PTP Wireless
EXTERNALLY-MOUNTED ANTENNA
Because of the distance we use, you will need a pair of good-quality high-gain antennas. Anything from a 1 km away up to 60km is possible if you do your homework first. Before you can select the correct antenna you need to determine the following:
- Is there Line of Sight (LOS) between both locations?
- Are there any other WISP’s in the area and if so are they operating in the band you wish to use? If there are any others using the same band you should hire a spectrum analyzer and see what channels are available.
- Can you get access to the site 24/7 in any weather?
How high should I mount my antennas?
If you have clear Line Of Sight (LOS) between the antennas, I would suggest that you fix the antenna a min 10 feet above the ground or roof. The higher you can fix a point to point antenna the better, but remember that you might need to work on it is bad weather so easy access to it is key. If you are planning on using 802.11g (54 Mbps) select an antenna with a higher gain (about 6 dBi more than you would need for 802.11b), 802.11g needs a stronger signal to get the speed.
As a rule of thumb, you can use the following table if you have clear line of sight.
0-4 km = 6 dBi each end.
0-10 km = 12 dBi each end.
0-35km = 24 dBi each end.
15km-60km = 32dBi dish each end.
Dishes give you a very thin beam, this will give you greater distance or on short shoots the narrow bean will filter out interference compared to a wide beam grid or panel.
ANTENNA CABLE
You should always use a short run of the cable between the radio and the antenna, the longer the cable is, the more loss you get. I always try to use a 3 foot LMR 400 on 2.4GHz or LMR 600 on 5.8GHz. Longer runs are possible, but try to keep the antenna cable as short as you can.
LIGHTNING PROTECTION
When the antenna is mounted above a roof or on a tower there becomes a high risk of lightning damage to the radio or the whole Access Point.
The tower or pole must to be grounded; I use 16 mm2 copper cables run to a good earth point. I would also strongly recommend lightning arresters on your LMR at the antenna end and surge protectors on your CAT5 (both ends). Don’t protect your system from static and lightning and you will need an endless supply of CM9’s & WAR boards.