a schematic diagram of an autotracking system 10 according to the prior art for tracking a signal source is shown in fig1 and includes a mode coupler 12 with a plurality of coupling arms 16 and 18 ( up to six additional coupling arms are conventionally used but are not shown ), an autotrack modulator 20 , a directional coupler 24 , and an autotrack receiver 28 . the mode coupler 12 can be a turnstile junction , an e - plane magic tee or a traveling wave coupler . a far field microwave signal 35 is focused by a reflector network 37 and 39 and collimated into a multimode feed horn 41 which can support higher order waveguide modes in addition to the fundamental sum mode . the mode coupler 12 couples the difference modes without significantly disturbing the sum mode ( σ ). the difference mode includes an azimuth tracking component ( δaz ) and an elevation tracking component ( δel ) which are extracted by at least two physically separate arms 16 and 18 . the difference modes carry the rf error signals while the sum mode carries the rf sum signal . the autotrack modulator 20 time - multiplexes azimuth and elevation error signals v . sub . δaz and v . sub . δel . the directional coupler 24 amplitude modulates the sum signal using the time - multiplexed error signals to generate a composite rf signal . the autotrack receiver 28 demodulates and downconverts the composite rf signal and generates if azimuth and elevation servo signals s . sub . δaz and s . sub . δel which are fed back to the autotracking servoloop to correct mispointing . where v . sub . δaz is the azimuth rf error signal voltage ; v . sub . δel is the elevation rf error signal voltage ; v . sub . σ is the rf sum signal voltage ; φ az is the phase difference between the azimuth error and the sum signals ; and φ el is the phase difference between the elevation error and the sum signals . because of the phase terms , the error and the sum rf signals must be tuned 0 ° in phase or 180 ° out of phase in hardware implementations . if the sum and the error rf signals are in phase quadrature ( i . e . 90 ° or 270 ° out of phase ), there will be no if servo signals because the cosine term is zero . as described above , each of the conventional couplers have drawbacks when used for a circularly polarized source . while relatively simple , the e - plane folded magic tee cannot autotrack circularly polarized signals because the sum channel only responds to linearly polarized sources . the turnstile junction approach responds to circularly polarized sources , but has complex construction and a large cross - section . most importantly , the turnstile junction has relatively narrow bandwidth , usually 2 % or less . turnstile junctions require tight ( high cost ) manufacturing tolerances and are sensitive to environmental changes . referring to fig2 and 3 , traveling wave couplers 43 and 44 , according to the prior art , typically include four ( fig2 ) or eight ( fig3 ) arms 46 depending upon whether the source is linearly or circularly polarized . each of the coupling arms 46 of the prior art traveling wave couplers 43 and 44 must be balanced to provide an accurate signal which lacks residual imbalance . in other words , each coupling arm 46 must generate a signal having phase and amplitude consistent with the phase and amplitude of other coupling arms in traveling wave coupler 12 in fig1 . in addition , prior art traveling wave couplers 43 and 44 require many hybrid circuits 48 . in fig4 an autotracking system 50 according to the present invention for tracking a circularly polarized source is shown . for purposes of clarity , reference numerals from fig1 are used in fig4 where appropriate . the autotracking system 50 includes a traveling wave coupler 54 with a single coupling arm 56 used to extract rf signals of the tm 01 mode . a 90 ° hybrid 58 is connected to the coupling arm 56 of the traveling wave coupler 54 . an autotrack modulator 20 time - multiplexes both outputs of the 90 ° hybrid 58 . the remaining elements of the autotracking system 50 parallel the prior art autotracking system of fig1 . circularly polarized signals include a vertically polarized ( vp ) component and a horizontally polarized ( hp ) component in phase quadrature . in other words , the ( vp ) component is leading or lagging the hp component by 90 degrees depending upon whether the ( cp ) source is left or right handed , respectively . electric field lines 98 of the tm 01 mode are illustrated in fig5 . the composite rf error signals carried by the tm 01 mode in response to a cp source and extracted by the mode coupler 54 into the side arm 56 is therefore the vector sum of the elevation and the azimuth errors , separated by 90 ° phase , i . e ., in mathematical form , ## equ1 ## the 90 ° hybrid 58 splits v . sub . δ equally between its two output ports 76 and 78 with 90 ° phase differences , i . e . ## equ2 ## if v h1 is phase - matched to v . sub . σ , the corresponding if servo signal s h1 will be proportional to v . sub . δaz only since the second term in eq . ( 4 ) is 90 ° out - of - phase with respect to v . sub . σ . while v h1 is phase - matched to v . sub . σ , v h2 is automatically phase - matched to v . sub . σ . if servo signal s h2 , corresponding to v h2 , is therefore also proportional to v . sub . δel only . the autotracking system 50 according to the present invention therefore obtains both azimuth and elevation servo signals with only one coupling arm 56 when the incoming signal is circularly polarized . referring to fig6 - 9 , the traveling wave coupler 54 for cp sources according to the present invention is shown in greater detail and includes a waveguide manifold 104 having an input port 106 adjacent