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COMPUTED ENVELOPE LINEARITY OF SEVERAL FM BROADCAST ANTENNA ARRAYS J. Dane Jubera

COMPUTED ENVELOPE LINEARITY OF SEVERAL FM BROADCAST ANTENNA ARRAYS J. Dane Jubera. 2008 NAB Engineering Conference. Complex Envelope Linearity: Ideal is flat amplitude and flat delay response (vs frequency). Report maximum deviation from ideal.

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COMPUTED ENVELOPE LINEARITY OF SEVERAL FM BROADCAST ANTENNA ARRAYS J. Dane Jubera

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  1. COMPUTED ENVELOPE LINEARITY OF SEVERAL FM BROADCAST ANTENNA ARRAYSJ. Dane Jubera 2008 NAB Engineering Conference

  2. Complex Envelope Linearity: Ideal is flat amplitude and flat delay response (vs frequency). Report maximum deviation from ideal. • Computed Results – No measured data, with apologies. • Antenna System Analysis MININECTM for Antenna Z and Radiation Characteristics all balanced-mode mutual impedances are considered MathcadTM for offline data reduction and network analysis 2008 NAB Engineering Conference

  3. General System Configuration 2008 NAB Engineering Conference

  4. “Antennas” and “Transmitters” to be Considered • FM Panel Array, 4 bay, 3 faces, Omni, CP • FM Panel Array, as above, with lateral offset & turnstile phasing • Single λ/2 dipole, LP • Resistive Load, non-radiating • Norton Equivalent Current Source, Zs= 50 Ω • Norton Equivalent Current Source, Zs= 500 Ω • Norton Equivalent Current Source, Zs= ∞ Ω • Linear System Analysis 2008 NAB Engineering Conference

  5. iBiquity Digital Corporation HD RadioTM Specification for Gain and Delay Flatness “The total gain of the transmission signal path as verified at the antenna output shall be flat to within ± 0.5 dB for all frequencies between (Fc – 200 kHz) to (Fc +200 kHz), where Fc is the RF channel frequency.” “The differential group delay variation of the entire transmission signal path (excluding the RF channel) as measured at the RF channel frequency (Fc ) shall be within 600 ns peak to peak from (Fc – 200 kHz) to (Fc +200 kHz).” [1] Doc. No. SY_SSS_1026s, Rev D, February 18, 2005, “HD Radio FM Transmission System Specifications” 2008 NAB Engineering Conference

  6. Top View of Panel System Feed Region Reflector Panel Dipole 2008 NAB Engineering Conference

  7. Isometric View of Panel System 2008 NAB Engineering Conference

  8. Top View of Offset Panel System 2008 NAB Engineering Conference

  9. Specify source locations. Specify source currents – one “on”, others “off”. Generate geometry of radiating structure. Save configuration file. Specify frequencies and far field directions. Duplicate configuration file for each source current location. Modify source currents. Execute analysis for each configuration file. Flow Chart for MININECTM Computations 2008 NAB Engineering Conference

  10. Collect all port voltage data and construct antenna port Y matrix at each frequency. Use network analysis to determine antenna feed currents when connected by model feed system. Compute CP mode fields. Compute delay. Collect all far field solutions. Scale by computed feed currents and superpose. Display results. Flow Chart For Off-line Computations 2008 NAB Engineering Conference

