Aerial photography - Ancient Gallery

This is a horizontally polarised Band III array, comprising two stacked six-element aerials. The phasing and matching arrangement is of interest. If a half-wave centre-fed dipole has a reflector and one or more director elements it no longer passes the collected energy efficiently to 75Ω cable. The usual way of correcting this is to fold the dipole into a thin loop, but there is another way, as shown here. If the feeder is connected not across terminals at the centre of the dipole but to two points some distance from the centre, a point will be found where the energy passes very efficiently to the feeder. This is called a ‘delta’ match. In this case we have a sort of ‘double delta match’. The arrangement serves the dual purpose of combining the outputs of the two aerials and matching the array to the feeder.

The main reason for stacking two aerials is to improve directivity and thus reduce ghosting, but that only applies if the arrays are stacked side by side. A vertically stacked pair like this will discriminate against signals from below and above so can be used to reduce aeroplane flutter, or tidal fading if the signal path is across the sea.

Previous

Next