national radio astronomy observatory green bank, … · authors: george behrens, gary anderson,...

NATIONAL RADIO ASTRONOMY OBSERVATORYGREEN BANK, WEST VIRGINIA

ELECTRONICS DIVISION TECHNICAL NOTE NO. 185

Title: SIX BAND FEED HORN PROPERTIFS AT 2025 TO 2120 MHZ andS-BAND FEED SYSTEM TUNING PROCEDURES

Authors: George Behrens, Gary Anderson, Galen Watts and Glen Langston

Date: November 15, 2000

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(11-00)

1SiX Feed Horn Properties at 2025 to 2120 MHz

and

S Band Feed System Tuning Procedures

George Behrens, Gary Anderson, Galen Watts and Glen Langston

00 November 15

Overview

This document describes measurements of the Green Bank Interferometer and 20 m antenna SiXband feed horn at the NASA S-Band Uplink frequencies. These measurements were made todetermine the suitability of this horn for use in a Satellite communications design. The S bandportion of the horn is dual circularly polarized and was designed to operate well in the 2200 to 2400MHz range, which includes the Satellite Downlink frequency range. Test data were needed in the2025 to 2120 MHz range to determine the feed horn beam shape, polarization purity and isolation.

The NASA standard S band Uplink band is 2025 to 2120 MHz and by convention the satellitedownlink is phase locked to the uplink frequency with turn around ratio 240/221. The downlinkfrequency range is 2200 to 2300 MHz. For the Japanese Halca Satellite, the uplink frequency is2084.4 MHz and the downlink frequency is 2263.6 MHz. Measurements indicate that the feed hornis suitable for use in the frequency range 2025 to 2120 MHz. The feed horn components were tunedfor improved performance in the transmit band.

Test Setup

The measurements were made on the NRAO Green Bank mtitenna range using a linearly polarizedfeed horn and also using two conical helix antennas. At the transmit horn an HP synthesizer wastuned to frequencies in the S band uplink range and power output was set to +10 dBm. A 20 dBdirectional coupler was used at the transmit horn to provide the Scientific Atlanta test receiver witha phase stable reference frequency.

The test receiver was located at the base of the tower holding the feed. The test receiver was lockedto the transmit frequency and generated a LO frequency that was equal to the reference frequencyplus or minus 45 MHz. The test signal received by the feed system mixed with the receiver L.O.signal at a coaxial mixer connected to one of the Orthomode Transducer (0MT) ports. The mixerdown converted the received frequency to 45 MHz. The test receiver was used to measure theamplitude and phase of the received signal. The test receiver is somewhat difficult to adjust, socare was taken to adjust the signal levels to maintain proper lock of the test receiver.

SIX Band horn tests at NASA S-Band Up Link Frequencies Page 2

Transmit Isolation Isolation AxialFrequency Helix 445 Helix 442 Ratio

(MHz) (dB) (dB) (dB)

2025 -11 -17.3 4.62050 -15 -16.0 3.72083 -82100 -9 -8.4 2.62120 3.02210 -16 -20.0 0.9

Table 1: Measurements of Polarization Isolation and Axial Ratio

Beam Pattern Measurements

The amplitude and phase beam patterns were made for the forward OMT port of the horn in boththe E and H planes. The forward feed OMT port was determined to be the RCP port, using theLCP polarized helix feeds. (Note that for RCP uplink transmissions, the LCP polarized, rear, portwill be used, when the feed is at the antenna prime focus.) The beam patterns for a number offrequencies in the band are shown in figures 1 to 10. The measurements at 2210 MHz, shown infigures 9 and 10, were performed to confirm the measurement process, by comparing the resultswith previous measurements. The new measurements were found to agree with the previous data.

