The feedline is still part of the antenna. The best you could do with this
setup (the first image) is to decouple immediately after the SMA connector
(ideally before the connector). 61 or 52 material ferrite clamp-ons? A
small diameter (1/4-inch or less) single layer coil of possibly three turns
right after the SMA might also do the job. You need to get the outer
surface of the coax out of the picture. As it is, it forms part of the
antenna.
With the setup in the first image, lightly grab the coax and run your hand
up and down the coax between the PCB and the VNA with your fingers around
and touching the coax. Does anything on the VNA change? If not, you're
sufficiently decoupled. If so, and things change on the VNA while doing
so, you need to decouple the outer surface of the coax shield from being
part of the antenna.
Dave - W?LEV
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On Wed, Feb 26, 2025 at 3:57?AM Nico via groups.io <nicolassimard=
[email protected]> wrote:
Wow ! Thank you so much to every one for taking your time in helping me. I
truly appreciate it.
@Siegfried @John
I finally did it. I've cut down my RG174 down to the shortest length I
could (35mm). I'v calibrated at the end of an RG316 I have, then connected
the "board-to-pcb adapter". I added a 180ps Edelay. I tried to double the
value (for round trip) but with no differences.
@WOLEV : The Whole board measures 100mm and the ground plane measure 74mm
in length. I'm realizing at writing this this could be a part of the
problem ? If this is a 1/4 wave monopole, then it should needs the
equivalent as a ground plane, that could mean my ground plane is not long
enough ? it should be 84 according to the datasheet. mmm I may have to
reorder some pcbs !
@Jim
1- No, unfortunately I don't have the evaluation board. I'm on my own, I
know almost nothing in RF !
2- This antenna is designed for high volume applications. In my case the
"high volume" will be 2 units !, Anyways, this antenna is sold in either
through hole or surface mount. There are squares as pads for the surface
mount version. I may have to remove them as I use the through hole one.
Unfortunately, I won't have access to any design tools in that field. Even
if I had one, I would not know ho to operate it to extract valuable data...
If my LoRa (20mW of extreme power !) module is not 50Ohms on the output, I
will never know and there won't be any ways I'll be able to measure it.
@Roger
1)My pcb stack is a standard 4 layer from JLC PBC with controlled
impedance option selected. They offer the calculator for the appropriate
trace width. As per the datasheet of the antenna, the transmission line
from the RF module to the antenna itself has a ground plane on layers 2 and
3 to offer the return path for the feedline. This is what is recommended in
the datasheet. This is what I did and know I understand the reason. So, in
theory, from the LoRa module to the antenna, this should be a transmission
line and not a part of the radiating element. Thanks for pointing out ;)
2)With my short 35mm adapter tonight, I tried 180ps and 360ps as Edelay
with no differences.
3) I tried yesterday to calibrate with the cable shorted, opened and then
loaded with a combination of a 47 + 3 ohms of 0805 chip resistor I had on
hand. I don't remeber exactly and I don't have any pictures but by memory
it gave me negative values as the R. Assembling two chip resistor to make
one, not the greatest idea. I will have to order som 50Ohms chip resistor .
Question : Can I use 0805 it should definitely be 0603 ? I've somewhere
there could have capacitance effect on a too big of a chip.
4) Thanks for the info on ferrites. I'll give them a look.
So here is my progress so far:
- Tonight I'v cut the cable to 35mm but calibrated at the end of my 30cm
RG316. Look at the pictures to see the results.
- I've realized that my ground plane may be too short and have to modify
the boards and order new ones.
- I've ordered an RF inductor and capacitor kit for the CLC matching
network. I'll give it a try and see from there where it goes.
To be continued...
--
*Dave - W?LEV*
--
Dave - W?LEV
