Electrically Small Antennas - IREAP Electrically Small Antenna • An electrically small, inductively

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  • Project Objective Development of an electrically small antenna, capable of ~ 1 MW cw power output, tunable from ~ 3 to 10 MHz

    Accomplishments / Progress • Experimental verification of antenna

    concept and tuning capability at 100 MHz (30 MHz to 100 MHz)

    • Experimental demonstration of full size antenna at 10 MHz (limited tuning range from 9.5 to 10 MHz)

    • Verified conventional antenna drive (sinusoidal source through 50 Ohm coax)

    • Verified direct drive approach • Radiated ~ 500 W at 10 MHz with

    approx. 90% efficiency (relative to DC power input)

    • Transportable “HAARP” scale-up

    Electrically Small Antennas

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Electrically Small Antenna

    • An electrically small, inductively coupled antenna design  k·a < 0.5

    • Highly resonant structure  Provides natural match to 50

    Ohm source • Up to 98% efficient • Small Loop Antenna (SLA)

    inductively couples to Capacitively Loaded Loop (CLL)

    2

    Stub length

    Stub Space

    SLA Radius

    Wire Diameter

    Width

    Height

    Dielectric

    GHz range antenna (no tuning): A. Erentok and R. W. Ziolkowski, “An efficient metamaterial-inspired electrically-small antenna,” Microw. Opt. Tech. Lett, vol. 49, no. 6, pp. 1287–1290, 2007

    Dielectric insertion for tuning or gap width tuning

    2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Antenna Concept Verified

    CAD Drawing Built Antenna

    Early 100 MHz ESA antenna

    dielectric tuner

    3 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Tuning with Dielectric Insertion

    • Resonant frequency adjusted by changing capacitance of CLL  e.g. inserting a dielectric

    • Eccostock HiK  εr = 15, tanδ = 2·10-3

     45- 100 MHz • Teflon

     εr = 2.1, tanδ = 2·10-4

     83 – 100 MHz

    4

    Experimental results of capacitive tuning only

    Resulting antenna: 5 – 10 times smaller than dipole

    Early 100 MHz ESA antenna

    2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION 5

    •E/H-Plane Gain Pattern  f = 99.35 MHz (No dielectric inserted)  Peak Gain = 2.82 dBi  3 dB Beamwidth = 158̊

    Measured Gain Early 100 MHz ESA antenna

    2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    10 MHz Antenna Pattern

    6

    • Small ground plane over soil (4.8 x 4.8 m) • Gain: 1.9 dBi • 124.5° HPBW • High losses into soil

    • Extended ground plane (9.7 x 7.2 m) (E x H direction) • Gain: 5.8 dBi • 114.4° HPBW

    2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Horizontal Gap

    • University of Maryland suggested modification • Larger gap possible due to increased

    capacitive area • Tunable by adjusting area of overlap

    – air tuning possible • Increased dielectric requirement for

    breakdown mitigation (large volume, high quality dielectric needed)

    7 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Horizontal Gap With Rollers

    Horizontal Gap

    8 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    • tanδ = 0.0065, μ = 50, 2 cm slabs – 85.6% radiation efficiency – 13.9% loss into ferrite – 81% Accepted Power – 69% Total efficiency

    Slab thickness exaggerated for visibility

    Low matching quality is observed

    Ferrite Tuning

    9 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Horizontal Gap Max Power (10 MHz)

    • Limited by – Air Breakdown

    • > 30 kV/cm at 50 MW • Assumes adequate insulation at feed point.

    – Teflon* Melting (2 MW input power) • Loss tangent: 2*10-4, 68.4 min at 9.4% loss (188 kW) • Loss tangent: 2*10-3, 18.8 min at 34.2% loss (684 kW)

    – Loss of original properties • Ferrites

    – Permeability changes with temperature, dependent on individual composition of soft ferrites (NiZn)

    – Typical loss at 2 MW excitation: 278 kW (2 cm thick slabs)

    * Teflon sheet: 15 cm thick, 200 x 300 cm approx.

    10

     1 to 2 MW should be feasible, air tuning preferred (6 kV/cm)

    2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Full Size ESA

    11

    10 MHz ESA

    2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Antenna Drive Methods

    Direct (Freq.– domain)

    Conventional (Freq. –domain)

    Direct (Time – domain)

    ESA

    10 MHz values: L1 = 1 µH, L2 = 2 µH, k = 0.154, C2 = 126 pF, R = 1.6 Ω, (L0 ~ 20 nH, C1 ~ 20 pF)

    12 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Freq. Domain Analysis

    • Analytical Solutions for standard drive

    Ladder network impedance S11 with 50 Ohm coaxial cable drive

    13 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Freq. Domain Analysis

    Compared to direct drive frequency domain (6 kV rms drive)

    Mutual Inductance adjusted to maximize direct drive performance

    Black: 50 Ohm, Red: Direct drive Dotted/Dashed: Reactive power

    Increasing k by 40 % changes the mutual inductance from to 0.34 to 0.48 µH. (freq. shift by 120 kHz)

    14 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Freq. Domain Analysis

    Compared to 3.3 MHz operation (only capacitive tuning)

    Mutual Inductance adjusted to reduce reactive component (60% increase)

    Black: 50 Ohm, Red: Direct drive Dotted/Dashed: Reactive power

    Increased power output in direct drive (20 kVrms source). Power and matching in standard drive (50 Ohm) also improved.

    15 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Spice Analysis, Freq. domain

    Tune this capacitor

    16 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    15 pF cap + stray

    stray only (~ 10 pF)

    130 pF cap + stray

    Freq. Domain Comparison

    17 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Direct Drive / Time Domain • 10 MHz ESA

    – 3 x 2.5 x 1.2 m – 4.8 m square ground plane, extended with steel

    wire mesh • Tunable via:

    – Dielectric insertion (polymer dielectric limits duration due to Joule heating)

    – Gap adjustment • ~ 4.7 dBi Gain • ~ 1 MW Breakdown Strength (air, 30 kV/cm) • Excited with square pulse train

    Dial in square pulse period from S11 frequency

    18 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Experimental Setup

    Switcher

    Oscilloscope

    Magnetic Loop

    Signal Gen

    Instrument Bench

    HV PSU

    Voltage Probe Current Monitor

    50 – 250V

    3... 10 MHz

    15m above antenna plane

    19 2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Direct Drive

    20

    Implementation with 1,200 V SiC MOSFET, mimicking PCSS for “low” voltage.

    2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Excitation

    • 1,200 V MOSFET switch • Antenna effectively filters out unwanted

    harmonics • Driving voltage: 100 V (50…250 V) • Magnetic loop placed 15 m centered

    above antenna

    • Approx. 500 W Radiated for 200V DC

    input • Approx. 90% efficiency

    21

    measured

    2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    Measured vs. Simulated

    22

    Current through exciting loop

    Received antenna signal Current through load R2

    2/10/2016

  • COLLABORATIVE RESEARCH ON NOVEL HIGH POWER SOURCES FOR AND PHYSICS OF IONOSPHERIC MODIFICATION

    10 MHz small scale model

    23

    L1

    L2 R2

    air capacitor

    C2 tunable

    ~ 5 cm diameter

    dcoil