Planning considerations for long distance wireless links

In response to a question related to setting up a 35Km wireless link, I offered the following comments:

35Km is a pretty long haul in the best of circumstances! ;-) 

For that distance, there are many considerations relevant to the design of equipment at each end. 

To start with, for distances over about 15Km, Fresnel zone starts to become your primary concern because over distance, ANY signal attenuation becomes significantly important!   If you are not familiar with Fresnel zone, take a look at the propogation section on the Link Planning article on our web site: http://shop.duxtel.com.au/article_info.php?articles_id=3 

For a 35 Km link, enter ‘35000’ in the value for ‘D’ and make ‘d’ the halfway point.  The result is that the Fresnel zone is about 20m /radius/ at the half-way mark.  That means that the direct line between the two antennas must have a clearance of at least 20 meters from any obstruction at that half-way mark.  Add to that a calculation to allow for the radius of the earth, which will tell you that you need at least 20m height at each end to /just/ see past the curvature of the earth.  So you need a total of about 40m altitude above the half-way mark (at both ends) for the link to be feasible. 

That 40m, of course is generally flexible for both ends.  For example, you can have one size 80m above sea level, and the other end /at/ sea level (assuming that the midway point is also at sea level!) or you can have one end at 60m and the other at 20m, and so on. 

Last thing you need to consider is your antenna gain.  Remember that you are not allowed to build a wireless transmitter with output power more than 36dBm (about 4 watts) EIRP.  Therefore, assuming that the transmit power at each end will be limited to 26dBm, the only way to improve the /receive/ signal at either end is to use a higher gain antenna!  (you will need to reduce the transmitter output power to compensate for the higher gain on the tx side, but that part is easy ;-) 

Look at the ‘free space loss’ calculator on the same article (above) and you will find that @ 2.4Ghz, you will lose about 131dB.  So for a tx power of 36dBm, you will end up with an effective signal of about -95 at the remote end.  If you choose a 32dBi gain antenna, you should be able to present a signal of about -63dBm, which is feasible.  Of course that also depends on what kind of background noise you might find in that vicinity! ;-) 

For 900MHz, that free space loss drops to about 122dB.  For 5.8Ghz, it is about 139dB. 

Keep in mind that antennas of equal gain are usually larger for lower frequencies – so a 32dBi antenna for 900MHz is likely to be significantly larger and heavier than an equivalent antenna for 2.4 and 5Ghz bands. 

Feel free to contact me with more questions!