Aerial photography - Ancient Gallery

A three element Band I aerial has had a ‘double six’ Band III fixed to its mast as high as possible. Both aerials are vertically polarised, and both are slightly unusual.

The Band III aerial is a stacked (or ‘bayed’) pair. There are considerable advantages in stacking two or more aerials, but the question arises of how to transfer the energy collected by both aerials to the downlead as efficiently as possible. The solution adopted here is a variant of the ‘delta match’. This design does away with the folded dipole (often used to bring the characteristic impedance of a multi-element yagi back to 75Ω) and instead taps the active half-wave element at the point where a good match to the feeder is obtained. So, if two aerials with delta-type matching are to have their outputs combined, the 2:1 mismatch that would normally occur at the point where the signals are combined can be compensated for by altering the points on the dipoles where the signal is tapped off. In theory this arrangement should be pretty well 100% efficient. One definite thing is that the signals from the two aerials will be combined exactly in phase, so the main front lobe should be truly to the ‘front’, thus gain should be as high as possible. I’ve done my best to explain it, but if it remains no clearer than mud I’m sorry.

The Band I aerial is unusual in that the dipole is not folded, despite there being a director element. Most three-element designs fold the dipole in order to achieve a reasonable match to the feeder impedance, as mentioned above. Possibly, since the dipole and reflector here are widely spaced, it was felt that the best match would be achieved if the dipole was left unfolded. I doubt very much that there was any sort of balun or matcher in the junction box.

The UHF aerial, by the way, is an example of the very heavy and strongly made products that were produced in the 70s for the TV rental industry and badged as ‘Telefusion’. Later versions had an Achilles’ heel in that the junction box could let water in.

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