Hi There.... "David A" asked (below), "Why dont they use the mast, the back up use's?"
In other words, why is the Harris TX, not used on the Taller mast?.... For a very good reason....

Practical Actual signal Strength measurements made by STx on a country lane, parallel to Orfordness
and outside the near field, showed a deep null (reduction of level) on a Northampton Compass bearing
Caused by the other 4 masts either side of the central radiating one

We Checked the country lane, before the measurements, using Google street-view for overhead Phone lines
which are known to reradiate and give false readings,
(Thanks to people on hear saying about poor levels, when one of the 5 taller masts was used)

When I worked for KCBC in Kettering, we had reports of poor coverage in one large village, so I drove
round and plotted the signal level every 10 degrees on a map, and found a deep null towards the village
(this was caused by electricity pylons re-radiating the signal, cancelling it in one direction)

So If the Back up mast (one of the taller 5 mast array) was used, would give a weaker signal in some directions
several nulls may exist in other directions, so the shorter single mast is normally used, to prevent this,

Also some of the Coax used at Orfordness is over 6 inches diameter, so to change to the other mast
is not exactly like swopping a couple of PL259's over!

Rod Watts