Taking bearings near a longwave yagiPieter-Tjerk de Boer, PA3FWM firstname.lastname@example.org
(This is an adapted version of part of an article I wrote for the Dutch amateur radio magazine Electron, April 2017.)
In the south east of France there's another longwave broadcast transmitter, used by Radio Monte Carlo, of which the photograph shows the antennas. The three large masts, each 300 m high, together form a yagi antenna for 216 kHz. The center mast is fed, and the other two act as director and reflector. All three masts are equally high, but with a capacitor and a coil, respectively, the director and reflector are tuned appropriately. This antenna broadcasts, depending on the time of day, between 900 and 1500 kW towards north-east, covering at least the southern half of France.
Besides the three large masts, some more masts can be seen in the picture, which however are farther away and therefore look smaller. The mast just right of the middle mast is the backup antenna for 216 kHz, a 350 m high omni-directional antenna. The five small masts at lower left are each 100 m high; together they form a directional antenna with a switchable radiation diagram on 1467 kHz, in use a few hours per day with 1000 kW for Trans World Radio.
Last summer I spent a few days in Montagnac, 4 km west of this station. Of course both transmitters could be received very well there. Seen from there, the outer masts of the yagi are separated by about 7 degrees, and the mediumwave masts are behind them. However, with a rotatable ferrite rod antenna the (sharp) nulls for both transmitters were not in the same direction; the null for the longwave signal was some 15 degrees to the right ("south") of the null for mediumwave.
This figure shows a map of the situation, including a possible explanation of the differing null directions. The three longwave masts all radiate the same signal, but with different phase and amplitude. The ferrite rod antenna receives all three signals, but from different directions and thus with different sensitivity. Thus, it is plausible that the net result is zero, despite the ferrite rod's real null direction pointing far beside the transmitter. For example, if the outer two masts are received with a 180 degree phase difference w.r.t. the center mast, and the amplitudes of the signals from the outer masts together are just as large as that of the center mast.
But I still doubt whether this is really the explanation. With a lot of puzzling one can guess the phases and amplitudes of the three masts such that a null occurs with the ferrite rod pointing 15 degrees besides the transmitter, but with most combinations of phase and amplitudes there is no sharp null. So it seems a bit too coincidental that this would be the explanation.