The polarization Isolation and Axial Ratio were measured and listed in table 1. A number ofrepetitions of the Isolation measurements were made due to inconsistency in the measurements. Itwas finally determined that the Helix feed serial number 445 from American Electronic Laboratorieshad poor properties in this frequency range. Helix feed serial number 442 from the samemanufacturer was used and found to give more consistent results. The measurements of Table1, Helix 442 results are considered most reliable. The Axial Ratio measurements were made using arotating linearly polarized transmitting feed horn. The Axial Ratio was determined by measuringthe difference between maximum and minimum received signal level as the polarized transmittinghorn was rotated ±180 degrees.

Return Loss and Isolation MeasurementsMeasurements of return loss and isolation were made in the lab, using a network analyzer. Thetest setup placed the transmit signal on the rear port, and the isolation was measured on the frontport. The return loss of the OMT ports could be adjusted by changing the iris width at the ports.The S band feed horn transition includes a metal "tuning ring" mounted inside a Polystyrene FoamSupport (PFS). Adjustment of this ring, by moving the PFS in and out, allowed improving thepolarization isolation in different frequency ranges.

The measurements of the return loss and polarization isolation in the frequency range 2000 to2500 MHz are are shown in figures 11, 12, 13 and 14. In figure 11, the feed is in the unmodifiedconfiguration. The return loss measurements match reasonably well with previous data, howeverthe polarization isolation is very fiat at about -10dB, somewhat poorer than was shown in previous


measurements in the 2200 to 2300 MHz region.

In figure 12, the same data were taken with the matching iris in the rear port widened from itsinitial 2.25 inch width to 3.28 inches, in order to improve the return loss in the transmit band. Thereturn loss is significantly improved in the transmit band, being no worse than -12.4 dB, but thereceive band shows between -8.8 and -6.3 dB return loss. The isolation in this configuration is notsignificantly modified.

In figure 13, the iris is returned to the original configuration, but the PFS containing the tuningring was moved back 2.51 inches from the front surface of the X band horn. In this configuration,the return loss remains high, -5.4 to -8.4 dB in the transmit band, but the polarization isolation isimproved to between -14.3 and -21 dB.

In figure 14, the wide rear port iris was used in conjunction with the repositioned tuning ring.Both the isolation and return loss are improved in the transmit band. The return loss is between-12.5 and -22 dB and the isolation is between -12.4 and -15.5 dB. The return loss is worse that theoriginal configuration in the downlink band, between -15.5 and -9.3 dB but the isolation is muchimproved, between -23 and -13.7 dB. The measurements of isolation shown in figure 14 are similarto the measurements made immediately after the feed was constructed.

Satellite applications will use narrow up and down link bandwidths (> I MHz), the return loss andisolation parameters can be tuned to yield good properties at specific link frequencies. In the fullytuned configuration (figure 14), for the HALCA uplink the return loss is -21 db and the isolation is-13 dB. For the HALCA downlink, the return loss is -12 dB and the isolation is -18 dB.

In figure 15, the return loss and isolation are shown with the tuning ring adjusted for maximumpolarization isolation in the uplink band. Notice that although the isolation in the uplink band isimproved to better than -17 dB, the isolation in the downlink band has degraded to between -12and -8 dB.

Conclusions

The amplitude and phase beam patterns at frequencies in the range 2025 to 2210 MHz weresymmetric and showed proper shape in the range ±60 degrees.

Measurement of return loss and isolation in the laboratory indicate that the S band horn tuningring had shifted from its original position. Readjustment of the tuning ring and modification of therear port iris yielded much improved performance. The placement of the tuning ring and size ofthe iris opening should be matched to the specific up and down link frequencies of the mission.


Appendix A: Procedure for tuning the S/X Feed

The following are some recommendations and comments regarding the tuning of the SiX Feedsthat are presently installed at the GB Interferometer, the 20 Meter Telescope and the one that wasinstalled on the the telescope in Kokee Park, Hawaii. The tuning is performed by first calibrating anetwork analyzer then attaching the forward OMT port to the transmit side of the network analyzerand attaching the rear OMT port to the receive side.

