ONLY if you test that coupon at 2,400,000,000 Hz!
Testing with the typical C Meter won't do it!?? The usual standard is 1000 Hz.
The Er changes with frequency when you get in the MHz.and up.?? Kent
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On Wednesday, July 27, 2022 at 07:04:11 PM CDT, Jim Lux <jimlux@...> wrote:
On 7/27/22 9:18 AM, KENT BRITAIN wrote:
? Hi Christian
I do lots of antennas on fiberglass PC Board, and typically the published Er value was measured at 1 kHz,
At 2.4 GHz, Er is going to be in the 3.7-3.8 range.? You can check their spec sheets for Er vs frequency plots, but that data is not commonly provided.
Most people do a test coupon, which is essentially what you're doing.
You have to pay BIG Bucks for PCB material that has little change with frequency.
Rogers Duroid(tm) is an example. It also has low loss at microwave
frequencies - the issue is the glass in FR-4. Taconic is another brand.
You can get them with epsilon all the way from 2 to 10.
For the record, you have not been able to buy FR-4 for some years.? It contained a Bromide Anti-Flammability compound that was banned by RoHS.??? Yea, lots of similar stuff on the market with slightly different part numbers.
Most fab houses still offer FR4
Note the epsilon they give for the various mfrs (3.2 to 3.92)
whether they are brominated, I don't know - there's lots of materials
that can make something fire retardant. I don't know that bromine is as
much a concern as lead, which is where RoHS gets to be a big deal.
has more info.? Polybrominated biphenyls aren't allowed in RoHS, but
that might not be a common problem.? There are "FR-4 halogen free"
materials too.
Best way to measure the Er at 2.4 GHz, bit out of the range for most Nano's, is to design a patch antenna for 2.4 GHz and see what frequency is really resonates at.? Then back into the calculations for a patch antenna using Er's that give you the actual frequency.?? Kent
? ? ? On Wednesday, July 27, 2022 at 10:49:19 AM CDT, steiniche@... <steiniche@...> wrote:
?
? Hi
I have a NanoVNA V2 Plus4.
I am trying to validate a PCB 50 ohm transmission line (trace) to be used for 2.4 GHz Wifi.
I am using a 4 layer stack-up (1.6 mm total) with the following configuration:
L1: Signal + ground pour
Prepreg 0.21 mm
L2: Ground
Prepreg 1 mm
L3: Empty
Prepreg 0.21 mm
L4: Ground
Board is FR-4 with Er = 4.6.
I have calculated the trace parameters using Saturn PCB design as a "coplanar wave" with a plane below.
I have attached 6 board files showing the actual board (board_image.jpg), rendered board (board_render.png) and the four layers (board_lx.png) + a screenshot of the calculations from Saturn PCB (calc.png). I use the following connectors from Molex:
I have calibrated the VNA in frequency range 2 GHz to 2.8 GHz with the provided SMA cable connected, see attached cal_load.jpg and cal_thru.jpg, direct_load.png, direct_thru.png
Now I connect the PCB trace and do a S11 and S21 measurement (see s11_s21.jpg). It seems to me that the result is not particularly good (see trace_thru.png and trace_load.png).
Can any of you spot any issues with the measurement method or configuration of the board? Also, if you can provide some realistic target results for such a configuration (two connectors + 82 mm trace) that would be great.
Any help is highly appreciated.
Thanks
br
Christian
? ?
