December 6, 2007
Because of the problem with node 10 from the day before, I removed the node 10 antenna and remounted it on the other side of the pole, thinking that the problem may be the way the coaxial feed cable was hitting the metal pole. I then aimed it again, double checked everything, and made sure it was plugged in.
Early in the morning (not early enough) we loaded one truck and sent three guys off toward node 14, while two of us drove to node 13 in another truck. At the node 13 site, we hooked up the antenna aimed at node 10 again (aka the 1310 antenna) and checked for a signal. Unfortunately, there was still no signal at all, even after re-aiming and re-checking everything. Based on the testing the day before, it seemed that there might still be a problem with the node 10 antenna or cable. I wonder if the way the antenna feed cable had been hitting the metal pole actually made a permanent kink in the cable, causing significant signal degradation. I have read that kinking coaxial antenna cables can easily ruin them. There are certainly other things which could have gone wrong too, such as the antenna aiming, computations for antenna aiming, computations for signal level at that distance, fog, etc. We had to abandon the connection between nodes 10 and 13, which meant that we could not access the Internet, and therefore could not do some of the VoIP testing which had been planned.
On all four antennas, aiming was done with an inclinometer and compass. I realize that in some cases the antenna's strongest signal may not be in exactly the same direction as the feed horn points, but it is a good starting point. For the 30 dBi antennas, we first used an inclinometer to adjust the vertical angle using the bolts included on the antenna mount, and then a compass to adjust the horizontal alignment with bolts which positioned a wooden block at the base of the mast and caused the entire mast to rotate. This method would not work if multiple antennas were mounted on the same steel pipe; is there a better way?
For the 24 dBi antennas, the mount did not include a tilt adjustment, and we did not build a rotatable mast, so both the horizontal and the vertical had to be adjusted simultaneously and then the bolts tightened carefully while hoping that the antenna did not move. There must be a better way to do this.
Because of an abundance of metal, the compass could not be close to the antenna. Two methods were used. For three of the antennas, we tied a string to the mount, ran it through the antenna at a small slit directly above the base of the feed horn (where the two halves of the parabolic dish join), and then out about two meters directly in front of the antenna. (This method wouldn't work if the antenna were mounted on a tower! How do telecom professionals aim antennas?) We then pulled the string tight, held the compass over the end of the string, and moved the string left or right until the string was lined up with the compass and the needle pointed at the computed compass direction. While holding this position, another person rotated the antenna until the feed horn was aligned with the string. For one of the 30 dBi antennas, a very similar process was used except that a mirror on the compass substituted for the string.
If anyone is 'trying this at home', the above technique will hopefully provide a signal strong enough to be detected by the radio card. Once this is achieved, iwconfig or other software can be used to fine tune the alignment. Another method for aiming, which could be used in addition to or perhaps instead of the above-mentioned method, uses a signal generator at one end and a spectrum analyzer at the other.
Before telling about the test results, I want to briefly mention some of the obstacles we faced (besides the technical ones and those already mentioned). First, coordinating schedules between the five of us was difficult because of things like preexisting commitments, constant threats of jury duty, and trying to avoid cold and rain (and we failed miserably at avoiding the cold and rain). Second, the 1310 antenna mast was unstable because we didn't take the time to make a solid cement base as we had done for the two 30 dBi antennas. Third, we miscommunicated about which power plug was needed to run a Toshiba laptop off of a 12 volt battery, and therefore had limited use of the laptop because its internal battery did not work well. Fourth, we had problems installing iperf on the Toshiba laptop and had left this step to the last minute, and thus could not do the connection speed tests which we had planned. Fifth, we tried a new Google Maps route to node 14 which should have been much faster but instead led to a locked gate. The men had to backtrack and take the route we had used before. Finally, as expected, everything took longer than expected, and we ran out of time. Because of this and the rain, we actually were not able to do any speed tests. Most of these problems could have been avoided by testing everything ahead of time with the actual equipment we had planned to use, or at least having backup equipment and plans. But testing takes a lot of time too.
The good news is that, against all odds, the 121 km connection worked! After aiming the two 30 dBi antennas with only the inclinometer and compass (no signal generator, spectrum analyzer, or tweaking with iwconfig), we plugged in both radios at 15:13 local time and established a stable link. iwconfig reported a signal level of -55 dBm at node 13 (-60 dBm at node 14). We did not have time to keep the link up for long, but we did log 424 pings (1 lost, 3.726 ms round trip average) and were able to establish a stable ssh session.
We plan to test again soon and hope to see if we can improve on the signal level by fine tuning the aim of the antennas, as well as perform a number of throughput and jitter tests with iperf to help determine how many phone calls the connection can support.
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