antenna radiation pattern measurement with a drone · 2017. 10. 12. · decreases as square of...
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Antenna Radiation Pattern Measurement with a Drone John Kean October 12, 2017
Cavell Mertz & Associates Inc.
Manassas, Virginia
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Why use a drone? • Since the beginning of
broadcasting, engineers have required information on antenna system performance
• Land-based methods • Mobile runs or “drive-test” • Chart recording of signal • Measurements at multiple
distances along a long radial to reduce terrain effects
• Curve-fitting data to estimate antenna performance
• Costly in time and expense • Can’t remove terrain effects
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Why use a drone?
• Later, portable equipment was developed to collect signal data in the air
• Drawbacks • Aircraft costly to operate
• Requires orbital distances that are subject to ground reflection, which are difficult to remove
• Some characteristics, e.g. vertical pattern measurement not feasible
Testing prior to Mt. Sutro SFO
tower measurement in 1990
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Why use a drone?
• In principle, drones • Avoid the terrain and clutter effects of ground-based
measurements
• Can collect measurements quickly, like helicopters
• Are less costly to operate than helicopters and pilots
• Use components with moderate costs
• May offer high measurement accuracy
• However…
• Require substantial development time
• Require an FAA-licensed drone pilot and engineer
• Require suitable weather conditions
• May not be allowed to fly at some locations
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Measurement drone diagram
• Complex system of digital computers, sensors, RF shielding
• Flight Control Computer • Holds desired flight plan
• Using GPS and sensors, maintains flight stability, orientation and location
• Monitors power and controls motors
• Accepts manual control via Radio Link
• Makes flight decisions in case of difficulties
Software Defined Receiver
Data Collection Computer
Flight Control Computer
GPS
Radio Data LinkBattery and Power
Control
Inertial Sensors
Measurement Antenna
Comm. Antenna
Motors
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Cavell Mertz drone • Custom Tarot-brand frame
• Carbon-fiber components • 6-motor/propeller system • 10,000 mAH 24V Li-polymer
battery can supply 20+ min.
• Custom flight controller • Inertial sensors for trim, a
flux-gate compass for orientation, diff.-GPS
• Autonomous (programmed) flight
• Live, wireless flight telemetry
• Bi-conical (50-1000 MHz) antenna
• Custom receiver/logger
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How it works
• SDR works like spectrum analyzer • Configurable span,
resolution bandwidth
• Measures signal (red) at regular intervals, along with location and altitude in elevation measurement (blue)
• Raw data must be processed to produce useful results, below
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Flight planning for an appropriate radius
• For horizontal plane measurements • Must choose a suitable
radius for array size (large arrays require larger radius)
• Easy when radius is within property
• If radius extends outside, over homes or roads, FAA waiver, if granted, may take 3 months
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Appropriate measurement radius
• Accurate measurement requires • the far field of the array • Inverse-square law
(radiated power density decreases as square of distance)
• Defined by Fraunhofer distance: 𝑟𝑓𝑎𝑟 =
2𝐷2
λ
• 6-bay FM array, rfar 150 m • Other considerations that
increase distance are • Vector sum of elements for
vertical radiation • High power density
rfar
D
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Pattern model for uniform linear array
• Periodic function for normalized array factor
• 𝐴𝐹𝑛 =sin 𝑛/2
sin 𝑛/2
• “Engineering Assessment of Potential Impact of Protections on AM, FM & TV” -Richard Tell, 1985
• Radiation pattern is product of AFn and single element pattern
• Excel workbook
• AFn for 2-bay 1λ:
• Cycloid single-element
• Multiplied pattern
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Ground reflection effects • May have severe effect on
any aerial measurements, especially at large distances (low reflection angles), however…
• Drones fly relatively close, so reflection angle is high, reducing specular reflection
• Depression angle is high, so downward radiation is usually low
• Specular reflection is reduced close-in, at high reflection angle
Measurement Drone
Depression Angle
Reflection Angle
0 to 90 degrees
H
V
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Ground reflection estimation • Two-Ray Model
developed in Excel
•λ²
(4π𝑑)²2𝑠𝑖𝑛
2π
λ
ℎ𝑇ℎ𝑅
𝑑²𝐺𝑇𝑃𝑇𝐺𝑅
• Takes into account LOS and ground reflection
• Good approximation for smooth well-reflecting terrain
• More accurate for d>5hThR
• Chart: λ = 0.3m, HR = 2m
• Yellow: free space • Blue: h=30m
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Ground reflection example • In this worst-case high refection is assumed, such as sea water
• Signal in brown may vary +6 dB and -25 dB; blue is free space path loss • Would be reduced by vertical radiation pattern, lower reflection coefficient
• Drone measurements must be checked for potential ground reflection effects • some sites may not have suitable geometry for aerial measurement
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Station measurement • Horizontally-polarized,
non-directional FM, side-mounted array on SW tower leg
• Horizontal plane measurement at radiation center elev.
• Relative field polar pattern, right • Maximum lobe at 335
degrees • Nearly optimal across 165-
260 degree arc • Minima at 25 degrees,
across tower, reducing ERP to ~10%
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Station measurement
• Elevation pattern from horizontal plane to ground (-90 degrees) for same FM, having 2-bay half-wavelength spacing of segmented rototiller
• Array factor calculation for array in dotted line
• Actual measured relative field pattern in green • Expected null at 30
degrees
• One (reduced) minor lobe
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Recommendations
• Drone measurement can be accurate, fast (only hours on-site)
• The right equipment is required • RF hardened (fields can exceed 20 v/m!)
• Known, accurate antenna and RF sampling
• Careful post-processing to reduce sampling noise and convert positional information
• Experienced flight crew • Commercial drone requires FAA-certified pilot, observes Rules
• Engineer to study and determine • proper flight distances
• Anticipate ground reflections that would degrade results
• Process and verify measurements
• Visual observation of entire flight and spotters as needed to keep flight area vacant