To determine antenna power and the effect of manufacturing tolerances, we built two versions of the tin-can antenna and had them measured by the Institute of Microwave Engineering at Leibniz University, Hanover. We drilled one tin-can antenna to a precision of approximately half a millimetre, but gave it a coupling pin that was about two millimetres too long. In the second tin-can antenna, the coupling pin is not exactly straight and the drilled hole for the exciter is one millimetre too far from the rear wall. Furthermore, its N-type socket is fastened with only two instead of four screws, which implies poor electrical contact with the can.
The Institutes tests showed that the differences in power of the two antennas, compared with an ideal simulation result, lie in the lower single-digit percentage range and are thus negligible. This shows that the tin-can antenna has an astonishingly high resistance to manufacturing errors, as long as its diameter lies within the optimum range.
The scattering-parameter diagram – above – shows the differing behaviour of the two antennas in comparison with a simulation result. It can be seen very clearly that the resonance point of the tin-can antenna with the over-long coupling pin has moved to a little over 2.3 GHz. This frequency matches a coupling-pin length of approximately 32 millimetres. With the second tin-can antenna, the resonant frequency is correct but reflection on the input side is clearly greater. These deviations can be explained by a combination of imprecisions in manufacture: a coupling pin that was not straight, an incorrect distance from the rear wall, possible deviation of the can from the ideal cylinder, and poor contact between the N-type socket and the can wall.
The really important thing is that the diameter be correct. Otherwise the proportion of the power reflected back to the sender, and thus not radiated, will rise. This happens with a can in the critical range of the illustration below, with a diameter of less than 84 or more than 111.5 millimetres.