Please refer to Figures 2 and 3 of the report that appeared in the Proceedings of the IEEE of May1994 entitled "Design and Implementation of a Low-Noise Prime Focus S/X Receiver System forRadio Astronomy" , by Roger D. Norrod, George H. Behrens, Jr, Frank D. Ghigo and Burton J.Levin. These figures are included here as Figures 16 and 17.

As shown in Fig. 16, the S-Band section of the Feed is composed of items 1 through 12. Adescription of these components is given in the paper_ However, a review of their purpose andinteraction is given here as an aid to understanding the tuning procedure.

The S-band Section of the S/X Feed System consists of three sub-assemblies- 1) the Feed, 2) thePolarizer and 3) the Orthomode Transducer (0MT).

The Feed:

The Dual Depth Corrugated horn (Item 1), is excited by the Coaxial Waveguide (item 5) where theX-Band wave:guide (Item 14) serves as the center conductor of the S-Band waveguide.

Impedance matching of the Corrugated Horn to the Coaxial Waveguide is achieved by the tuningring (Item 3) which is held in place by the Polystyrene Foam Support (PFS).

The Polarizer:

The S-band Polarizer (Item 6) is comprised of two cross-linked polystyrene plates that also act assupports for the X-Band Waveguide. The length of the polarizer is such that it provides a differentialphase shift of approximately 90 degrees for waves that have E-Fields parallel and perpendicular tothe polarizer plates.

Impedance matching of the polarizer to the circular waveguide is achieved by the steps milled onthe ends of the polystyrene plates.

The Polarizer plates are oriented such that the E-fields excited by the two Orthogonal Slot CoupledT-Junctions (Items 7 and 9) are rotated 45 degrees with respect to a plane pa:raliel to the polarizerplates. Such orientation causes that component of the E-field parallel with the Polarizer platesto lag that component that is normal to the plates by 90 degrees-the criteria required for circularpolarization.


The Orthomode Transducer (0MT):

The S-band OMT Sub-Assembly, is comprised of Items 7 thru 12. The Forward Slot coupled T-Junction (Item 7) couples energy from the S-Band WR-430 Waveguide to the Coaxial Waveguide.Item 7 is oriented with respect to the Polarizer Plates to provide Right Circular Polarization atthe Feed. Similarly, the Rear Slot Coupled T .-Junction is oriented with respect to the Polarizer toprovide Left Hand Circular (LHC) at the Feed.

The Septum (Item 8) acts as a back short for the Forward Junction and channels RHC energyreceived by the Feed to the RHC WR-430 waveguide. As RHC energy propagates down the coaxialwaveguide it is converted to a linear polarized wave by the polarizer. The polarization of the E-Fieldis such that it is parallel to the septum and therefore is reflected back towards the feed. Howeversince the septum is located approximately 1/4 wavelength beyond the Forward Junction, it appearsas a high impedance as seen at the junction and little energy propagates beyond the junction.Essentially all the energy is then coupled to the the RHC rectangular waveguide.

In the case of LHC energy received by the Feed, the polarizer converts the circularly polarized signalsuch that its E-Field is oriented normal to the Septum and therefore the Septum appears essentiallytransparent to the signal. The signal then propagates to the Rear T-Junction (item 9) where it iscoupled to the LHC WR-430 waveguide after being reflected at Back Short (Item 10).

Tuning of the feed system over a limited frequency range is achieved by setting the feed impedancematch and the OMT match.

S-Band Feed System Tuning Procedures

Feed Impedance Match:

As mentioned above, the S-Band Feed is matched to the Coaxial waveguide by adjusting thedimensions and position of the Tuning Ring (Item 3) which is held in place by the Foam Support(Item 4). During the original tuning of the Feed it was optimized over the 2.2-2.3 GHZ Band. Thedimensions and position of the Tuning Ring was determined empirically by measuring the Isolationbetween the RHC Coupler (Item 7) and the LHC Coupler (Item 9). The dimensions of the TuningRing and its position were changed until maximum isolation was obtained.

