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Making the most out of your antenna

Nick Lidington

Posted 11/11/2016
  Read more from Nick

Antennas are simple things but most likely it is the last item to think about when designing or using wirelessly connected products. A small amount of thought beforehand on what antenna type you should use and the compromises that that might bring will pay off later. We are often receive phone calls from people who need to sort out their antenna situation as its not going well……
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Making the most of your antenna

Antennas are simple things but most likely it is the last item to think about when designing or using wirelessly connected products. A small amount of thought beforehand on what antenna type you should use and the compromises that that might bring will pay off later.

Typical direct connect half dipole antenna often used with little thought. Siretta Delta 2DThe many phone calls we receive at Siretta are where the caller now realises he needs an antenna and wants a ‘high gain’ antenna of some sort. It is clear then that little thought has gone into how the antenna is to be deployed (internal or external, combined or separate antennas where multiple antennas are required). Many seem to think that the antenna has to be physically outside the equipment to be effective or seen to be effective.Typical half dipole PCB antenna with short cable. Siretta Echo 14Where the equipment is in a normal plastic enclosure it makes no difference whether the antenna is on the inside of the box or the outside in terms of what might not work as well in the box. Removing this preconceived view then enables a wide range of PCB style antennas to come into the picture that can be used inside the box to good advantage. Firstly they are thin and can therefore be placed in free spaces within the box. There are many shapes and sizes of PCB antenna with different attributes depending on the frequency(ies) required. Many of them have a good gain profile compared with the average screw on external antenna.Siretta Alpha 20 – flat blade antenna with 3M cable. Intended for covert adhesive mountingAdditionally, the cable length with a pcb antenna is generally very short (50-150mm) and therefore any losses involved with cables are minimal using PCBs. This is something many people ignore – the losses in the cable can be significant. We often get asked for internal type antennas like T-bar or flat blade types but with 10m and in one case 25M cables. If the electronic equipment is sited in a permanent place but internal to a building and needs an antenna then reviewing where the best place to site an antenna is the next thing to think about. If the wireless system is internal to the building like with Wifi or ISM bands then the best place for the antenna is not necessarily in the electronics box but in a place that minimises building signal attenuation like stone or brick walls. But in doing this due consideration needs to be given to cable length again. Placing the antenna for best signal but losing it all over a long length of cable is counter productive. For T-bar, flat blade, magnetic and puck style antennas that can be placed anywhere the most popular length of cable is 3M with some using 5M. These type of antennas generally use RG174 cable which does have an attenuation factor of around 2dB per metre at wifi (2.4GHz) frequencies. The attenuation is worse the higher the frequency and lower for lower frequencies. For example, the same RG174 has an attenuation factor of 0.98dB per metre at 900MHz. Considering that a 3dB loss of signal is in fact a halving of the signal, its quite clear that a 3M RG174 cable at wifi frequencies cuts the signal to less than a half of what was received at the antenna. A 5M cable in the same scenario would lose 10dB of signal along its length which is cutting the signal level down to one tenth of what was received at the antenna – or- the antenna gets to radiate one tenth of what the electronics sent down the cable to the antenna. Its clear then, having a short cable and /or low loss cable is a must for antennas that are not sited within or on the electronics by more than a metre or so. Consequently, if a remote antenna is really needed, like on the side of a house, a more specialised antenna of higher gain nature, like the Siretta Oscar 1A and Oscar40, to improve the signal at the antenna is needed and then fed with low loss cable (Siretta LLC200 and 100 cables) to reduce the cable losses. This is very different antenna situation altogether.Siretta Oscar 1A wall mounting antenna

Often it is necessary to site an antenna outside its electronics box to avoid walls and metal objects and boxes that will impede wireless signals. The above detail on cables still applies but removing wireless impediments by careful placement of the antenna needs to be considered. This is where the flat blade and puck style antennas come into their own. Additionally they can be placed covertly to avoid any tampering with.

One little thought about aspect of practical antennas is that mostly they are half of a full dipole antenna. A theoretically correct antenna is a full dipole antenna and made up of two elements where the feeder coax cable has its central core connected to one half of the dipole and the outer sheath is connected to the other half. This is not so practical for many antennas in real life – like the direct connect types many people use where the physical antenna is just one half of the dipole – a half dipole. The compromise here is that the electronics that the antenna is connected to becomes the effective other half of the dipole in that antenna application. It then depends on your electronic system as to how good it is in being the other half of the full dipole with your physical antenna attached. Siretta Echo 2 – full dipole PCB antenna – with short cableThis is the compromise that is used all the time to lesser or greater effect depending on the antenna and the electronics to make up a full dipole and therefore an antenna that has some performance. A PCB antenna is very often a full dipole in itself because it can be easily, thereby reducing the compromise introduced by a half dipole antenna requiring the electronics to perform the function. Panel mounting antennas like the Siretta Tango range antennas get around this compromise neatly by making the panel they are mounted on the other half of the dipole (ground plane) – assuming it is metal of course.
Siretta Tango 41 – through hole / panel mounted antenna
Choosing an antenna then is not the simple task of picking one you like the look of , or, just the cheapest or the smallest. Consideration needs to be given to the aspects above to be able to specify what sort of antenna and cabling will be needed for you particular application. Thereafter some trials with suitable alternatives within the type and style will confirm which type will suit and then the detail spec which will be the best compromise for your application. To make the point further, if you have not considered internal antennas beforehand you preclude their use once your electronics is designed and built – in these days of miniaturised electronics. You need to make those decisions, antenna decisions, at the time of designing your equipment. Treating the antenna like a mains plug (3 pins, any shape, any colour, they all work ok) will ensure your equipment will not function as you expect. Of course, you can give us a call at Siretta to ask our opinions on what we would advise for your projects – many do and we have many satisfied customers around the world as a result.

For further reading – see another of our antenna blogs: A basic description of what an antenna is.

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