Antenna placement

I developed a device that can be mounted inside a waste container. The device is battery-powered with an expected lifespan of about 2 years and publishes hourly data. It is designed to monitor the following things:

  • Container collections by the truck
  • The fill-rate of waste inside the container (i.e. 0% is empty, 100% is full)
  • Temperature (for potential fires)

My issue: I have been testing 15 units of this device for a while now in various areas - while it works, for a few locations the connectivity was excessively poor, which I attributed to bad antenna placement. I tried tackling this issue with a new design (see image), but would greatly appreciate additional feedback first.

In this design I will:

  • Use an SMA TG 22.0112 antenna located at right side of the enclosure away from metal parts
  • Partially run the antenna trace from the Electron trough the PCB to avoid having to run a long cable
  • Move the battery further to the left (away from the antenna)
  • A large area of the PCB GND planes will be removed underneath the antenna

The devices are mounted almost upside-down inside the container, so the signal would mostly travel through the PCB.

Are there any improvements I’m forgetting about that could improve the connectivity more here?

I’m not sure I understand this point. Having a single coax cable from antenna to Electron is likely to be better than having antenna to carrier PCB to connector to coax cable to Electron. The longer cable could be a cost issue, but unless the cable is meters long, the RF performance of a single cable is likely to be better than all those connections, each of which has the potential for small amounts of signal degradation.

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That’s true…I can add additional holes in the PCB, and run the cable underneath the PCB (sandwiched between PCB and enclosure).

Other than that, is the placement of the antenna in this image ok? I’m not an engineer and I don’t have specialized equipment to test the reception. The next step would be to hire an engineering firm to develop an antenna for my device, but I’d first like to give it a try myself first as it’s obviously cheaper. The battery is about .250’’ away from the antenna - would that be close enough to affect the signal a lot?

It’s hard to tell without testing. I can tell you the things I would be worried about with that antenna placement:

  1. It looks like your antenna will be horizontal (i.e. parallel to the surface of the earth) in your final application. Most cellular signals are polarized +/- 45 degrees to approximately match how people hold their phones when talking. Changing the antenna angle could increase signal strength. This is a fairly minor effect but could make or break reception in certain cases.

  2. You have that big battery and its metal bracket near the end of the antenna. The battery has a conductive metal case that acts as a shield for the antenna so that almost no signal will be able to come from the left side in your picture above. If you can get the battery more than say three antenna heights away from the antenna, you will get better results. If you could put the antenna outside the box so that the battery was below the connector end of the antenna, you could use that battery as a virtual ground plane which would be helpful. As it stands I think you are blocking a lot of signal with that battery placement.

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  1. Relative to the Earth, the position of the antenna would differ by about 60 degrees. Would it be best if this position was upwards, or downwards? In the image, it would be 60 degrees downwards. I can rotate the position 180 degrees to give it a 60 degrees upwards angle.

  2. Unfortunately, due to space constraints the battery can’t be moved away, and the antenna can’t be placed outside the enclosure (damage + waterproof reasons).

Antenna height = is this the height of the antenna relative to the PCB, or the thickness of the black part of the antenna?

With the current placement, the connector end (silver) of the antenna would be almost 1’ higher up than the top part of the battery.

  1. +/- 45 degrees with the connector end of the antenna closer to the earth is ideal. You can have any orientation and get signal, but the max will be at those points if you can arrange it.

  2. The longest dimension of the antenna is what matters. I guessed that as about 35mm which for 800-900MHz band would make the near-field distance around 6.5 to 7.35 mm. You need to be significantly farther away than that, say 10x or around 70mm, to get the best signal. The large metal close to the antenna does two bad things–it blocks the signal from that direction but it also de-tunes the antenna slightly making it less efficient.

It doesn’t sound like you can change it anyway, so it seems you are stuck.

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