1- Would you recommend to clean all 3 elements of the matching network, as
I don't know which component will go where ?
Can you send me a schematic of the matching network? Possibly I can figure
it out. Is the matching network from the supplier or derived from you VNA
measurements?
I'm annal, but could you give me a clean S11 measurement from 902 through
928 MHz (the entire ISM band) of the unit in its case. Clearly the
dielectric of the plastic case is strongly affecting the behavior of the
antenna.
Looking at your previous postings, I can only conclude the antenna designer
developed that out of the plastic case. I seriously doubt he/she evey
evaluated the result that is published in the plastic case. I base that on
the relatively good S11 outside the case only requiring a single 10 nH
series inductor against the relatively poor S11 when inside the case.
Yet another request: Measure and send to me an S11 measurement of ONLY the
antenna. Attach the ferrite decoupled coax at the point on the PCB where
the transmitter energy is connected directly to the antenna with no
intervening components. I wat to determine how the antenna behaves both
inside and outside the plastic case.
2- If you look at the trace between the components, you'll see I made the
trace the same width as the pad. Is this a good idea ? The pad's center to
center is 4mm. The trace 0.9 wide, with a surrounding ground plane and via
fencing I treat it as a coplanar wave guide and the rest of the trace and
clearance are in accordance with that. The idea behind making the trace
between the component is to avoid width bumps.
Since these low power "transmitters" are pretty insensitive to SWR (or
reflection coefficient), I believe you did the right thing by keeping the
trace width the same as the pad for the series inductor. Of course, this
avoids an impedance bump in the path to the antenna.
3- Is my via fencing overkill putting them shoulder to shoulder ? Would a
1/10th of 1/4 wave spacing be enough for a coplanar wave guide ?
We typically use 1/10 spacing between the vias. However, that is not the
free space spacing but the spacing within the dielectric, Vp. Vp in a
dielectric goes as the reciprocal of the SQRT of the relative dielectric
constant of the dielectric medium. Since you are using FR-4, in round
numbers, the dielectric constant would be {SQRT [avg(3.8 and 4.8)]}^-1 =
{SQRT [4.3]}^-1 = 0.482. This multiplied by the free space spacing will
give you the "in-dielectric" spacing.
Yes, the via "fence" is always recommended to confine fringing fields from
propagating throughout the dielectric.
4- Could you suggest me a type of inductor in particular that would
appropriate in the 0603 package for what I'm doing ?
Typically, these days as a result of the cell phone and WiFi industries
operating up to and including 5 GHz, most of these parts are OK for the 900
MHz ISM band and through the 5 GHz ISM band. However, I still recommend
cleaning any copper from under the SM inductors and/or capacitors.
BTW, Yesterday I took a reading with the board flat outside its case. I had
installed 2 ferrite in a row along the cable. I had a VSWR of 1.19:1 all
across the bandwidth from 900 to 930MHz which is better than the spec
sheet. If only I could get below 2 while in its case...
This is why I'm requesting measurements of both the antenna only inside and
outside the case with your ferrite decoupled coax leading to the VNA.
Recently, I've had a couple of (paid) jobs where the plastic case
significantly altered the behavior of the intended antenna. You're not
alone, be rest assured.
We are finally touching on the design "gotcha's" regarding PCB design at ?W
frequencies. Possibly we should take this off the nanovna group. But,
it's still good information for those unfamiliar with these techniques.
Please let us (I'm one of the moderators of this group) if this thread bugs
anyone. It's good information for everyone concerned and certainly touches
on applications of the NANOVNA and any VNA.
Dave - W?LEV
On Thu, Mar 20, 2025 at 12:53?AM Nico via groups.io <nicolassimard=
[email protected]> wrote:
Dave,
Thanks for the hint.
1- Would you recommend to clean all 3 elements of the matching network, as
I don't know which component will go where ?
2- If you look at the trace between the components, you'll see I made the
trace the same width as the pad. Is this a good idea ? The pad's center to
center is 4mm. The trace 0.9 wide, with a surrounding ground plane and via
fencing I treat it as a coplanar wave guide and the rest of the trace and
clearance are in accordance with that. The idea behind making the trace
between the component is to avoid width bumps.
3- Is my via fencing overkill putting them shoulder to shoulder ? Would a
1/10th of 1/4 wave spacing be enough for a coplanar wave guide ?
4- Could you suggest me a type of inductor in particular that would
appropriate in the 0603 package for what I'm doing ?
BTW, Yesterday I took a reading with the board flat outside its case. I
had installed 2 ferrite in a row along the cable. I had a VSWR of 1.19:1
all across the bandwidth from 900 to 930MHz which is better than the spec
sheet. If only I could get below 2 while in its case...
Thanks,
--
*Dave - W?LEV*
--
Dave - W?LEV
k6whp
