Antenna Radiation Pattern Measurement with a Drone · 2017. 10. 12. · decreases as square of distance) ... •Drone measurement can be accurate, fast (only hours on-site) •The

Antenna Radiation Pattern Measurement with a Drone John Kean October 12, 2017

Cavell Mertz & Associates Inc.

Manassas, Virginia

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

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

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

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

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

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

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

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

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

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

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

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

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%

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

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