Hi, I'm Dustin Morris. I'm an RF Antenna Engineer at DGI Wireless Design Services, and I'm going to talk to you today about some antenna fundamentals and some key antenna parameters to get you started on your next off the shelf antenna integration or custom antenna design.
So, starting off with wavelength, which is defined as the speed of light over frequency. We can see that as frequency increases, wavelength decreases. I have a couple examples here say at 3 gigahertz, your wavelength is 10 centimeters , and at 900 megahertz, your wavelength is 33 centimeters. So, as your frequency is decreasing, your wavelength is increasing. So, why is that important? Well, to have a well-performing antenna, the physical dimensions of the antenna are going to have to be an appreciable factor of a wavelength, typically a quarter wave or larger. So, you need design enough space around the antenna and a ground plane length that's long enough to allow an efficient antenna.
Frequency has a direct impact on your device size and your antenna performance. Antenna Impedance is defined as the voltage over the current at the antenna input terminals. Now, most data sheets are going to tell you that their antenna is 50 ohms, which is really not the case. And that's why there's metrics that define how well the antennas match the 50 ohms. There's Voltage Standing Wave Ratio, VSWR, and Return Loss. Now, VSWR should be less than two, and Return Loss should be less than minus 10dB, and that correlates to a 90% of transfer efficiency .
We're only losing 10% in our reflections. Now, a key takeaway from this is that you're going to need an impedance matching network between your antenna and your radio. Now, if you look at the antenna data sheet and the guidelines, they're going to tell you what kind of components which are typically inductors and capacitors that you're going to put between the antenna and the radio. And you want to put these as close as possible to the antenna. And this is going to maximize your power transfer from your radio to your antenna. Antenna efficiency is by far the most important antenna parameter for these small wireless embedded devices, and it's defined as the power radiated over the power input to the antenna. Basically, the higher the antenna efficiency, the better the antenna.
Think more bars on your cell phone. I mean, this is a really key parameter that you want to look for in these data sheets. If the datasheet does not mention the efficiency of the antenna, find another datasheet. Your target or goal should be somewhere around 50% but just know that this is highly dependent on the RF environment and the ground plane of your device. If your ground plane is a lot smaller than the evaluation board of the antenna, you're going to see this number drop considerably. If you have absorbers like batteries or human tissue next to your antenna, this number's going to drop considerably in your implementation. Directivity is defined as the the ability of the antenna to focus or concentrate energy in any given direction.
You'll have low directivity antennas like a dipole that will have this omnidirectional radiation pattern, where the bulk of the energy is spread out equally over all distance or all angles from the antenna. And you can have high directivity antennas that focus the energy in like a single beam so think like satellite dish antenna. Gain is defined as the efficiency times the directivity.
Gain is always going to be lower than directivity. As we mentioned over here, the efficiency and we were targeting at 50%, you're probably not going to get anywhere close to 100%, but it's really dependent on the antenna design. So, for an example say your satellite dish antenna has a 40 dbi directivity antenna and the gain is probably going to be somewhere between 38, 39 dbi. Right? There's going to be some losses here. You're going to have conductor losses. You're going to have dielectric losses and then, of course, we're going to have our mismatch losses, and don't forget you're going to have losses in impedance match network. Now, for your omnidirectional antenna, the same thing. You have a dipole with 2.2 dbi directivity. The gain is probably going to be closer to 0 dbi. And all that correlates to an antenna efficiency of about 60%, so this is still a well performing antenna even though the gain is 0 dbi.
Another key takeaway is that a low Gain antenna can still be a very well performing antenna. Now, all of these parameters are only usable over a finite frequency range, and that frequency range is the bandwidth of the antenna. So, you're going to have an impedance bandwidth, you're going to have an efficiency bandwidth, you're going to have a gain bandwidth . So basically, you can't use a GPS antenna for Bluetooth, you can't use a WiFi antenna for cellular. You need to pick the right antenna for your application because they all have a finite frequency range that they can operate over. I know this is a short crash course in antenna fundamentals. There's only six antenna parameters here, and there's many more antenna parameters, but, you know, these are key antenna parameters for small wireless embedded antenna design. And I hope this short talk will help you with your next off the shelf antenna selection or start you on your next custom antenna design.