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Topic: RF PCB traces (Read 1 time) previous topic - next topic

PaulRB

That's a neato PCB, Paul! It's like a project board with radio built in, right?
Yes, it is a sensor node using LoRa radio for use on TTN or similar. The prototype area allows the user to choose type of sensor.

BogusException

Yes, it is a sensor node using LoRa radio for use on TTN or similar. The prototype area allows the user to choose type of sensor.
Just curious... How does that radio do against others you've tried? I have a few sitting around that i never used. I do know the range/ etc. of tbe nrf24... I need to make similar to get rid of my breadboards scatteted around the house!

ElCaron

#2
Jun 06, 2018, 11:24 am Last Edit: Jun 06, 2018, 11:25 am by ElCaron
With the RFM95 @ 868MHz, I got 20-30m in a building and a few hundred meters to a kilometer outdoors with spring antennas.

Compared to the nrf24, it is a completely different league.

PaulRB

#3
Jun 06, 2018, 06:08 pm Last Edit: Jun 06, 2018, 06:17 pm by PaulRB
Data packets from my prototype LoRa sensor have been picked up 11Km away. But that was by a proper LoRa multi-channel gateway with a high gain outdoor antenna. The sensor's antenna is just a length of wire. My self-built single-channel "gateway" doesnt seem to be able to pick up signals from the same sensor at more than a few tens of metres, about the same as WiFi. I suspect the antenna I have on my single-channel gateway is rubbish. It was sold as 868MHz and claimed a 3dB gain, but it arrived in a plastic bag with no labels on it and looks identical to the 2.4GHz antennas sold for use with WiFi routers.

ElCaron

It was sold as 868MHz and claimed a 3dB gain, but it arrived in a plastic bag with no labels on it and looks identical to the 2.4GHz antennas sold for use with WiFi routers.
The 868MHz antennas I got from Aliexpress are much longer than most wifi antennas. They work for my rfm69 MySensors network, with a longer range than Wifi. I use them on the gateway. All the sensors and the test equipment for the rfm95 use spring antennas and it works fine. If I did the calculation correctly, at max power, more than a spring antenna isn't allowed in Europe anyway, at full rfm95 transmission power.

PaulRB

The 868MHz antennas I got from Aliexpress
Can you post a link please ?

ElCaron

#6
Jun 10, 2018, 07:37 am Last Edit: Jun 10, 2018, 07:38 am by ElCaron
SMA
Spring
Don't forget to take care of trace impedance to the SMA connecor.

PaulRB

Thanks, I've ordered the sma antenna to try
Quote
Don't forget to take care of trace impedance to the SMA connecor.
How? I am trying to keep the PCB track between the LoRa module and the sma socket or wire antenna hole as short as possible. What else should I do?

ElCaron

#8
Jun 10, 2018, 10:20 am Last Edit: Jun 10, 2018, 10:20 am by ElCaron
Thanks, I've ordered the sma antenna to tryHow? I am trying to keep the PCB track between the LoRa module and the sma socket or wire antenna hole as short as possible. What else should I do?
Set trace width, distance to groundplane to the sides and groundplane on the bottom such that you get a trace impedance of 50Ohm: http://spok.ca/index.php/resources/tools/99-cpwcalc

Use a line of vias on each side to connect he side gp o the boom gp. I usually put a ground fill area with adjusted settings around the trace.

PaulRB

#9
Jun 10, 2018, 11:03 am Last Edit: Jun 10, 2018, 11:25 am by PaulRB
Thanks. So I need to find out the PCB thickness and dialectic constant, and the track height that easyEDA offer, then I can adjust the antenna track width and clearance to get 50R using the calculator on your link.

Quote
he side gp o the boom gp
Sorry, you lost me there! Did you mean "the side ground-planes to the bottom ground-plain"? I guess I am trying to form something similar to a coaxial cable using the tracks, vias, ground planes and substrate.

PaulRB

#10
Jun 10, 2018, 11:54 am Last Edit: Jun 10, 2018, 11:57 am by PaulRB
From easyEDA's basic board offer:

Height = 1.6mm = 63mil

Thickness = 1oz = 1.4mil

Substrate = FR4, so I'm assuming dialectic constant ~ 4.5

I get ~50.8R impedance using track width 24mil and clearance of 5 mil.

Am I doing it right? Feels like a very wide track and very narrow clearance!

allanhurst

#11
Jun 10, 2018, 12:18 pm Last Edit: Jun 10, 2018, 01:03 pm by allanhurst
Co-planar? ie is there an earth plane underneath? And FR4 is normallly Er 4.7 - 4.8. Check with your PCB supplier.

It all makes a difference

Draw us a picture of your transmission line  and we'll figure out it's impedance - or there are plenty of calculators on the net.  Microwaves101 is an excellent source.

What frequency are you at? above about 5GHz FR4 is rather lossy.

Allan

PaulRB

#12
Jun 10, 2018, 02:22 pm Last Edit: Jun 10, 2018, 03:05 pm by PaulRB
Thanks Allan,  yes, ground plane on underside of board, 868MHz LoRa. I am using the calculator posted by ElCaron in post #43 which suggested FR4 was 4.3-4.8 so I took the mid point.

easyEDA's spec says
Quote
2 Layers


FR4,1.6mm,1 oz,HASL,Green Solder Mask,White silkscreen
Here is the topic I started asking for help. @ElCaron @allanhurst would you mind reviewing it? The exported .jpg picture from easyEDA is not the best representation, but you should be able to see what I'm attempting. The antenna connection is bottom right.

allanhurst

#13
Jun 10, 2018, 08:34 pm Last Edit: Jun 10, 2018, 08:43 pm by allanhurst
All I see is a picture of a via. 

You have to reckon this as inductor of 0.2 - 0.5 nH depending on the hole diameter, length and plating thickness. It's bad practice to use them in series with an antenna  because of this variability . If you want an earth use many in parallel or eg a fan terrnination over an earth plane.

Keep everything on one layer.

I'll have a look at your link and get back to you.

edit : had a look, and it's irrelevant.. Correction? /edit

You can model this sort of stuff with eg HFSS and Sonnet etc , but the seats aren't cheap. and they're not easy to use.

Allan


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