VP Wireless Range Considerations

This page does its best to provide a guide to realistic expectations about the likely range of wireless operation with VP2 stations and about how to achieve the best practicable results with any given installation. However, it bears repeating that the circumstances of virtually every wireless installation are unique and it is impossible to completely guarantee the results at any given location.

A wireless system is usually simple to set up, provided that you keep the wireless distance comfortably within recommended limits and follow the advice below relating to obstructions and interference. More care is needed if you are trying to span larger distances, or where reception problems may be anticipated for other reasons. If reception might be problematic and laying a cable is a viable alternative, in some cases we have encouraged customers to consider using a cabled system.

In principle, the distance for wireless communication between console and sensor unit (ISS) can be regarded as falling into one of two zones: a limited range over which good reception should be robust and relatively unaffected by secondary  factors (as defined below); and a longer range where successful reception is routinely possible, but where it is necessary to ensure that a range of secondary  factors are optimised, at least as far as is practicable. Inevitably, the borderline between robust and less reliable reception is not easy to define because it will vary with each installation, but it is still perhaps useful to think in terms of these two zones even though the reception quality will still need to be investigated for each individual installation.

We would obviously recommend that users aim to operate within the robust range for reception. It should be possible to stray into the longer range zone without too much trouble, provided the influence of the secondary factors is appreciated, but, clearly, the greater the number of adverse factors that might be in play in a particular installation, the less likely reliable reception will be.

It is also worth noting that wireless reception can deteriorate quite substantially before the display of weather data on the VP2 console becomes noticeably disrupted. This is because while data is broadcast from the sensor suite (ISS) transmitter in packets every 2.5 seconds, the console can afford to miss many of the packets and will maintain its display values based on the last good data packet received. The only generally noticeable effects will be that parameters that may be changing rapidly such as wind speed will no longer follow changes as promptly as usual. However, we don’t recommend that stations be set up for routine operation under such marginal reception conditions (eg <50% data packets successfully received), because there is little leeway for adequate reception if any of the secondary reception factors temporarily take a turn for the worse. Also, from time to time the console may think that it’s lost contact with the ISS completely, which may eventually cause updating of the console display to stop.

The key to understanding likely wireless range is that, at the radio frequencies used by the VP stations (around 868MHz in most of the world outside North America), the signal between transmitter and receiver travels in a direct straight line between the two points, but is weakened (‘attenuated’) by any object such as a wall that it must pass through in its direct line of transit.

The two primary determinants for good reception will be:

1. The total straight-line distance between transmitter and receiver (eg ISS unit and console).
2. The number, thickness and nature of obstacles in direct line-of-sight between the two.

These are the overriding factors and the more thought that is put into minimising signal attenuation due to these factors, the better the chances of good reception. Remember to try to minimise the effect of wall thickness by positioning the console such that the signal passes through the wall at right angles, wherever possible. Passing through a wall obliquely can double its effective thickness and it’s also not difficult unwittingly to position the console such that the signal may need to pass along the whole length of a wall (eg placed on a window sill), a situation that may sometimes give zero reception even in a room which has otherwise acceptable or good reception.

Practical experimentation with monitoring of the reception quality (see below) will often be necessary to establish the optimum positioning. But while a window sill can be a poor choice in some configurations, equally it can often be an excellent choice in others. If the console can be positioned so that it can ‘see’ the transmitter (eg the ISS) through the window then this will typically give the best results because of the low signal attenuation of the glass in the window compared to the surrounding wall. [NB Be aware though that ‘high-tech’ glass coated with a thin metallic film, for example to reduce solar glare, may weaken wireless signals.]

The nature of intervening walls can also be an important factor. In general of course, thick concrete, brick or stone walls will prove much more of an attenuating barrier than lighter stud walls, but this is not always the whole story. For example, in modern houses, it is becoming increasingly common to find widespread use of rigid insulation backed with metal foil in stud walls and roofs. This type of metallic material can be a complete barrier to wireless transmissions.

Overall, the message is to consider these primary factors carefully when positioning your console, but to remember also that that not all of the effects of construction materials will be obvious and that experimentation and simple trial and error is likely to play a substantial part in finding the optimum location for your console!

The secondary factors include:

