Application Specific Antenna

Choosing an antenna can be a difficult task, there are several parameters that need to be taken into consideration. It is imperative to examine several physical, electrical, and economical options to find the perfect antenna for a given application. While this is not a “one size fits all” topic, and much of this may be rudimentary for the RF engineer, hopefully this will shed light on many factors that need to be considered when choosing an antenna.

Desired frequency range

A very convenient part of antenna design is that, once you know your application, you will then know which frequency band you will be working with. Many common frequencies that are used today are the 902 - 928 MHz band for things like amateur radio, 1575.42 -1610 MHz for GPS, and 2.4 GHz and 5.8 GHz for Wi-Fi. This by no means covers all wireless applications, but it paints a good picture of some of the major ones.

The frequency that you will be working with will play a role in the size of antenna that your application will call for. Recall that an ideal antenna will be some fraction of a wavelength, typically either ½λ or ¼λ with a ground plane. Ground planes help to reduce the necessary size of an antenna. While we would ultimately like to see ½λ, we can get around this by adding a ground plane for the antenna to radiate against. This can be as simple as a metal layer on a PCB which ultimately makes up the other ¼λ.

Since higher frequencies have smaller wavelengths they can use much smaller antennas. This is a very good thing for the engineer working on an embedded design where space is very limited. An example would be the 2450AT43B100E from Johanson Technology Inc. This antenna is 7 mm x 2 mm and can handle WLAN, home RF, Bluetooth, and 802.11 b/g applications.

Using the equation to calculate wavelength (see Equation 1), we can see how the size of the wavelength corresponds to the size of the antenna. The speed of light is 299,792,458 meters per second, and a 2.4 GHz signal has 2.4 billion cycles per second. To find the wavelength we need to divide the speed of light over frequency giving us 0.125 meters, or 12.5 cm.

Equation 1: Wavelength (λ) equals the speed of light (c) divided by frequency (f).

Equation 2: Calculation of 2.4 GHz wavelength.

Ideally, we would be looking for ½ λ or ¼ λ of this for the length of our antenna and 7 mm is just a little over half putting us about where we want to be. The 2450AT43B100E datasheet explains why this would be a good antenna for a 2.4 GHz application, but not particularly well suited for another frequency. This can be illustrated below by looking at this graph that charts the return loss by frequency:

Figure 1: Typical frequency performance of the 2450AT43B100E chip antenna. (Image courtesy of Johanson Technology)

As you can see, the center frequency of this antenna is showing about -23.976 decibels of return loss at 2.47 GHz with values of -10.246 decibels at 2.58 GHz and -10.151 at 2.38 GHz. This is an example of an affordable antenna that is well suited for an embedded design. Since return loss equates to the amount of signal that has not been radiated from the antenna, but rather sent back to the generator or amplifier, the lower the return loss the better.

Johanson has created a chip antenna selection guide that can be very useful in choosing the proper product. If we were to examine the same frequency with a different application, the proper antenna may look much different. Is this antenna going to focus more on cost savings, or quality? One is not necessarily better than the other for your bottom line. Imagine how many sensors are being used wirelessly in today’s world. Many applications that use antennas do not require the most sophisticated equipment to get the job done. The antenna you use for your Bluetooth is not as critical as the antenna on top of the air traffic control building at your local airport. This would be a good time to mention another way of getting around size restrictions. Ground planes help to reduce the necessary size of an antenna. While we would ultimately like to see ½λ, we can get around this by adding a ground plane for the antenna to radiate against. This can be as simple as a metal layer on a PCB.

