Thursday, 4 October 2012

169 MHz ISM band antennas - a reality check!

The introduction of the EU harmonized ISM band at 169 MHz offers utility businesses a viable alternative to higher frequencies for wireless metering. Using the 169 MHz VHF frequency band, wireless metering can be deployed for longer range applications at relatively low power. This has many advantages for operations and billing organisations.

The challenge however comes with specifying 169 MHz antennas to go with the wireless meters. There are already a number of ISM-band 169 MHz module vendors with released products on the market and I am sure there are many more in development. From a RF perspective the module is relatively straightforward, but to supply compact, discrete antennas is not as easy.

A wavelength at 169 MHz is approximately 1.8M in length and therein lies the problem, for even a 1/4 wave monopole would be half a metre in length once the antenna had been housed in a sheath and connectorized.

For metering applications in domestic environments, discrete, tamperproof antennas are desirable, but not only is making a very low profile 169 MHz hard to achieve, it would also need a large groundplane. This is not particularly achievable on a small-sized utility meter. The options really are to use 1/4 wave 169 MHz helical antenna either bulkhead mount with a RF connector or through-hole mount so the antenna cannot be unscrewed from the meter. Both options would be approximately 120mm in overall length and would ideally need a minimum groundplane size of 85mm x 85mm. This size of groundplane could be possible on most meters, but a larger groundplane required by lower profile antennas are just not going to be possible.

Here are two images of the 169 MHz Helical with SMA and the 169 MHz through-hole mount:

169 MHz helical with SMA-Male connector
169 MHz permanent mount with N-Female connector

For concentrator applications (where a number of remote meters communicate with a hub using VHF communications), a wall-mount 169 MHz antenna might come in useful. Designed for outdoor use, ideally these will be of fibreglass construcion to withstand the elements. Here below is an image of the 169 MHz wall-mount antenna:

 169 MHz fibreglass wall-mount omni with bracket

It is also important to be wary of very small 169 MHz antennas as invariably some parameter will be compromised, for example small 169 MHz PCB antennas with a gain of -17.6 dB. It is useful to remember that the laws of physics are not going to change and that the wavelength is always going to be 1.8M. Very small antennas are going to massively compromise RF performance, defeating the very object of the exercise.

In conclusion, the 169 MHz band is a real opportunity for wireless metering and related applications, however the antenna selection and design-in for the meter must be considered early and with the appropriate level of planning to ensure that a suitable antenna design can be accommodated by the meter in terms of available groundplane etc. without comprising performance or aesthetics.

Related links:

An offering of 169 MHz antennas can be found here:

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