After thinking about yesterday's post, I went up into the loft to look at my TV aerial. Sure enough, the elements are horizontal. Apparently that will be the direction of the electric field in the electromagnetic wave. Internet research tells me that this is a Yagi-Uda antenna, after the Japanese people who invented it nearly 90 years ago. The actual detector is the bit that the wire is attached to. The horizontal bits in front affect the field in such a way as to make the aerial directional. You can't see out of the loft but my gut feeling is that it might be pointing more towards Sandale transmitter than Caldbeck! The bit nearest to us on the picture is the reflector. The dipole detector is actually 2 separate sticks of metal with a small gap between then inside the junction box with the wire. The electromagnetic wave makes electrons in that wobble so the signal is detected and then amplified. In theory, the dipole aerial is half a wavelength long. I measured it as 20cm, which would suggest 750MHz waves using the wave equation c = f x lambda. Wikipedia quotes Caldbeck as up to about 500MHz. In theory, a longer dipole might be better. I studied this at university 27 years ago and reading the work on the Internet, it does sound familiar but I'll need to read up more to make sure I'm getting this right.
