Proposed 3.1 KM 2.4 GHz Wireless P-t-P Hardware Setup

I am about to extend my wireless network to a new location 3.1 KM away (point to point bridge). I have direct line of site. I have another point to point link that uses 5 GHz Alvarion with built in antenna as seen here (Can't seem to post URL's but directional antenna 80 degree beam 21 dBi +18 dBi on radio.)


The distance on this link is 1.7 KM and each antenna has 21 dBi Gain. Signal is strong at -69 dBm. Direct Line of Site. The radios are idled back to 18 dB each so total gain each radio = 40 dB (correct me if I am wrong no loss due to cable). I have had great luck with this setup.

The Alvarion is a little out of my price range for this next project so I have specked out the following and need help to be sure I have a good amount of gain and enough transmit power etc. I do not want to be way over on gain either, but lack the know how to do the calculations.

Frequency 2.4 Ghz (More loss over distance than 5 Ghz but more forgiving to terrain or so I hear )
Distance 3.1 KM
Direct Line of Site Confirmed
Radio’s 2x EnGenius ENH200EXT
(no URL sorry)

Antenna’s (Still up in the air so to speak)

Wade h1024 antenna

I want a focused beam but unsure what Helical means. I was told if the beam is focused in one direction your loss over distance is not to the power of 2 any more (like with an omni antenna). The Alvarion was what seemed to be a good 80 Degree horizontal beam and I didn’t lose much but it was ½ this distance, a higher freq, and 20 db instead of the proposed 15 db antennas.

for loss calculations I have 5 feet of cable from radio to antenna on each end. The current Alvarion solution is ISP Grade wireless gear. The Engenius not so much but it has 500mw max output (27d. What is a WRT54G 120-180 mw with a firmware hack? Now my Alvarion is set at less than 200mw at 18 dB as far as I can tell. It seems as you add mw (say 300-400) you are only getting a few more dB. From 400mw to 500mw it is not much at all but from 50 mw to 200mw is a big jump.

I have lots of specs if more info is needed. Don't pretend to know everything about this or I wouldn't be posting this. Any epic fails here? Please only replies from those who have used directional Yagi antennas or panels like the alvarion antenna (directional) at ranges over 1 KM. Thank you !!

Very helpful was the 103 Long Link How To. This calculator gave me what I think are good results after I put in all the variables.

Free Space Loss 109.9 Rx Signal Level -54.7 dBm
Theoretical System Operating Margin 34.3

Is this a good Number? This is with radio on max transmit at 26 dB. If I idle it back to 21 dB which looks to be the lowest setting the results are;

Signal Level -59.7 dBm
Theoretical System Operating Margin 29.3.

Do these numbers look ok?

any SOM over 20 should be fine. my lap[top sees -50 dbm in my living room 6 feet from the router, so 50 something signal strength is excellent

Thanks man! This is making good sense now. the calculated signal level is no different then the figure I see on a home network under wireless signal strength (Net Stumbler etc). (50's great, 60's good, 80+ could get harry etc). The Rx Sensitivity on these radios blows away most network adapters with a range from -73 (Full Bandwith) to -95 at 6 Mbps.

I have for giggles fired in the numbers for the ENH202 with built in 10 dBi directional antenna. It looks like this radio is 800mw not 500 so even with the built in antenna numbers are;

Rx Sensitivity -60.9 But a Theoretical System Operating Margin of 24.1. Even this looks like it will work properly saving me 100's of dollars on antennas. Radio will be wide open but none the less work within a good range.

that helps me a lot ,thanks