referring to fig1 a transmission antenna system comprises a plurality of antenna elements 1 each fed with a respective signal from respective outputs of a beam forming circuit 2 via a signal path indicated in general by reference numeral 3 . each path consists of a power amplifier 4 , a diplex circuit 5 and a conductor in the form of a coaxial cable 6 connecting the diplex circuit to the antenna element 1 . with such an arrangement , the beam forming circuit 2 will transmit suitable signals to the antenna array elements 1 such that a desired beam shape will be produced . however , this will only occur if there is exact phase matching from the point of beam forming , through the power amplifiers , diplexers and upfeed cables to the individual antenna elements . this exact phase matching cannot be guaranteed simply by using apparently identical components and consequently it is proposed to add a closed loop phase adjustment of each signal path so as to compensate for any phase errors in the signal path . this is achieved by providing a signal monitor which feeds a signal indicative of the phase of the signal at a selected one of the antenna elements back to a phase error detection circuit 8 where the output phase is compared with the phase of the desired signal output from the beam former 2 in order to produce an error signal which is then fed to a respective phase control circuit 9 which in turn is used to control a phase adjustment circuit 10 in the respective signal path . the process is repeated for each antenna element . in the embodiment shown in fig1 the output signal monitoring is achieved by placing at a single monitor probe 12 in the near field of the antenna array . the output of the probe 12 is fed as an input to the phase detection circuit 8 via a single non - phase critical cable so as to avoid errors . in use , the system preferably uses a short &# 34 ; quiet &# 34 ; period in the normal transmission . this might consist of a natural break in transmission based on an allocated time - slot or some other format characteristic . a more complex alternative would be the super - imposition of a low level coded transmission identifiable by the phase detector circuit 8 which low level coded transmission could be separately fed to each element of the array . it is preferred to utilise a special test signal fed through the beam former and into a selected antenna array element in turn but it is equally possible to utilise a suitable part of an actual signal to be transmitted . on installation , all values , in terms of the relative phase between each signal path and one taken as a reference are recorded in memory using the probe 12 . any departure from these initial values of differential phase in operation will represent a drift in one of the signal paths . if a change in any of the differential phases is noted , a correction will be made by the phase control mechanism in a manner which will be described later . turning now to fig2 this shows a second embodiment of the present invention which is a modification of the first embodiment . in this second embodiment the same reference numerals are used to indicate the same parts as in the first embodiment and consequently will not be described in detail . the difference between this embodiment and the first embodiment is that signal couplers 14 and a switch 15 are used to replace the monitor probe 12 the switch 15 enables each individual signal path to be separately monitored but it will be understood that the switch could be incorporated within the phase detector circuit itself if this was more convenient . it is important to note , however , that the monitored signal which is fed back to the phase detector should be fed back using a single non - phase critical cable so as to avoid any errors . any cable whose characteristics will not alter over the period of one correction will suffice for this purporse . this feature results from the fact that phase samples from each element are handled in sequence and it is the basic aim of the invention to monitor and correct the relative phase of each path . while the above description has been given in relation to monitoring a phase antenna array when in the transmission mode , it will be appreciated that the array could be monitored in a receiving mode . this would principally entail supplying a known pilot signal to a suitably mounted probe from a signal generator and then comparing the received signal with the input signal . this is shown in more detail with reference to fig3 . fig3 is a composite diagram showing the two modifications which are required in order to implement the monitoring of an array of antenna elements by injecting a test signal into the array of elements and detecting the signals at the end of the signal paths adjacent the beam former . the test signal to be injected into the antenna array is derived from a signal source ( 16 ) which is connected to either the switch ( 15 ) and hence the signal couplers ( 14 ) or else the probe ( 12 ). the source ( 16 ) also sends a test signal to the phase detector where the phase of the test signal is compared with the received signals . referring now to fig4 this shows in more detail how the phase control is carried out by the control circuit 9 . again , the same reference numerals are used for the same parts shown in the other drawings as this type of phase correction can be used in all embodiments described above . the phase detector ( 8 ) receives an output a from the signal path at the beam former and an output b from the monitoring system at the antenna array and compares them to produce an error signal c . the error signal c is fed to a look - up table ( 20 ) where the value of the error signal results in a specific output signal d being produced which is applied to the phase shifter ( 10 ) of the signal path associated with the antenna element being monitored . a selector circuit ( 21 ) is provided to ensure that the output signal a from the signal path and the phase shifter ( 10 ) are the correct ones for the antenna element to be monitored . in this diagram , all the signals from the signal paths are supplied to a first switching circuit ( 22 ) and the desired signal a is fed to the phase detector circuit ( 8 ) under the control of a control circuit ( 23 ) which also controls a second switching circuit 24 which directs the output d of the look - up table to the appropriate phase shifter 10 . the control circuit ( 23 ) has a further output which is used to control the switch circuit ( 15 ) if used . the use of a look - up table allows the use of non - precision phase shifters as the signal required to make a specific phase shifter can be predetermined during initial installation of the antenna array and the calibration thereof so that when the output of the phase detector ( 8 ) produces an error signal this is directly converted by the look - up table into the appropriate predetermined signal in order to produce the desired phase shift from that specific phase shifter . also , the precise pattern formed by the array of antenna elements can be maintained even when the length of the signal path from the beam former ( 2 ) to the antenna differs . it will be understood that with a look - up table , it is not necessary to try to produce a zero error signal from the output of the phase detector ; any desired predetermined error signal can result in an output from the look - up table which has now effect on the phase shifter concerned . equally , different values of error signal can result in the same value being output from the look - up table if desired .