a flange 107 for connection to the multimode horn 41 and an output port 108 adjacent a flange 109 for connection to the directional coupler 24 . the traveling wave coupler 54 includes one coupling arm 112 connected to an outer surface of the waveguide manifold 104 . the waveguide manifold 104 is sized to support both the circular te 11 and tm 01 modes . the coupling arm 112 is dimensioned to support only the fundamental rectangular te 10 mode . the waveguide manifold 104 has a circular cross - section while the coupling arm 112 has a rectangular cross - section . the coupling arm 112 is also sized to ensure that the phase velocity of the te 10 mode in the coupling arm 112 is the same as the phase velocity of the tm 01 mode in the waveguide manifold 104 . microwave energy of the tm 01 mode in the circular waveguide is easily transformed into the rectangular te 10 mode in the coupling arm 112 due to the phase velocities of the rectangular te 10 and the circular tm 01 being the same . microwave energy of the circular te 11 mode is not easily transformed into the rectangular te 10 mode due to different phase velocities of the circular te 11 mode and the rectangular te 10 mode . the traveling wave coupler 54 includes orifices 120 provided in a common wall 124 shared by the coupling arm 112 and the waveguide manifold 104 . orifices 120 in common wall 124 define a coupling region 126 . specifically , microwave energy transferred in the tm 01 mode in the circular waveguide manifold is coupled into the coupling arm 112 because its phase velocity is identical to that of the te 10 mode in the coupling arm 112 . on the other land , the orifices 120 cause negligible effects on the te 11 mode if the orifice size is not excessively large . the microwave energy transferred in the te 11 mode therefore passes through the coupling region 126 with little leakage into the coupling arm 112 . an approximate relative distribution energy is indicated by the relative length of vectors 128 . in other words , maximum energy transfer occurs through orifices 120 centered between the input port 106 and the output port 108 . minimum energy transfer occurs through orifices 120 located adjacent the input and output ports 106 and 108 . the coupling arm 112 includes input and output ports 132 and 134 . the input port 132 terminates in matched load , i . e . the characteristic impedance of the coupling arm 112 . the coupling arm 112 includes a connecting flange 136 adjacent the output port 134 . referring to fig8 orifices 120 defining the coupling region 126 are shown in greater detail . the orifices 120 shown in fig9 are rectangular in shape although other shapes , such as circular or elliptical , are also acceptable . center points 140 of adjacent orifices are preferably equally spaced a distance &# 34 ; d &# 34 ;. the rectangular orifices preferably decrease in size from a center orifice 120 - 1 . in other words , the orifices 120 - 2 and 120 - 2 &# 39 ; are smaller than orifice 120 - 1 , the orifices 120 - 3 are smaller than 120 - 2 , . . . , and the orifices 120 - n are smaller than the orifices 120 -( n - 1 ). alternatively , two center orifices 120 - 1 and 120 - 1 &# 39 ; having the same dimension may be used . in an embodiment operating at ka - band , a 15 &# 34 ; circular waveguide manifold is used and includes 48 orifices ( two center orifices 120 - 1 and 120 - 1 &# 39 ;) spaced a distance &# 34 ; d &# 34 ;= 0 . 24 . the circular waveguide has an inner diameter of 0 . 41 &# 34 ; and operates between 25 . 56 and 27 . 56 ghz . the orifices 120 - 1 and 120 - 1 &# 39 ; have a dimension of 0 . 148 &# 34 ;× 0 . 074 &# 34 ; and the smallest orifices 120 - n and 120 - n &# 39 ; have a dimension of 0 . 06 &# 34 ;× 0 . 03 &# 34 ; or greater . shorter or longer waveguide manifolds can also be used depending on bandwidth required . as can be appreciated , the above dimensions relate to traveling wave couplers operating at approximately 26 ghz . using scaling , operation can be obtained for other frequencies . at a minimum , operation from 16 ghz to 60 ghz is readily obtainable through scaling . in use , a circularly polarized source transmits electromagnetic signals which are focused using the reflector network 37 and 39 into the feed horn 41 . the traveling wave coupler 54 excites the te 11 and tm 01 modes . the te 11 mode or sum signal is coupled to the coupler 24 . microwave energy of the circular tm 01 mode is transformed into the rectangular te 10 mode in the coupling arm 112 which is coupled to one input 70 of the hybrid 58 . the hybrid 58 generates the azimuth and elevation error signals and outputs the elevation and azimuth error signals to the autotrack modulator 20 which time multiplexes the azimuth and error signals . the coupler 24 amplitude modulates the sum signal using the time multiplexed error signals and outputs the time - multiplexed , amplitude - modulated composite signal to the autotrack receiver 28 . the autotrack receiver 28 demodulates the composite signal and generates the sum signal and the azimuth and error signals for use in the servoloop . as can be appreciated , the traveling wave coupler 54 according to the invention vastly reduces weight and cost while providing performance superior to conventional traveling wave couplers . balancing of the coupling arm 112 is not required . weight reductions realized using the traveling wave coupler 54 can be especially important to satellite applications which must be launched into orbit . other features and advantages will be readily apparent . the various advantages of the present invention will become apparent to those skilled in the art after a study of the foregoing specification and following claims .