Enclosures & Antennas
Many RPi900 applications will require a permanent outdoor installation. For this situation you’ll need to properly house your electronics to protect against the environment. You’ll also need to give consideration to your antenna.
An antenna is used to improve the efficiency of your radio signals. By providing some directionality to the signal, improving the sensitivity in some directions at the expense of others. It is important to note that an antenna does not provide any increase in overall power. Instead it offers an effective gain, specified in dBi, which is a comparison of the antenna’s directional sensitivity relative to an isotropic, or uniform, antenna. The directional characteristics of an antenna are more fully described by its radiation pattern, often shown as a polar plot.
For point-to-multipoint radio networks, an omnidirectional antenna is often used. This provides gain by “flattening the sphere,” providing improved sensitivity in the plane of the horizon but limited to a few tens of degrees either side. For a base station or repeater, omni antennas provide some gain while still covering radios in different locations.
For endpoint radios and simple point-to-point connections, you should consider a more directional antenna. These include Yagi and panel/sector antennas. Compared to an omni antenna, they can achieve the same range with lower power.
Other things to consider when choosing your antenna include its weather resistance, its mounting mechanism and the type of coaxial connector it uses.
The DNT900 radio module operates in the 900 MHz ISM band, allowing license-free use in the US, Canada, South America, Israel, Australia and New Zealand. Strict regulations nonetheless exist for the maximum power level at which you operate your radio. The antenna gain will have an influence here. Regulations typically stipulate a maximum EIRP, where:
EIRP (dBm) = transmit power (dBm) + antenna gain (dBi)
In Australia, clause 45 of this document restricts EIRP to 30 dBm (1 W) or less. So for example, when using a 5 dBi omnidirectional antenna, you would have to set the radio transmit power to 24 dBm (250 mW) or less. Slightly more generous restrictions apply in the US; FCC part 15 allows EIRP up to 36 dBm (with transmit power capped at 30 dBm). It is your responsibility to understand and abide by the regulations in your country.
Outdoor electronics enclosures come in many sizes and materials. I recommend aluminium enclosures as they are sturdy and easy to drill. For weatherproof enclosures, look for a rating of at least IP66, or NEMA 4. Ideally it will include an internal mounting plate (or at least holes for one) where your electronics will be mounted.
You will need to drill holes in the enclosure to connect exterior equipment. This will include your antenna connector and probably your solar panel and/or battery cable. A step drill works very well for drilling these holes. For passing cables into the enclosure, cable glands are the way to go. These are easy to fit and provide a good seal. (You may need to purchase the gasket and/or interior nut separately.)
The best way to connect your chosen antenna is to use a bulkhead connector pigtail. This cable is terminated on one end with a U.FL connector which couples directly to your DNT900 radio. The other has a larger RF connector which provides a waterproof (bulkhead) connection through a wall of the enclosure. Many types of connectors are available; type N connectors are common and a good choice. Use self-amalgamating tape to protect the exterior connector once installed.
Caution: The U.FL connector used by the DNT900 and RPi900 is fairly fragile, its only mechanical support being surface-mount solder joints. Take great care when removing cables from these connectors, as it is possible to tear the connector from the board. Try to minimize the number of times you change the antenna cable. Use vertical force only. A paperclip hooked around the cable might help; an overpriced tool is also available.
Ironically, having gone to lengths to weatherproof your enclosure, it may be a good idea to drill a small ‘weep hole’ in its floor. This allows inevitable condensation inside the enclosure to drain properly rather than pooling.
Mounting the Board
The Raspberry Pi includes two mounting holes. You can use these to mount the RPi900 stack inside the enclosure using standoffs. However, RPi900 uses one of these holes from above for its own mounting post, so you’ll need to accommodate this by stacking the standoffs (which are available in male-male, male-female and female-female). Use #4-40 screws and standoffs as larger sizes will not fit through the holes.
When used without a Raspberry Pi attached, the RPi900 board has 3 mm mounting holes on three corners for mounting the board with standoffs or spacers. The holes could also be used to mount the RPi900 upside down with Raspberry Pi attached. (Connectors would be difficult to access in this configuration however, and long standoffs would be needed.)
A final, easy option is to mount RPi900 directly to the floor of your enclosure, using the SMA antenna connector as a bulkhead connector. (Mounting nuts and a gasket are supplied.) When using a Raspberry Pi this configuration is not possible due to the projection of the SD card beyond the board. A neat workaround is to use a low-profile microSD card adaptor, available on eBay and elsewhere.