Aerial Photography - Modern

These two aerials are not stacked as you might at first think. The one on the left is for Group A (channels 21–37) and the one on the right is for Group E (channels 35–68). If you look closely you can see the difference. We tried a wideband aerial (channels 21–68) but the results were poor, with quite severe ghosting on the Group A analogue channels and inadequate signal levels on the higher multiplexes and analogue C5. The attenuation caused by the trees was much worse on the higher channels. The installation was used to feed signals to the head-end of a small distribution system and the limited budget precluded the use of channel filters to equalise the channels. The Group A aerial had a masthead amplifier with 12dB gain, whereas the Group E aerial had a masthead amplifier with 24dB gain. The two were combined with a good quality diplexer. This equalised the signal levels quite well, but the extra gain of the Group E masthead meant that the diplexer (which in theory had inputs accepting 21–32 and 36–68) couldn’t prevent ghosting received by the Group E aerial spoiling reception on ch31. A diplexer can’t completely reject signals that are just outside the passband. A ch31 notch filter on the Gp E line solved that problem, and the distribution system has continued to provide good reception to date.

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