Choosing a LoRa Antenna
Where should I put my antenna?
The antenna’s position should always come first; remember that LoRa works best on line of sight. The radio waves can get through some buildings but will not go through the ground which means that hills will block the whole signal. It is important to mount the antenna as high as possible with as clear a view of the horizon as possible.
It is often necessary to mount the antenna separately from the device and that a patch or extension cable is therefore needed. All cables negatively affect performance, the better quality the cable the less negative affect, the shorter the cable the less negative affect. Putting the antenna in the best position with the shortest cable is ideal.
Vertically polarised antennas should always be mounted vertically.
Gain; high low or medium?
Antennas don’t create or amplify radio energy, they only divert, direct, or concentrate it in some direction. This is directional feature is called gain. Please remember, no new energy is created, it is simply redirected or given directivity. The amount of intensification in a preferred direction is quantified as gain.
Antenna gain is measured in dBi, as antennas are not amplifiers a higher gain is not necessarily better. Imagine the radio waves being emitted from the antenna as a doughnut. A lower gain antenna will give a smaller fatter shape, as the gain is increased imagine the doughnut being squashed so that it is thinner and wider, with the highest gain antennas the shape is more like a pancake. The shape from a theoretical 0dBi antenna would be a perfect sphere. More gain does not give more doughnut, just a flatter wider shape. Also the higher the gain of the antenna the more distortion is introduced. Too high a gain antenna may even overshoot some transceivers that are nearby. All this means that ideally you need the lowest gain antenna that you can get away with in your location. This is different for every location.
What is an Omni Fiberglass Antenna?
Omni just means that it is omnidirectional and aims to transmit and receive signals in a 360 degree arc; fiberglass means that it has a fiberglass cover. Almost all antennas are omnidirectional, or at least aim to be unless they are directional. See below.
What about a directional Antenna?
Very few people will need a directional antenna. I have ones on the way with a 180 degree beam angle for those that live in flats or apartments in large buildings where there is almost no chance of transmitting or receiving through the building. Using the doughnut analogy, these cut the doughnut in half, but don’t throw away half, it is folded on top of the first half.
What does VSWR mean?
The parameter VSWR is a measure that numerically describes how well the antenna is impedance matched to the radio or transmission line it is connected to. VSWR stands for Voltage Standing Wave Ratio. The VSWR is always a real and positive number. I always think of it as a measure of the signal lost by the antenna. The smaller the VSWR is, the better the antenna is matched to the transmission line and the more power is delivered to the antenna. The smaller the VSWR is, the better the antenna is matched to the transmission line and the more power is delivered to the antenna. The minimum VSWR is 1.0. In this case, no power is reflected from the antenna or lost, which is ideal.
How are antennas tested?
I use a specialist highly directional 15dBi antenna with a device called a Spectrum Analyser to measure the strength of the signals coming from a device in different directions at different distances with different antennas. I don’t have a laboratory to measure the exact dBi of each design but I can make very accurate comparisons. I have been building private LoRa networks for a number of years for monitoring health and safety and security data in large scale outdoor events and so have significant experience in this area.
Help and advice
If you need any help or advice on which type of antenna is best for your device in your location, please do not hesitate to get in touch.