• Optimal orientation of the transmitter and receiver aerials. Once installed in its permanent position, the ISS aerial will remain relatively constant in orientation, but the console aerial will be less fixed – moving the console around the room, or cleaning it, could adversely affect the reception.
• Height relationship of ISS and console aerials. The situation where the ISS is substantially above the console unit (ie at a steep angle of elevation) should be avoided, because signal transmission won’t be efficient in this configuration. Reception is likely to be best when both aerials are at roughly the same height, or when any angle of elevation between the two is relatively shallow. In practice, a good configuration might be for the ISS to be located at a height of four feet and for the console to be indoors at this same height or slightly higher, eg perhaps on a window sill on the first floor of a building, provided the mutual angle of elevation remains fairly shallow. This arrangement, while by no means essential, should help to minimise the effects of potential obstacles such as dense vegetation, people, chain-link fences, garden buildings etc in the path between ISS and console;
• Prevailing weather conditions. Most obviously, the range for good reception may be significantly reduced during heavy rain or hail, but might also be affected to a lesser extent in other conditions such as fog;
• Electrical power available at the transmitter. The ISS wireless transmitter is powered by a solar panel, backed up by a lithium battery. Under conditions of bright sunlight or with a fresh battery installed, range will obviously be maximal. But with an old battery, on a very cold winter’s night perhaps following a period of several days of little sunshine, as sometimes happens in the UK, signal strength may inevitably suffer a little;
• Objects around the console . This is especially true of metal or metal-coated objects, such as filing cabinets, refrigerators etc, but can even include humans. Sometimes even standing nearby to look at the console can influence how well the signal is received, which can be either better or worse. If this is the case then the signal is probably too weak for reliable routine use, but, as ever, you will come to your own conclusion about what is the optimum solution for your installation;
• Interference. Other electrical appliances can generate radio signals in the same frequency band that the VP console uses and so the console will have difficulty hearing the ISS signal above the background noise. Sometimes this may be spillover interference from an appliance operating on a specific nearby frequency such as a mobile or cordless phone. But there are also more general radiators of radio-frequency interference such as fluorescent light fittings or appliances like PCs with a plastic (or no!) case. The VP2 console does have reasonably good resistance to many common sorts of interference, but if say a wireless phone of some description  is used very close by the console there could be adverse effects on signal reception;

While it is possible simply to monitor console reception by checking whether any weather readings are missing or slow to update, this is is tedious and vague process. The most immediate test is to check the console display to see if an ‘X’ is present in the bottom right had corner flashing on and off regularly at 2.5 second intervals. If reception is lost temporarily an ‘R’ will be displayed. If reception is lost for about 5 minutes then an ‘L’ will be displayed. For a more precise approach there are two radio transmission diagnostic screens on the VP2 console. Use of these screens is described in detail on the Understanding and checking VP2 wireless transmission page.

The maximum range suggested by Davis for the ISS to console distance is 1000′ (300m), but this is obviously with all the secondary factors listed above fully optimised and is through air only. Our  experience suggests that, in practice, a maximum range of 400-500′ (120-150m) for reliable reception under varying weather conditions may be more advisable.

Overall, our recommendation is that the wireless stations are best regarded as a means of avoiding the inconvenience of laying a run of cable externally and of passing that cable and its plug through an external wall and not, primarily, as a way of siting the ISS unit some considerable distance (ie >100-150m) from the console (though this is indeed possible if wireless repeaters are used – see below). Best results will be obtained when signal attenuation is minimised by limiting the transmission distance and line-of-sight obstacles as far as is possible consistent with good sensor exposure. Remember that the ISS and console aerials should preferably be at roughly the same height.

As a specific recommendation, the console should ideally be placed on a window sill or close by, such that it has a direct unobstructed ‘view’ of the ISS transmitter at a distance of up to 100m. If this is not feasible then the console should be positioned such that the signal has to pass through a minimum number of brick/concrete walls. The presence of one such external wall is probably unavoidable in many UK installations and is still likely to be compatible with a robust overall range around 50-60m. But further such walls in the signal path will inevitably result in further degradation of the signal quality. A wireless repeater will often help when a signal is too weak because of range or obstructions.

As noted above, routine transmissions well beyond these distance and attenuation limits are perfectly feasible, but reception quality will start to suffer (though probably with no visible effects on displayed weather data at first) and/or progressively more care will need to be taken in optimising the secondary factors that affect reception quality. One key observation is that while the wireless range in free air is excellent considering the low VP2 transmitter power of 8mW and is generally in accord with the Davis specification, thick walls do, not surprisingly, attenuate the signal substantially.

Wireless range can be increased, after the local primary and secondary factors have been optimised as far as possible, by the use of one or more wireless repeaters

A repeater is placed intermediate between wireless sensors and console and essentially acts as a staging post for the signal. These repeaters can be very effective and extend wireless range quite considerably, though obviously represent an added expense. In general, the ISS to repeater wireless link will be outdoors in line-of-sight and through air, with possibly only a limited amount of vegetation to attenuate the signal. Under these circumstances, range closer to the maximum specified by Davis can be expected, for example an extra 100-150m per repeater, possibly more. For longer distances, a chain of multiple wireless repeaters can be used.

There are actually four different types of VP2 repeater, which fall into two pairs:

• The repeaters are available in either AC-powered or solar-powered versions. The AC-powered type is obviously only suitable for use close to a mains power socket, which typically won’t be available out in the field. But sometimes using a repeater close by a building can be very useful in increasing the signal strength within the building and in this case the cheaper AC-powered repeater version may come in very useful;
• There is also a different pair of repeaters (AC- and solar-powered) termed long-range repeaters. These have identical circuitry to the standard repeaters but instead of the input and output feeds being connected to the standard single whip antenna, they are implemented as two wired connectors (one for receive and one fore transmit) to which external antennas – specified and purchased separately – can be attached. If high-gain yagi antennas are chosen here, the range can be extended to 1km or more;
  NB The gain control which was previously available in the VP1 generation of stations has been discontinued in the VP2. In practice, this was only ever of very limited use for increasing wireless range because it tended to increase background interference and noise by as much as the signal strength was increased, therefore giving little overall benefit to the signal to noise ratio.