Wednesday, 25 September 2013

Antenna options for ISM 433 MHz frequency band - a short guide

The ISM (industrial, scientific, medical) frequency band at 433 MHz is a popular frequency for control, telemetry and access applications. The choice of antenna types is not as great as at 2.4 GHz due to 433 MHz having a longer wavelength than 2.4 GHz, however in most cases there are enough off-the-shelf external antenna options available to address most customer requirements. We'll take a look a some of them here.

In terms of antenna connectors, we are going to assume for the benefit of this overview that the radios are fitted with SMA-Female Jack connectors. In practice, the connector may be different, RP-SMA or BNC or TNC for example.

433 MHz Whip antennas  - often a moulded 1/4 wave whip antenna. It offers more bandwidth than a stub or helical antenna and is often better suited to mobile applications because the inherent flexibility of the whip antenna (they can withstand knocks a bit better than rigid antennas). A typical 433 MHz whip antenna will be approximately 170mm in overall length and will look something like:

433 MHz Whip antenna

The moulded antennas can often have what is known as a skirt - this is the length of moulding that extends past the end of the barrel of the connector (this can be seen in the image above). The 1/4 Wave whip requires a groundplane to operate correctly. Whips can also be permanent mounted with a flying lead for applications where a bulkhead mounted antenna is not desirable.

433 MHz Helical antennas - often a moulded 1/4 wave helical antenna. A typical 1/4 helical antenna is approximately 75mm in overall length. Like a whip antenna, it needs a groundplane to operate effectively, but due to the design the bandwidth of the antenna is smaller than the whip antenna. A typical moulded helical antenna will look something like:

433 MHz Helical antenna

433 MHz Stub antenna - usually shorter than a 433 MHz Helical antenna, the stub is actually also a 1/4 wave helical antenna, but the pitch of the radiating coil is smaller. This affects bandwidth and gain, but when the overall length is 55mm, the small size of the antenna may make the compromised in performance worthwhile. A typical 433 MHz stub antenna would look like this:

433 MHz Stub antenna

433 MHz Wall-Mount - wall-mount antennas come in various flavours, high gain or low cable and with options for integrated cables. As a minimum the wall-mount antenna typically offers 2 dBi gain and is generally designed to feed via a cable to the radio. One example of a wall-mount 433 MHz antenna is shown below:

433 MHz Wall-Mount antenna

433 MHz High Gain Omni antennas -  usually designed for pole-mouned applications, these antennas are typically constructed using a fibreglass tube and can offer 5 dBi or 8 dBi gain. However the compromise for high gain omin antennas is the length of the antenna (they are not discrete), the 5 dBi antenna is approximately 140cm long and the 8 dBi nearly 3M long. These antennas will give you much more range that terminal mount or the wall-mount antennas and some applications require this performance. Below is an example of a high gain omin antenna for 433 MHz:

433 MHz High Gain Omni antenna

433 MHz Yagi antennas - directional antennas offer much higher gain than omni antennas as they radiate broadly in one direction. At 433 MHz frequencies, a Yagi antenna will be similar in size to a traditional TV antenna. 433 MHz Yagi antennas can vary in gain depending on the number of elments the yagi has, but typically the gain can range from 7 or 8 dBi to 14-15 dBi. These antennas are usually pole mounted with a jumper cable to the radio or modem and are designed for outdoor deployment. A typical six element yagi looks like this:

433 MHz Yagi antenna (6 Element)

433 MHz Panel antenna - a panel antenna is also directional in its radiation pattern, more discrete than a yagi antenna, but not offering as much gain, typically gain at 433 MHz could be 6 to 10 dBi. The panel antenna can usually be wall or pole mounted and is fed by a jumper cable. A typical panel antenna looks like this:

The above antennas are the common options for 433 MHz applications - there are of course more specialist antennas such as robust low profile magnetic mounts or collinear vehicle mount antennas - but in many applications a terminal mount, an omni or a directional antenna will fit the bill. The challenge is to understand the trade-offs in performance and select the correct antenna for your application.

For further information, please visit our websites or contact us via e-mail at