It is assumed that the feed is optimally matched under this condition because when the test signalis injected into the RHC Coupler and propagates thru the polarizer it gets converted to a RHCpolarized signal. This RHC signal continues to propagate to the feed. If the feed is perfectly matchedall the energy will be propagated into free space. However if the Feed is not perfectly matched, aportion of the test signal will be reflected at the Throat of the Feed (the Feed/Circular WaveguideInterface). Since reflections of Circular Polarization signals are converted to the opposite sense, thereflected portion of the RHC test signal becomes a LHC signal and appears at the LHC (Rear)Coupler. Hence, the Isolation measurement is directly related to how well the feed is matched.

If the Feed is to be matched for a specific frequency, measure the isolation between the RHC andLHC Couplers and slide the Foam Support which houses the Tuning Ring until maximum isolation

S/X Band horn tests at NASA S-Band Up Link Frequencies Page 6

occurs over the desired band. Measure the Isolation using the Network Analyzer in the Dual ChannelMode so that both Return Loss and Isolation can be observed simultaneously. Ensure that whenadjusting the tuning ring, that there is little change in Return loss and that S12 is the same as S21and that the Return Losses Sll and S22 are similar.

OMT Matching:

The RHC and LHC Couplers are matched to the WR-430 Rectangular Waveguide by empiricallyadjusting the back short (10), the Septum (Item 8), and Tuning Irises (Items 11 and 12).

In the original development of the OMT, the OMT was terminated with a sliding Coaxial WaveguideLoad. The Network Analyzer was calibrated in WR-430 waveguide and the Admittance lookinginto the RHC port was measured for different positions of the Septum. The Reference plane duringcalibration was the junction of the WR-430 waveguide and the RITC Coupler. The Septum positionwas adjusted to cause an admittance locus that could be tuned by using a Inductive Iris at theJunction or at some point away from the junction and inside the WR-430 Waveguide. The widthof the Iris was adjusted to obtain the best match over the 2.2-2.3 GHZ Band.

The LHC Port (Rear Port) was tuned in a similar fashion except that the Back short was adjustedinstead of the Septum. A location of the Septum was found that would permit a Shunt InductiveIris in the WR-430 waveguide that would optimally match the LHC Port over the 2.2-2.3 GHZBand.

If the OMT is to be tuned for another Band, it is recommended that the total feed system beconnected and that the Network analyzer be calibrated for a Full Two Port Measurement. Connectthe Network analyzer to both ports and measure simultaneously the Return Loss (S11) and Isolationbetween the RHC and LHC Port (S12) by using the dual channel mode on the network analyzer.Vary the width of the window to obtain the best Return Loss and observe S12 to ensure there is nointeraction. Wider openings in the window of the iris will permit lower frequency operation.

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Figure 11: S Band Horn return loss (top) in the frequency range 2000 to 2500 MHz. Polarization isolation is shownbelow, indicating nearly constant isolation value of -10 dB. The rear port iris opening width was 2.25 inches.

Figure 12: S Band Horn return loss (top) in the frequency range 2000 to 2500 MHz with the rear port matching iriswidth increased to 3.28 inches. Polarization isolation is shown below, indicating the isolation is not effected by thefront port matching iris.

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Figure 14: S Band Horn return loss (top) in the frequency range 2000 to 2500 MHz with the rear port matching iriswidth increased to 3.28 inches, and tuning ring adjusted as above.

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From "SIX Receiver System for Radio Astronomy"

X—BAND SECTICIN

Figure 16: Block diagram of S /X Band system.

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DETAIL A Si = 3.65852 = 1.016b = 1.270t = 0.159

SECIION THRU X—X CORRUGATION DETAILS

DIMENSIONS IN CENTIMETERS

Figure 17: Cross section diagram of SPC Band system.

national radio astronomy observatory green bank, … · authors: george behrens, gary anderson,...

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