Imagine you are contracted to locate an antenna for a taxi company or municipal bus service that needs GPS capability in their vehicles so they can keep track of their fleet. This presents challenges that the previous design did not. Previously we were challenged by size restrictions, now we are not as limited by space, but challenged by the environment and noise. We will be looking to find a panel mounted antenna so that it can be attached to the vehicle, and since it will be outside it must have at least an IP67 rating. The MA650.ST.AB.003.DZ by Taoglas Limited is an example of an antenna that can provide this level of protection while meeting the frequency bands for cellular and location-based services. There are a couple of center frequencies for this antenna, the 1575.4 MHz band is used for GPS and the 1602 MHz band is used for GLONASS (Russian version of GPS). This has an IP69K which means that it is going to be able to resist dust and high temperature, high pressure washing. This is extremely useful on a vehicle that may go through a car wash. Another point to highlight with this antenna is that it will require power due to the LNA (low noise amplifier) it has. While this antenna has a 3 dB gain on its own, with the LNA it has additional gain; this will be determined by the voltage the antenna sees. This unit can be powered with a DC source from 1.8 V - 5.0 V.

Antenna characteristics

LNA characteristics

Figure 2: Antenna and LNA characteristics for the MA650.ST.AB.003.DZ antenna. (Image courtesy of Taoglas Limited)

Like anything in electronics, it all boils down to what the application calls for.  Sometimes a custom antenna is the best solution. Taoglas offers services that may be helpful for those looking to work with a cellular application. Cellular design will typically be higher volume, at which point a custom design may end up being a lower overall cost. This allows you to work with other engineers who specialize in antennas to solve physical restraints and meet the stringent network requirements. Different cellular carriers will have different approvals. Verizon and AT&T will have different approvals, and there will still be many FCC regulations that will need to be followed. A custom solution will likely carry a high minimum ordering quantity, but for the engineer tasked with high volume production, it could end up being the most practical solution.

Impedance

Since an antenna is one component in an entire circuit, we need to think of how it will interact with the rest of the design. Antennas are meant to radiate electromagnetic waves into free space, and to do so efficiently we will want maximum power transfer. This means we will want to have a termination on our antenna that has the same impedance as the transmission line. While this is rudimentary in the RF world, and most antennas today will call for a 50 Ω termination, there are some insights that may help in picking the best connector for your antenna. If you are dealing with an antenna that will need an actual coaxial connector on it, once you know the impedance requirements you will still have a vast list of possible connectors that could work. If possible, picking a more popular connector style may be prudent. Many walkie-talkies will use SMA or RP-SMA connectors, and many mobile devices will use UHF connectors, many Wi-Fi modules will use U.FL connectors.

Another point of consideration if you need to have a length of cable attached to the antenna is how long you want your cable to be. Since RF transmission lines change their impedance every ¼λ, a small amount of cable can make a fairly large difference in the antenna’s performance. Examining the MA140.A.LB.001 from Taoglas Limited, the datasheet shows options with different cable lengths (Figure 3).

Figure 3: Return loss in free space for the Taoglas MA140.A.LB.001 antenna. (Image courtesy of Taoglas Limited)

These graphs show the same antenna with 30 cm, 1 m, 2 m, 3 m, and 5 m cable lengths and their respective return losses. As can be seen, the 30 cm antenna is going to have the lowest return loss. This does not mean that longer is better, but rather that for the given frequencies this antenna was meant to handle, the extra length gets us closer to the purely resistive part of the Smith Chart. We ultimately want to remove as much capacitance and inductance as possible from our transmission line and having the accurate length of line greatly effects this.

Gain and Directivity

While antennas are passive devices, they still have gain. This term is not used the same way that it would be with an Op-Amp, or any kind of active device, rather, this is a ratio that corresponds to the directivity of the antenna. “Corresponding to the directivity factor, the gain G is the ratio of the radiation intensity Fmax obtained in the main direction of radiation to the radiation intensity Fio, that would be generated by a loss-free isotropic radiator with the same input power Pin.”, this is measured in dBi. (From Antenna Basics White Paper – Rhode & Schwarz). There are several other ways to measure gain in an antenna, dBi is one of the major categories, this could also be expressed in dB, in which case 10 dB would be 10 times the energy in relation to an isotropic antenna. Another example would be dBd, which is in relationship to a dipole antenna. An isotropic antenna radiates power evenly in all directions. While this is an ideal antenna, it does not exist; it is used as a reference to measure the gain of other antennas by comparison. The reason that some antennas have more gain than others is because they are directional and have more power transmitted in a given direction than an isotropic antenna would. A good example of an antenna that has a very directional radiation pattern is a dish antenna. While the gain is high and the range is long, this is only going to be functional if the antenna is pointed at what it is trying to communicate with.

