The antenna plays a huge role in a communication system. The whole availability and the sensitivity of a communication system depends on the antenna. A malfunctioning or ill-fitting antenna could disrupt the whole system. Therefore, it’s important to choose the perfect antenna for the application.

With the development of wireless technology and Internet, the usage of antennas has grown rapidly. The variation and diversity of the applications is vast. They vary from a single baby-monitoring home appliance to a target-tracking device with machine learning using GNSS. So, in order to fulfill all of those cases successfully there must be lots of research and development in antenna designing and innovations.

When you’re choosing an antenna for a specific application, there are many possibilities available to you. However, it is important for you to choose the most efficient antenna for its purpose. This guide will provide you with the important things that you should consider when you choose an antenna. 

1. The User Case or the Application

Of course, it all depends on your application. You need to be certain about why you need the antenna, specifically. There are many categories and types of antenna which give the same performance.

For example, there may be a dipole antenna and a printed microstrip dipole with the same performance. However, according to your intended application, the antenna you need might be different despite performance being similar. If the intended application is a water level monitoring system, you may need an adhesive antenna instead of a dipole.

First, you should define the goals that you wish to achieve, and then you can search for your antenna with the goal in mind. You should consider the following:

  • Why you need an antenna?
  • What is the improvement you wish to achieve?
  • Orientation and mounting place of the antenna.
  • RF parameters of the system.

2. Working Frequency

An antenna has one specific frequency, or it has one or two bands of frequencies. That means, it is designed to perform at its best at that defined frequency. It’s likely to be a disaster if you choose an antenna in a wrong frequency band. The wrong frequency could decrease the signal strength rather than increasing and may damage the component due to RF power reflection.

All the general-purpose applications, mobiles communication or IoT appliances share a few common frequency bands to operate. ITU (International Telecommunication Union) and the respective telecommunication regulatory body for each county allocate frequency bands and define them for applications. Using an unallocated band is illegal and you need to make sure that the application you plan does operate in one of those frequency bands.

Let’s assume the user is choosing an antenna for boosting the received signal strength of a 4G router. There are 44 bands in 4G with different bandwidths. Out of those, each country uses different set of bands. Then, in the cellular networks, different cells use those different bands. They follow the concept, “Frequency Reuse”.

When choosing a 4G antenna or even a 3G antenna, the antenna could transmit and receive in all the possible bands. Therefore, you need an antenna, which was designed specifically for 4G or 3G applications.

The following are the frequency bands in cellular transmission:

Most of the IoT appliances use one of many ISM bands (Industrial Scientific and Medical). An ISM frequency band is a license-free band. That means, anyone can use this frequency band for low power and short-range applications without permission. Of course, there are power limitations. In fact, some countries have restricted using some ISM bands. So, you need to consider this when choosing the application.

All the RC controllers, almost all the IoT appliances, RF and Microwave heating applications, Bluetooth, Lora, ZigBee, RFID, NFC and many general-purpose applications use one of the many ISM bands.

The most common ISM bands are as below:

  • 13.553MHz to 13.567MHz
  • 40.66MHz to 40.7MHz
  • 433.05Mhz to 434.78Mhz
  • 902Mhz to 928 MHz
  • 2.4GHz to 2.5GHz
  • 5.72GHz to 5.87GHz

Some applications only use one of these and some use many.

For example, RFID operates on all the ISM bands according to the applications. You should carefully choose the PCB antenna for RFID application. On the other hand, short-range applications like ZigBee operate on 2.4GHz and 915MHz ISM band only. Nevertheless, the antenna you use for 2.4GHz ZigBee is different than the antenna for 2.4GHz Wi-Fi.

SRD (Short Range Device) is another license-free frequency band. Lora operated in either ISM band 915MHz or SRD band 863-870MHz. Again, this is due to the regional restrictions of the frequency bands.

When you consider Wi-Fi antennas, Wi-Fi uses either 2.4GHz or 5GHz separately. Most Wi-Fi routers are dual-band routers. That means, they can operate in both 2.4GHz and 5GHz. But the running Wi-Fi protocol decides the operating frequency band. When you choose a Wi-Fi antenna, the antenna should be in the correct frequency.

So, when you purchase an antenna in the ISM band, be sure to check which band you are operating in.

3. Polarization

Polarization indicates the orientation of the electric field of the Electromagnetic wave. For the perfect signal reception, the polarization of the transmission wave should match with the polarization of the antenna. Otherwise, there will be polarization power loss in the antenna for the receiving signal.

