Author Topic: Considerations on Wireless Range  (Read 149 times)

Nick_W

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Considerations on Wireless Range
« on: September 09, 2016, 12:38:48 pm »
I have been looking at the range of the modules.

I was having problems connecting from the first floor of my house to the ground floor. Knowing that Low Power 2.4 GHz has trouble passing through obstacles, I was thinking this was simple attenuation.

Looking at the antenna design further though, I believe there are some fundamental antenna design considerations to take into account.

The module antenna seems to be 1/4 wave monopole (via a QFN balun) of the inverted F design.

I'm not sure exactly where the ground plane for the antenna is (I can't see it, but it has to be there).

So, things to take into account to get the best range from the module.

1) surroundings.
By this I mean things close to the antenna. Material close to the antenna will effect it's impedance, and hence it's matching (and thus reflected power). This includes plastic, wood, people, and metals. Antennas work best for the environment they are matched to (at design time). I don't know if the antenna was designed for a free air environment, but placing the module on a surface or in an enclosure can cause problems.

One reason I believe that I have some problems with range to a door sensor, is that it is attached to a door! Worse, the door is a steel door. Short of attaching a wire antenna, there is not much I can do about this though.

2) Ground Plane
A monopole antenna needs a good ground plane, but it must be a minimum distance from that ground plane. A steel door for instance a few mm from the antenna will not be good. I also noticed the coin cell slightly overlaps the antenna area. moving the coin cell may help improve range, or using the module with a separate supply (to remove the coin cell completely) may help. Avoid placing the module directly on a surface (close to the antenna), raising the module up from a surface a few cm could make a difference.

3) Shielding
The antenna must not be shielded by something conductive (like a faraday cage). An example would be a metal fridge or freezer, or a metal range hood. A metal wall box would be bad, use a low voltage plastic one. You can put the modules inside a metal enclosure, but the range is likely to be severely reduced.

4) Attenuation
Objects between the module and gateway will refract, reflect and attenuate the signal. Try to avoid objects in the signal path if possible. I know, people, walls, doors etc. are not something that can be planned for, but if you have the choice, minimize obstructions as far as you can. This could be as simple as avoiding the areas of walls/doors that have wood/metal behind them (like wall studs for example).

5) Orientation
An ideal monopole is isotropic (ie radiates in all directions equally), in reality this is rarely true. so the module/gateway will have greater range in some orientations than others. Due to the principal of reciprocity, if the transmit signal is better in one direction than another, then the receive gain will also be better in the same direction (ie the transmit and receive functions for an antenna are the same). This is why boosting the power/gain at one end of a radio system does not work, you have to either boost both tx power and rx gain, or boost tx power or rx gain at both ends.

Slightly adjusting the orientation of a module/gateway can make a connection better or worse, and is worth some experimentation with long range modules.

Polarization may also be important. Monopole antennas are polarized in the plane of the antenna, usually horizontal, or vertical. If the polarization is different between a module and the gateway, there will be a signal loss. Theoretically a 90 degree difference (ie module vertical and gateway horizontal) could result in a 20dB signal loss. In reality it's probably less than this, but it is a factor!

In summary, I have found that some module I though were out of range, could be brought back in range by adjusting the positioning/orientation of the gateway. this will be a compromise, but it is worth considering in the placing of modules/gateways.

The quoted 200m range is obviously under ideal conditions, and is the maximum possible. In the real world, a range of 30m would be considered good.

These are my thoughts on range and antenna considerations, it's a long time since I was designing antenna's, so please excuse any errors, also feel free to correct any bad assumptions or mistakes I have made.

FYI I believe the design of the antenna is based on this: http://www.ti.com/lit/an/swru120b/swru120b.pdf

I hope you find this useful.
« Last Edit: September 09, 2016, 01:43:58 pm by Nick_W »

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mc-Josh

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Re: Considerations on Wireless Range
« Reply #1 on: September 09, 2016, 04:30:32 pm »
Nick, thanks a lot for this detailed post. I just have a few comments to add below.

