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Practical testing

Zoom The toilet-brush tin-can antenna, too, had to go through a practical test. Under optimal conditions, its net throughput was better than 8 Mbit/s at a distance of one kilometre.
The finished antennas had to prove themselves in a practical test. We selected a radio link one kilometre in length, with line-of-sight contact, and a 30-metre connection within an office building, without line-of-sight contact. We selected two WiFi routers in the Linksys WRT54G series as our test setup, and we installed OpenWRT 7.07 "Kamikaze" as the router operating system. Throughput was measured with the iperf network tool: in a measuring window of 128 kilobytes, we took the mean of five half-duplex test runs, each of 20 seconds. The router and the tin-can antenna were connected to each other via cable three metres long.

With standard stub antennas set up at both ends, no connection was achieved on the longer link. But with a tin-can antenna at just one of the two routers, we received a link. Iperf determined an average TCP throughput of 2.3 Mbit/s. Using tin-can antennas at both routers raised throughput to 8.5 Mbit/s.

Zoom The tin-can antenna was put through its paces at Hanover University. The receiving antenna can be seen on the right. It was rotated around the vertical and longitudinal axes.
It should be remembered that we were testing radio communications under ideal conditions: line-of-sight contact and an almost completely free Fresnel zone; this is the spatial area between two antennas in which the energy is transmitted from the sender to the receiver. It has the approximate form of a stretched football which, in the case of a WiFi, has a diameter of approximately one-third of the square root of the distance in metres – about ten metres at a range of one kilometre. If the Fresnel zone is not completely free, say because there are trees or houses too close to the line of sight, considerable losses are to be expected, particularly in rainy weather.

In an office environment, a free Fresnel zone is impossible. Even given the short distance, the existence of several walls ensured that no connection was made with stub antennas, but one tin-can antenna was sufficient to give a connection with a throughput of 3.2 Mbit/s. With a tin-can antenna at each end, it jumped to 7.1 Mbit/s. Our configuration, particularly the cabling, could be optimised further. Specially made cables, as short as possible, could reduce losses.

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