Most of the dipoles, monopoles, Yagi antennas or PCB antennas for Mobile Applications or any ISM applications are linearly polarized. There are some special cases in IoT applications, which use circular polarization cases. Examples are GPS or GNSS applications. Generally, satellite communication uses circular polarization because the ionosphere can cause distortion in linear polarized waves. For that type of application, you must use an antenna with circular polarization. It can be either RHCP (right-hand circular polarization) or LHCP (left-hand circular polarization).

The antennas must align with the incoming polarization of the EM wave. The rule of thumb is that the orientation of the transmitting antenna should be the same as the orientation of the receiving antenna. For example, you should use the RHCP GNSS antenna for an RHCP application or the polarization loss will be very high.

To avoid the polarization issues, GSM, 3G or 4G transmitter antennas in the communication towers use cross polarization with a 90° distance apart. Then, the orientation of the antenna doesn’t matter.

4. RF Power and Gain

Most of the IoT device and emerging wireless technologies tend to draw low power. This helps to accomplish reduced power wastage and reduction of High RF power damagers for humans. However, when the transmitting RF power is low, the operating range decreases and the receiver sensitivity of the receiver should be very high. That is a main feature of modern low-power, short-range IoT devices and wireless technology appliances.

To achieve this sensitivity, transmitter and receiver antenna directivity plays an important role.

Antenna directivity indicates how the antenna concentrates the radiated power towards a particular direction, compared to an Isotropic Antenna with the same power. That means, an antenna can reduce the power towards an undesired direction and direct it towards a desired direction.

The term “Gain” is also the same idea. However, the gain is considering the power loss in the antenna itself. Nevertheless, in practice, we use the term “gain” for this concept.

The gain we want depends on the application itself. If the gain is higher, the directivity is higher, and the covering beamwidth decreases. Then, the antenna can cover only a small portion of the 360° plane.

According to the gain pattern or the radiation pattern, there are omni antennas and directional antennas.

For example, if you want to boost the received signal strength of your 4G router, you only need to direct the antenna towards the transmitting tower. Then you can use a Yagi antenna with a gain of 10-16 dBi. However, you can’t use a directional antenna as the WLAN antenna of the router. You must use an Omni antenna, which radiates power towards the whole horizontal plane.

If you want a DAB receiver antenna to mount in your car, it must be an Omni antenna because the vehicle always moves, and you need signal reception from any direction.

Most of the IoT applications need to cover whole 360° in the horizontal plane. Even the adhesive antennas you need for mobile applications need to cover 360°  in the horizontal plane. However, they don’t have to cover all 360° in the vertical plane. So, most of the monopoles or dipoles in the IoT applications have a small gain of 2dBi in the vertical plane.

When you’re transmitting or receiving a signal, you should have an idea about the power levels that you are dealing with. This is valid for both low-power and high-power cases. For example, let’s assume someone needs an antenna to increase the received signal strength of an appliance. Then he can use an antenna for that. However, the antenna is a passive device. It’s not an amplifier. An antenna can’t amplify the signal strength. An antenna resonates to an incoming EM wave. Before you install the antenna, you need to check the link margin. That is,

Received Signal Power/dBm=(EIRP-Path Loss)/dBm+Receiver Gain/dBi

You may not be able to change it. But with a certain gain, you can increase the received power level to a certain extent. However, if that’s still low compared to the noise, there’s no point in using an antenna.

Also, high-power can damage the antennas, cables or connectors. If damaged they will not behave in the same manner as before and the power losses will be higher.

On the other hand, most of the modern IoT applications are using low-power wireless protocols such as:

  • Bluetooth
  • ZigBee
  • BLE
  • 6LoWPAN
  • NB-IoT

Therefore, you don’t have to be concerned about high-power damages with that type of application.

5. Impedance

Unlike in DC electronic applications, impedance is a particularly important concern in high-frequency applications such as antennas. At the connecting interfaces, you need to be concerned about impedance matching.

Here, the question arises, “Why do we need to match the antenna to the transmission line?” The answer is, when we are dealing with high frequencies, the current along the line can no longer be considered as always constant at a specific point. It changes according to the sinusoidal manner following the frequency of the transmission wave.

So just like any travelling wave behaves, EM waves also tend to reflect at connecting interfaces of two different mediums. Therefore, if the impedances are different, signals tend to reflect.

6. VSWR

(Voltage Standing Wave Ratio) indicates the amount of the power reflected at the interface. The higher the VSWR, the higher the reflected RF power. This can be a problem in two ways,

  • Power loss
  • The reflected RF power can damage the components

In order to reduce the VSWR, the impedance of the two connected mediums must be equal. Then, the EM wave does not see two mediums. Moreover, the wave propagates in the forwards direction without reflecting. That’s why you need perfect matching.

In practice, it’s impossible to achieve perfect matching with VSWR=1. It is good to have a VSWR in the range of 1.5–2.0.