You are correct, we are using an inverted-F type antenna based on the TI DN0007 design note. The ground plane for the antenna runs the entire area of the mcmodule directly under it.

1) Surroundings: You are correct that the materials in the near field of the antenna have a large impact on the impedance which effects the matching. The antenna on the mcmodule is matched to an open air environment. Different materials have different effects on the antenna matching, plastics and wood have minimal impact whereas metals, liquids, and people (holding the device in your hand) have a huge impact on the matching. Placing the mcmodule on a drywall or wood surface will have minimal impact on the RF performance whereas placing the device on a metal door with have a LARGE impact, try placing the mcmodule as far away as possible from the metal surface for best results.

2) Ground Plane: We have done a quite a bit of RF testing of the mcmodule in an anechoic chamber enviroment. We did testing of the mcmodule both with and without the coin cell with no notable degradation of the RF performance with the coin cell installed.

3) Shielding: Placing the mcmodule in a metal fridge or freezer will have a huge impact on the rf performance. The only way for the RF signal to be received or transmitted would be through the non-metal gaskets of the freezer. Also, unless you are using a battery that is designed to work at lower temperatures the voltage of the battery will be lower with will effect the tranmitted power of the device.

4) Attenuation: Again, different materials attenuate RF signals worse than others. Metals, liquids, and people(mostly liquid) have a much greater effect then wood or plastics.

5) Orientation: Polarization of the antennas also has a large effect on the range. In our testing in an anechoic chamber environment we have seen differences of 9~10 dBm between horizontal and vertical polarization. Simply rotating your mcmodule by 90 degrees (or changing the position of the mcGateway) can have a huge impact on the range.

The 200m range we have tested is line of sight in open air.

The next iteration of the mcgateway will include a PA/LNA which will increase the power output and also the receiver sensitivity of the mcgateway. We also have a horizontally and vertically polarized antenna we can switch between to provide antenna diversity which will also increase the range. In addition to this, the mcMod120 includes a radio with slightly improved power output and receiver sensitivity as well.

If you have any other questions please post them.
 
mc-Josh

kbrooking

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Re: Considerations on Wireless Range
« Reply #2 on: September 09, 2016, 09:46:06 pm »
Great information. Thanks to both Nick and Josh for providing.

Any idea on how much further the range will be extended with PA/LNA?

Nick_W

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Re: Considerations on Wireless Range
« Reply #3 on: September 09, 2016, 10:32:24 pm »
Typical gains from a LNA are in the 10dB range, and in theory an increase of 5dB would double the range.

Don't get too excited though, there are other considerations. The main problem here will be crosstalk and interference. You will notice that the module frequency is 2.4GHz, the same as the WiFi frequency. Getting the on-board WiFi not to densensitize the module receiver will be a challenge. Probably a good reason not to use the gateway on WiFi if you can help it (maybe they have a clever co-existence scheme though).

Also the 2.4 GHz spectrum is jammed with signals, from WiFi to Bluetooth to Microwave ovens, extracting signal from this mass of noise is not easy.

I doubt anyone will give you potential range values without a lot of testing.

Having said that, if anyone is looking for a beta tester of a new gateway, I'm willing!

Nick_W

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Re: Considerations on Wireless Range
« Reply #4 on: September 19, 2016, 11:32:16 am »
Just a quick note here.

I have found that wood does make a difference to the signal level. In my (not very scientific set up), moving the gateway away from a wooden table makes 10 points difference on the RSSI readings, and an open or closed door also makes about 10 points difference.

My problem door sensor (on the steel door) was reading -97 Rssi, changing the orientation of the gateway slightly, and moving it away from a wooden (actually wicker) decorative strut produced an Rssi of -76. In the attached screenshot, it's the module C2 .
« Last Edit: September 19, 2016, 11:41:19 am by Nick_W »