Design and Performance of Concealed Enclosures for SATCOM and Telecom

Transcription

1 Design and Performance of Concealed Enclosures for SATCOM and Telecom D. J. Kozakoff 1, J. Fitzhugh 2 and W. Pounds 3 Affiliations: 1. Consultant and Subject Matter Expert, 2. President and CEO of ConcealFab Corporation, and 3. Vice President of Engineering of ConcealFab Corporation

2 What this presentation is all about For zoning or security reasons it is often desirable to enclose telecom antennas and radios within an RF invisible enclosure to conceal the antenna(s) from public visibility. An RF invisible enclosure, when properly designed, will have negligible effect on the enclosed antenna.

3 What you will learn RF transparent construction materials: parameters that are important. Important antenna parameters to preserve. Mathematical approaches to computing the effect of the enclosure. Unique problems that sometimes arise. Other issues.

4 Small Structures

5 Small Size Structures

6 Medium Size Structure

7 Large Size Structure

8 Small Cell Deployment Poles Environmental Specifications 142 mph continuous wind speed, 3-second gusts of 180 mph (ASCE 7-5/10). TIA-222-G Structural standard for antenna supporting structures and antennas. AASHTO standard specification for structural supports for highway signs, luminaires and traffic signals, 6 th edition.

9 Medium Enclosures Environmental Specifications Medium enclosures are 12 by 12 by 12, or smaller. Structural frame supporting RF transparent materials can also be of an RF transparent material. Structure must conform to the latest version of the International Building Code (IBC). 75 mph basic wind speed with 125 mph gusts. -40 to +125 degrees F ambient rating. Must handle up to 30 pounds per square foot of snow load.

10 Large Enclosures Environmental Specifications Large enclosures are larger than 12 by 12 by 12. Structural frame supporting RF transparent materials must use steel members (non RF transparent). Structure must conform to the latest version of the International Building Code (IBC). 90 mph basic wind speed with 125 mph gusts. -40 to +125 degrees F ambient rating. Must handle up to 30 pounds per square foot of snow load.

11 Some Candidate Thermoplastic Materials Material Dielectric Constant Loss Tangent Acrylic Polypropylene ABS Polycarbonate Polyethylene Teflon Polyvinyl Chloride Nylon Celtek (expanded PVC)

12 Forms of Thermoplastic Materials Candidate structural wall or roofing materials can be flat or corrugated panels. Some candidate materials such as the twin wall polycarbonate roofing material (right) are reduced weight, effective dielectric constant and loss.

13 Other Candidate Materials Some thermoset wall materials are a fiber reinforced plastic (FRP) Potential resins include epoxy, polyamide, silicone, cyanate ester Potential reinforcement fibers include fiberglass, quartz, polyester A-sandwich constructions offer better broadband performance Candidate skin materials include fiberglass, quartz, polyester or Dyneema (UHMWPE) Most viable core material is a polyethylene core (low cost, low weight)

14 Dielectric Parameters of these other Materials Material Dielectric Constant Loss Tangent Epoxy Fiberglass Epoxy Quartz Cyanate Ester Quartz Dyneema (UHMWPE) Closed Cell Polyethylene Foam Material

15 Method of Analyzing Performance of Multilayer Dielectric Walls

16 Matrix Solution for N Layer Wall

17 Applicable Coefficients

18 Matrix Equation Manipulation + A B E + 0 N + 1 C D 0 0 E = E From which the loss in db is given by: LOSS( db) = 20LOG10( 1 ) A

19 Matrix Equation Manipulation And the front face reflection in db is given by REFLECTION ( db) = 20LOG10( C / A)

20 A look at worse case and best case material loss behavior Epoxy FG = worse Polypropylene=best

21 Loss of 0.25 Epoxy FG at 1 GHz

22 Loss of 0.25 Epoxy FG at 30 GHz

23 Loss of 0.25 Polypropylene at 1 GHz

24 Loss of 0.25 Polypropylene at 30 GHz

25 For electrically thick, loss only due to reflection

26 Loss data summary Loss generally is greater for the material with the highest loss tangent Loss increases with frequency Loss increases with angle of incidence For low loss materials that are electrically thick, the reflection loss component dominates

27 A look at worse case and best case material insertion phase (IPD) behavior Epoxy FG = worse Polypropylene=best

28 Relative IPD for 0.25 Epoxy FG at 1GHz

29 Relative IPD for 0.25 Epoxy FG at 30 GHz

30 Relative IPD for 0.25 Polypropylene at 1GHz

31 Relative IPD for 0.25 Polypropylene at 30 GHz

32 Relative IPD summary Relative IPD increases with angle of incidence Relative IPD increases with frequency Difference in IPD between the parallel and perpendicular polarization is responsible for depolarization ideally the IPD should be the same for both the perpendicular and parallel polarizations