  11. Results, Configuration 1 Source Impedance: 50Ω 2008 NAB Engineering Conference

  12. Antenna Input Impedance, Γ Plane 2008 NAB Engineering Conference

  13. Return Loss, Antenna Input ≈ 18 dB over 3.5 MHz 2008 NAB Engineering Conference

  14. Far Field Behavior, Single Channel Δ = 0.05 dB Δ = 0.3 ns 2008 NAB Engineering Conference

  15. Far Field Behavior vs Azimuth, 3 Channels Worst Case Δ = 0.7 ns Δ = 0.09 dB 2008 NAB Engineering Conference

  16. Far Field Behavior vs Azimuth, Magnitude, Polar 2008 NAB Engineering Conference

  17. Results, Configuration 1 Source Impedance: 500Ω 2008 NAB Engineering Conference

  18. Load Impedance Presented to Transmitter ≈ 500 ft Transmission Line Γ Plane 2008 NAB Engineering Conference

  19. Far Field Behavior, Single Channel Δ = 1.87 dB Δ = 251 ns 2008 NAB Engineering Conference

  20. Far Field Behavior vs Azimuth, 3 Channels Worst Case Δ = 251 ns Δ = 1.87 dB 2008 NAB Engineering Conference

  21. Results, Configuration 2 Source Impedance: 50Ω 2008 NAB Engineering Conference

  22. Antenna Input Impedance, ΓPlane 2008 NAB Engineering Conference

  23. Return Loss, Antenna Input 2008 NAB Engineering Conference

  24. Far Field Behavior, Single Channel Δ = 0.2 dB Δ = 2.2 ns 2008 NAB Engineering Conference

  25. Far Field Behavior Vs Azimuth, 3 Channels Worst Case Δ = 3.49 ns Δ = 0.25 dB 2008 NAB Engineering Conference

  26. Far Field Behavior vs Azimuth, Magnitude, Polar 2008 NAB Engineering Conference

  27. Results, Configuration 2 Source Impedance: 500Ω ≈ 500 ft Transmission Line 2008 NAB Engineering Conference

  28. Load Impedance Presented to Transmitter ≈ 500 ft Transmission Line Γ Plane 2008 NAB Engineering Conference

  29. Far Field Behavior, Single Channel Δ = 0.31 dB Δ = 11.3 ns 2008 NAB Engineering Conference

  30. Far Field Behavior vs Azimuth, 3 Channels Worst Case Δ = 11.3 ns Δ = 0.31 dB 2008 NAB Engineering Conference

  31. Single Dipole, 98 MHz ± 200 kHz • Table above shows performance of a single λ/2 dipole antenna fitted with a low Q matching circuit with which to adjust impedance. • Assumed transmission line length is 201 feet. Not as much gain and delay variation as seen with 500 feet of transmission line. Source ZΔ GainΔ DelayAntenna Return Loss 50 0.04 dB 0.01 ns 16.3 dB 1.44 dB 81 ns 16.3 dB 0.54 dB 32 ns 26.4 dB 0.30 dB 19 ns 32.0 dB 2008 NAB Engineering Conference

  32. Resistive Load, Non-Radiating • Resistive Load (RL + j 0) • Long Transmission Line, Lossless • Current Source (Zs = ) • Evaluate voltage on load resistor vs frequency • ρ=|Γ|, Γ = (RL-Z0)/(RL+Z0) • For sufficiently long transmission line (≈ 600’ @ FM) Δt = 4ρ(L/v)/(1- ρ2) ΔG = 20 log(VSWR) = 20 log [(1+ρ)/(1-ρ)] (L/v is 1-way transit time in transmission line) • Example 1: For ρ=0.2, L/v = 720 ns ( ≈ 700 ft) => Δt = 600 ns & ΔG = 3.5 dB Example 2: For ρ=0.126 (18 dB RL), L/v = 508 ns ( ≈ 500 ft) => Δt = 260 ns & ΔG = 2.2 dB 2008 NAB Engineering Conference

  33. Summary of Results • Contribution to envelope non-linearity is primarily via the antenna input mismatch, length of transmission line, and transmitter source mismatch. • Systems using transmitters which are source matched to the transmission line show very good performance in all cases studied here relative to HD Radio specification of 1 dB gain variation and 600 ns delay variation. • Systems using transmitters with high VSWR relative to line impedance require low antenna VSWR to achieve similar envelope linearity performance. 2008 NAB Engineering Conference

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