Can the radio experts tell me if a mag mounted antenna is using the metal sheet it is connected to as a ground plain, or as a director/reflector. I don't get the fact that it is not electrical bonded to the base of the antenna, yet it seem to be vital the screen is bonded to the magnet. Can electricity jump through paint⚡️😳
Ian,
I wouldn't call myself an 'expert', but the simple answer is 'YES', the ground plane of a magnetically mounted antenna, fitted on a steel car roof for example, is capacitively coupled to the 'earth' braid of the cable feeding the antenna whip or 'monopole' and forms an integral part of the system. For this reason, magnetic antennas (or any other monopole whips) should be mounted as near as possible to the centre of the ground plane. Failure to do so causes the transmission pattern to be 'skewed' with maximum transmission effectively across the longest length of ground plane away from the antenna itself.
Without a ground plane, a high proportion of the energy from a monopole
transmitter will be radiated at a very high angle and horizontal propagation will be limited to a relatively short distance. The antenna is also unlikely to prove a good electrical match to the coaxial feed line - setting up standing waves which can wastefully (and in some cases destructively) reflect power back into the transmitter.
Unless capacitively connected, it is therefore essential that the antenna ground plane of a transmitting antenna is
electrically connected to the outer braid of the coax at the feed point.
A good ground plane both reduces the angle of maximum transmission (effectively providing 'gain' to the antenna) and improves 'impedance matching', lowering standing wave ratio (SWR) and ensuring that maximum power is transferred from the transmitter feed line into the antenna. With a 'perfect' ground plane extending infinitely in all directions around the monopole however, antenna impedance will be about
35 Ohms, which is not a very good match to coaxial cable. For transmit antennas, the impedance of a monopole antenna is often increased to match 'standard' 50 ohm coax by replacing the horizontal ground plane with a set of 'radials' angled downwards at approximately 60 degrees (see attachment).
With
receive only antennas, the same principles apply, but matching is generally less critical, though the closer to the ideal we can achieve, the stronger the signal passed to the receiver itself is likely to be.
Lots of reading out there if you want to get technical.
Regards
Peter