When you purchase an antenna, the value for the input impedance is given. You can find that in the product specification. It’s the input impedance at the feeding point of the antenna. Therefore, you need to buy an RF cable with the same impedance as that value. That’s known as the characteristic impedance of an RF cable. There are cable standards with standard characteristic impedance. Most of the practical values are 50Ω and 75Ω.

The manufacturers design antennas to have a standard impedance at the feeding point. Of course, the input impedance depends on the feeding point and the frequency of operation. That’s another reason why choosing a wrong frequency can damage your system. A quality product has the standard input impedance at the feeding point. So be sure to check the manufacturer’s specification for the product.

These are valid for microstrip or stripline antennas too. Standard RF cables are used to connect those antennas to devices. Therefore, the impedances must have that value.

7. Matching

Some antenna categories cannot achieve an RF cable standard impedance values. For example, a folded dipole in a Yagi antenna has an impedance of 300Ω. Sometimes, the impedance may change due to practical reasons.

In these kinds of cases, we can use a matching component to match the antenna to the RF cable. They transform the unmatched impedance to be matched at the connecting interface.

In PCB antennas, the matching circuit is printed on the PCB itself. In that case, you don’t have to worry about matching. Even for the folded dipole, there are cables with that characteristic impedance. However, when you need to match the antenna to the RF cable, you can use matching techniques such as:

  • Balun Transformer
  • Stub matching
  • Capacitive or inductive matching

8. RF Connectors and Adapters

Every antenna uses a standard connector. The most important point when using RF connectors is that the connection should be tight. A loss connection can increase the VSWR and it can damage the components. So whichever connector you use, make sure the connection is nice and tight.

There are a few standard RF connectors. When you buy an antenna, manufacturers give the type of connector you have in the antenna cable. The common connector types are:

  • SMA Connectors

A Sub-miniature connector, which is the most common RF connector.

Recommended up to 18GHz of working frequency.

There are many variations according to the mounting position, such as thru-hole, edge mount, bulkhead, etc.

  • BNC Connectors

A lightweight, miniature connector with a locking mechanism.

Recommended up to 4GHz.

Most testing and instrumentation devices have BNC connector.

  • TNC Connectors

Medium-sized RF connector.

Recommended up to 11 GHz.

  • N Connectors

Medium-sized connector with threaded coupling mechanism.

Recommended up to 18GHz.

  • MCX Connectors

Small connector with snap-on/off mating to quick connect/disconnect.

Recommended up to 6GHz.

MMCX is the micro version of MCX with the same performance.

It’s good to have the same type of male and female RF connectors for cable mating. The power losses will be lower. However, if the connector-types are different, you must use RF adapters. There are RF adapters between all types of RF connectors.

9. RF Cabling

Most of the RF cables are co-axial type cables. RF current shows the behavior, “Skin Effect”. The current travels on the surface of the RF cable. Therefore, you must use RF cables like Co-axial cables for that. There are RF cable standards. Depending on the application and the antenna, you need to choose the best RF cable. The most common, standard characteristic impedance are:

  • 50Ω
  • 75Ω

Another important factor of RF cable is the power loss or attenuation. The cable manufacturer provides the details of the attenuation per unit length. It’s given in decibels. If the attenuation is 3dB, then the power decreases by half. Therefore, you need to account for the attenuation loss of the RF cable too. Sometimes, antenna come with long RF cables. Then, the power loss could be higher. If you don’t need a long cable, you’re wasting power by using one. The other important thing is not to try to cut RF cables and solder. There’s a good chance that you may not be able to achieve perfect matching.

When you buy an antenna, think about the length of the RF cable you want and make sure to pick the most efficient value.

So, when you buy RF cables for the antenna, check these factors:

  • RF connector of the cable
  • RF connector at the other connecting end
  • The input impedance of the antenna
  • Whether there any matching components

10. Protection and Mounting

Once you’ve purchased an antenna, you need to make sure that it performs the way it is supposed to. For that, you need to mount or install the antenna to check. When you chose the antenna, you considered a few factors and chose the antenna to achieve those. So, if you chose an antenna for a monitoring device to hang on the wall, you need to make sure you mount the antenna the same way you planned. Make sure that the antenna has a line-of-sight between the transmitter and receiver and the obstacles between them are low.

You also need to consider the matching polarization to reduce losses. If you have a VSWR meter, you can check the antenna performs the same way as the specifications. If the signal level is low or VSWR is high, make sure the cable mating and RF connector are not loose.

Conclusion

If you want to choose an antenna, you have a clear idea about the considerations. The most important point is that you must choose the most suitable antenna for your application. That will help you to get the most efficient performance out of the antenna and the application.

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