The Antenna Size Equation - F3 Wireless

## The Antenna Size Equation

When you’ve worked on hundreds of wireless products, you start to see patterns. Radio performance problems specifically brought on by the size of the product is something many of these devices have in common.

All wireless devices need a way to radiate their signal well. 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 on which your information is modulated) and a good antenna radiates a large proportion of 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 (cellular for instance) may use a range of frequencies, it’s the lowest, with the longest wavelength, that 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 narrowband radio like a GPS receiver, you can 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 part of the above equation. The bandwidth is dictated by the fact it’s a cellular radio as is the frequency of operation. This all means that your only remaining variables are size and efficiency. If you make it smaller, it will perform worse.

Seems pretty constrained right? All those factors hold for cellular anywhere in the world. 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 maintain a minimum level of performance for devices on their network in order to ensure the system actually works. Since the efficiency is then constrained for a cellular antenna on a North American network, that means your device antenna size is pretty much set. The one caveat here is that it’s actually the antenna volume that is constrained. Some antenna structures are inherently planar but some are not and using all 3 dimensions can help performance.

So here’s the punch line. 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 one 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 and location of major system components such as display, connectors, and batteries. If your antenna performance isn’t good enough simply because of these mechanical 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 be done again, because they simply couldn’t be made to perform well enough. Scrap the mechanical tools, the packaging, the ID and start over. My favorite example is a certain coffee maker with a cell device in it that had 30k units built before they even considered doing 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 resolve. By the time you realize there’s a problem, all you can do is make major changes.

So what do you do about it? It depends. There is no simple rule, and every product is different, which means every solution is custom. At F3 Wireless 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 a landmine with your first step forward. Even if you have internal engineering and your project is in progress, a quick review by our experts can make all the difference.

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