33 A look at worse case and best case material behavior with circular polarization Epoxy FG = worse Polypropylene=best

34 Copol loss for 0.25 Epoxy FG

35 Copol loss for 0.25 Polypropylene

36 Axial ratio (AR) for 0.25 Epoxy FG

37 Axial ratio (AR) for 0.25 Polypropylene

38 Corrugated epoxy fiberglass A material type where changes of AOI over the surface of the material result in an effective AR for the entire sheet. AR is important in circular polarized SATCOM antennas

39 Circular polarization summary Copol loss and AR both increase with AOI Copol loss and AR both increase with frequency Corrugated surface siding requires good engineering in order to maintain AR below a desired level especially important with circularly polarized SATCOM antennas

40 Other Electromagnetic Considerations The effect of an enclosure on an antenna pattern is determined by comparison of the computed antenna pattern with and without enclosure. Complex transmission coefficient for rays intersecting various portions of an enclosure wall are determined by a boundary value, multilayer dielectric solution.

41 Antenna pattern from A by B size rectangular aperture (no enclosure) Where I(X, Y) is the internal aperture distribution and Kx = k Cos θ Cos φ Ky = k Sin θ Sin Ф

42 Antenna pattern from A by B size rectangular aperture (with enclosure) j( KxX + KyY ) E( θφ, ) I( X, Y ) T ( X, Y ) e dxdy = Where T(X,Y)I(X, Y) is the external aperture distribution and Kx = k Cos θ Cos φ Ky = k Sin θ Sin Ф T(X,Y) = Transmission Coefficient for X,Y ray

43 Difference between internal and external antenna aperture

44 Sample Antenna Patterns Satellite Downlink Frequency (Ka Band) Satellite Uplink Frequency (Ka Band)

45 START 3D Radome Program MAIN (EXECUTIVE PROGRAM) INTERNAL APERTURE EXTERNAL APERTURE INTERCEPT COMPUTE PATTERNS WALL END

46 Reduction of Antenna Data For an antenna in a fixed position, comparison of the internal aperture antenna pattern with the external aperture antenna pattern allows quantification of all important structure effects on the antenna performance. For an antenna that is movable or scanned, data can be represented over upper half space for all possible antenna pointing angles.

47 Data Representation for Scanned Antennas Create data contours based on no more than 15 degree computation increments in both AZ and EL. The methodology is somewhat labor intensive.

48 Parameters for Contour Plots Linear Polarized Antennas Principal polarization loss (db) Orthogonal polarization loss (db) Boresight error (MRAD) Registration error (MRAD) Increase in antenna VSWR Increase in antenna noise temperature Circular Polarized Antennas Copol transmission loss (db) Xpol transmission loss (db) Axial ratio (db) Boresight error (MRAD) Registration error (MRAD) Increase in antenna VSWR Increase in antenna noise temperature

49 Other problems with multiple antennas within enclosure

50 Sidewall reflection versus Transmission Reflected energy introduces noise into adjacent antennas Absorptive baffle between antennas may be a solution

51 Sidewall reflection versus antenna voltage standing wave ratio (VSWR) Sidewall reflected energy can increase antenna VSWR For sidewall reflection below -20 db the increase in antenna VSWR is negligible.

52 Backlobes at 734 MHz (-45 deg. Polarization) WITHOUT SUPRESSOR WITH SUPRESSOR db

53 Measured F/B at Low Band RED = WITHOUT SUPRESSOR GREEN = WITH SUPRESSOR

54 Measured F/B at High Band RED = WITHOUT SUPRESSOR GREEN = WITH SUPRESSOR

55 Cell Antenna with Backlobe Supressor

56 Conclusions 1 With modern electronics systems, minimizing the effects of an enclosure is more than just minimizing loss and requires careful design and analysis. Loss through a siding material is comprised of both reflection loss and dissipative losses within the material. One is generally limited to commercially available siding materials (types and thicknesses) a tradeoff between electrical and mechanical requirements.

57 Conclusions 2 Most thermoplastic materials have excellent electrical properties but are not strong mechanically large enclosures using thermoplastic materials require a steel frame to support the siding reflections from steel frame could be a problem if not masked. At large angle of incidence, many siding materials depolarize the signal if not properly designed: increased cross polarization for linear polarization or increased axial ratio if circular polarization

58 Conclusions 3 An antenna with fixed pointing is easiest to implement; an antenna that can be pointed over a wide range of AZ and EL angles may encounter aperture blockages that increase far-out sidelobe levels for instance SATCOM antennas. Multiple antennas within the same enclosure can interfere with each other if not properly treated: absorbing baffles to treat sidewall reflections, use of electromagnetic bandgap (EBG) or broadband waveguide chokes to minimize back radiation.

59 Thank You for Viewing Our Presentation Have a Nice Day! D. J. Kozakoff dr.kozakoff@yahoo.com J. Fitzhugh jfitzhugh@concealfab.com W. Pounds wpounds@concealfab.com