Low transmit gain performance:
The performance of the dish may be substantially degraded if the weight of the radio assembly (Feed horn, OMT, ORU, BUC, LNB, etc) at the front causes the shape of the main reflector to be distorted.
At low and moderate elevation angles, the weight of the feed pulls the sides forwards.
The error may be measured by using two lengths of fishing line (or strong thin thread). Put one length down the front of the dish and make tight by tying off or using sticky tape at the back. Put the second line across the front sideways, where the side struts attach, and secure tightly behind.
Take a rule and measure any gap where the lines cross.
The gap should not exceed 1.3mm (1/16th of wavelength at 14.5 GHz).
Effects of the distortion are poor focus and low gain, particularly in the higher frequency transmit direction.
Correcting the distortion:
If the dish is of the Andrew type there are short rear side struts.
The short side struts go between the rim of the dish and the central al/el adjuster box. The purpose of these short struts is to stop the dish distortion.
The end holes in the short struts nearest to the az/el box are elongated to permit a small adjustment.
In my experience you need to set up the antenna at its operating elevation angle with the feed radio assembly complete. Then measure the gap, if any, between the lines at the front. Then, with the help of two people, lift the feed, pull the sides back and tighten the rear strut bolts all at the same time. Repeat the lifting, pulling and tightening until the lines only just touch.
Now finalise the dish pointing.
The three pictures below showing the rear side struts. In all cases here, the elevation angle is low, about 30 deg, and the weight of the radio assembly tends to pull the dish sides forward unless the rear site struts are tight in tension.
The above images have been kindly provided by El Molino Systems S.L.,
Communication Systems UK phone till end Aug 2014: (+44) (0)20 71830128,
Feedback: If anyone actually follows the idea above I would be pleased to hear from you (email me at email@example.com ) and tell me your comments. Pictures welcome.
The same problem can apply to other dishes, see correcting distortion in VSAT antennas due to heavy feed assemblies.
Page started 23 Nov 2007, amended 4 August 2015 ECJ