So, is high gain a good thing? The honest answer is it depends upon the application. If you are using a directional antenna, then yes, it is. In designing a device, antenna placement can be a key to success, especially if you are using a very directional antenna like a dish. If you are using a device like a cellular telephone that needs to search out several cell towers in different directions, then having a higher gain may not matter because you will not be permanently directing your antenna at anything. Specifications of gain found on a product’s datasheet will only show what the antenna is capable of. It will ultimately be the design of the circuit which will determine the gain in the real world.

Bandwidth

There are several different frequency range configurations in the antenna world. If you are looking for cellular coverage you will need a broad bandwidth. Many antennas, while having a wide overall bandwidth, will have smaller center bands. This is convenient for the engineer with the need for a multi-band solution. The FXUB70.A.07.C.001 from Taoglas Limited has a range from 698 MHz to 3 GHz and has six bands that it is designed to operate at. This may not be the best antenna for any given frequency, but it offers a broad range of usable frequencies in an economic and aesthetically pleasing package. Who would want six antennas taking up space when one can get the job done? While some applications call for multiple bands, others call for one band. If you were operating in an ISM band like 915 MHz and were planning on using an antenna for only that frequency, you would be better suited with something like the TI.09.A.0111 from Taoglas Limited. This antenna has a very low VSWR at 915 MHz as illustrated in Figure 4.

Figure 4: The voltage standing wave ratio (VSWR) of the Taoglas TI.09.A.0111 antenna. (Image courtesy of Taoglas Limited)

While having a low VSWR is desired, it only helps if it is low at the frequency you are working with. While this antenna can boast a VSWR of 1, it can only do so at 915 MHz. VSWR is frequency dependent so you will always want to know what your center band is when you take this number into account.

Modules with Antennas

Since we are on the topic of application specific antennas, I thought it would be a good point to bring up transceiver modules. Many embedded designs will not only need antennas, but will also need to integrate end user applications. Instead of purchasing an antenna, a microcontroller, and hours of engineering time, you could steer your project towards a transceiver like the BLE113-A-M256K by Silicon Labs. This module uses an integrated chip antenna and works well for Bluetooth applications. The module has several configurable I/O ports taking the place of a microcontroller.

There are several other transceiver modules, but with the IoT boom, Wi-Fi would be another good example to investigate. The ATWILC1000-MR110PB from Microchip Technology uses a PCB antenna and operates in the 2.4 GHz band. This transceiver not only has the ability to communicate in this band like an antenna, but has the embedded technology to understand the language of what it is receiving. As you can see in Figure 5, this transceiver can handle several different modulation schemes.

Figure 5: Some radio performance specifications for Microchip’s ATWILC1000-MR110PB wireless module. (Image courtesy of Microchip Technology)

Conclusion

Ultimately, antennas are an interface between two devices; they just happen to use free space as their medium. There are many different frequency bands that you may want to use, and once you know which band you want to use you will be able to start choosing an antenna. Depending upon your application, you may want to go with an embedded solution that is more cost effective, or a more robust solution that will have solid quality that can hold up against the elements. Some antennas will have a small bandwidth and are great for a single purpose, while others have wider bandwidths that can handle several applications. Regardless of what your application is, an antenna is just that, an antenna. It is the duty of the RF engineer to make a circuit that can utilize an antenna to its fullest potential. 

Resources

1. Frenzel, L. (2016). Electronic Communication Systems. New York, NY: McGraw Hill
2. Reckeweg, M., Dr. Rohner C. (2015). Antenna Basics White Paper. Rohde & Schwarz
3. Donovan, J. (2012-11-08). Selecting Antennas for Embedded Designs.
4. Understanding Antenna Specifications and Operation. (2011-03-03).