The Antenna Size Equation

 In f3 wireless

So when you’ve worked on over a hundred wireless IoT (and M2M and home automation, and radio telemetry etc.) end devices, you start to see patterns. I’ll talk a bit about these more in the blog as we go along but today it’s the thing many of these devices have in common, radio performance problems specifically brought on from the size of the product.

All wireless devices need a way to radiate their information signal. This is true for transmitters and receivers, antennas are reciprocal devices so the antenna performance is just as important for a receive only application like GPS as it is for cellular or WiFi. That radiating element is the antenna. You stimulate the antenna with RF energy (which is nothing but a high frequency sine wave) and a good antenna radiates a large proportion that RF energy as photons.

The size problem comes into play because the size of the antenna is directly proportional to the wavelength your radio has to use. For most commercial products, you have to use whatever frequencies are dictated by the radio you choose and those are usually chosen for good propagation performance, not small implementation size. While a radio may use a range of frequencies it’s the lowest, which has the longest wavelength and dictates the size of the antenna.

When it comes to antennas, the major performance criteria for a given frequency are size, bandwidth and efficiency. You can have any 2 of those 3 things. For a single frequency narrow band radio like a GPS receiver, you can play tricks to shrink the antenna and still have good efficiency but the bandwidth of that antenna, and thus it’s resistance to being de-tuned by nearby objects in the environment like people, car seats etc. is poor. For cellular where most radios have to operate over a huge range of frequencies, you’ve already lost the bandwidth and part of the above equation, the bandwidth is dictated by the fact it’s a cellular radio as it is the frequency of operation. This all shakes out to mean that your only remaining variables are size and efficiency. If you make it smaller, it will perform worse.

Seems pretty constrained right? All that holds for cellular everywhere. To an extent it holds for any wide bandwidth radio service like WiFi as well. Now for cellular in North America, here’s where it gets worse. You as the product developer don’t actually get to define what level of performance is good enough. The cellular carriers have to ensure a minimum level of performance for devices on their network in order to ensure the system actually works. Since efficiency is then set, for a cellular antenna for use on a NA network, that means you device antenna size is pretty much set. The one caveat here is that it’s the antenna volume that set. Some antenna structures are inherently planar but some are not.

So here’s the punch line. For a NA cellular device, your mechanical engineer dictated if your device will ever comply with carrier certification when they made their initial sketches and defined the initial size of the product. This is the first reason it’s critical to have expert guidance from the beginning of a wireless development project. Antenna performance is tightly coupled to mechanical factors like device size, location of major system components such as display, connectors, batteries. If your antenna performance isn’t good enough simply because of these machnical factors then your device won’t ever pass certification without changing those factors. I’ve seen many dozens of products that were “done” that had to get done over. Scrap the mechanical tools, the packaging, the ID and start over. My favorite being a certain coffee maker with a cell device in it that already had 30k units built before they even considered doing the cellular radio testing. They ended up scrapping all the electronics boards in those 30k units and completely rebuilding them. The moral here is that most of the risks in wireless product development don’t have immediate obvious symptoms you can detect and react to. By the time you realize there’s a problem, you have to make major changes to do anything about it.

So what do you do about it? Here’s that thing I say everybody hates, It Depends. There is no simple rule, every product is different so every solution is custom. At F3 Engineering we have proprietary high speed, low cost processes to mitigate these risks. These are things you only do with wireless products and you only know to do them if you’ve done this before. Working with us on your product ensures you don’t step on that first landmine with your first step forward. Even if you have internal engineering and you’re project is in progress, a quick review by our experts can make all the difference.

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