brass sheet over an brass ground plane with air dielectric.cutter knife oThis is a common way to make a patch antenna for ham radio.The radiating element can be made i of a sheet that is a few tennth of a mm and the groundplane ca be a mm thick.By doing it this way you are able to adjust the distance to the groundplane for optimum Z0 and radiation.Another option is to use a sheet of PTFE and copper tape.The PTFE can be machined down to the correct thickness and the coppertape can be cut with a sharp box cutter knife.I assume that your university got a machineshop with a mill trim the PTFE to correct thsickness.Other materials than PTFE can be used if the dielectric values are avalable.

author: hagster
author_rank: Frequent Contributor
timestamp: July 08, 2020, 05:37:46 am
content: FR4 can be fairly lossy at higher frequencies, which is why you don't see many patch antennas on thick or stacked FR4. Normally they use an air gap(or foam gap) as this has the lowest losses. For an educational project this might not be important and you can obviously experiment with the number of layers to tune your antenna.An alternative approach is to use 3D printing to create the spacer. You can still use thin FR4 for the top metal. The hard part here is to find a filament with known dielectric properties. If you happen to have a stratasys machine available they do publish basic permittivity and loss tangent data. You can also get RF quality filament from PrePerm with various properties and low loss. Otherwise you might need to experiment. The great thing about 3d print is you can vary the infil to change the properties. E.g. if you print a 50%infil the effective permittivity will be the average of air and you plastic.Here is an interesting paper on the subject.http://radio.feld.cvut.cz/conf/poster/proceedings/Poster_2018/Section_PE/PE_043_Vesely.pdf&ved=2ahUKEwiPwf3u9bzqAhXGQhUIHQ76DesQFjACegQICxAH&usg=AOvVaw29WgLT3hfP1cPnHeiKtL7y