Antenna Controller

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Antenna Controller

• ECE 4532-01 Senior Design I
• Mississippi State University
• November 22, 2004

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Team Assignments
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Amateur Radio

• Popular worldwide hobby
• Technically oriented
• Emergency communications
• Many modes of communication
• Licensed by the FCC in U.S.

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Background

• Mobile amateurs operate on HF bands between 3.5
  30MHz
• Height restrictions require the use of short
  antennas
• Electrically short antennas have high capacitance
• Screwdriver antennas use inductance to cancel out
  the capacitance
• Early screwdriver antennas required manual
  inductor adjustment

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Screwdriver Antenna

• 12V DC motor built into bottom of antenna
• Motor drives a variable inductor inside the
  antenna
• Reactance is removed from antenna circuit
• Results in a tuned antenna at desired resonant
  frequency

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Frequency Response
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Far Field Plot
Plot of radiation pattern of the Screwdriver
antenna at 14MHz
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Problem

• Existing controllers not very accurate
• Amateurs must manually hold switch until the
  antenna is tuned
• Makes tuning while driving unsafe

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Technical Design Constraints

• Tuning Accuracy
• /- 2 inductor turns will provide tuning
  consistency
• Voltage Regulation
• 8-16 volts will produce 5 volts
• Temperature Requirements
• Operating range -40C to 80C
• Memory
• Non-volatile, stores positions
• Relay Specifications
• Will pass 5 amps for 1 antenna cycle

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System Block Diagram
User Interface
Power
Voltage Regulator
Memory
Microcontroller
Sensor
Relays
Antenna
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Microcontroller

• PIC16F73
• 5/8-bit A/D
• 4K memory
• 22 I/O pins
• 20 MHz

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Software Process Model
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User Interface
4 Digit 7 Segment LED Display shows the current
position of the antenna
Up/Down Switch allows manual adjust of the
screwdriver antenna
Toggle Switch for selecting On/Off
Four memory buttons for saving and recalling
positions
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LED Display Circuit
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Memory Buttons

• Four buttons to save and recall positions
• Resistor ladder reduces number of microcontroller
  input pins

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Memory Buttons Resistor Ladder Simulation
Button 4
Button 3
Button 2
Button 1
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Regulation Circuit

• Design requires a constant 5 volts within /-5
  with 8-16 volts input
• Selected LM7805 regulator

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Voltage Regulation Tests Results
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Motor Control

• Relays
• Allow microcontroller to switch motor on/off
• Pair of relays allow current flow control

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Problem 1 Relays

• Relays using 5V coil unavailable
• Solution
• Use relays with 12V coil
• Transistor needed to activate coil

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Relay Circuit
Current Flow
Current Flow
5V
5V

• Schematic of motor control system
• Increase/decrease antenna inductance

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Relay Tests

• Connected relay to 12 and 2.4? load
• Tested for 5 minutes (2X cycle time)

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Problem 2

• Sensor input connected to microcontroller
  external interrupt pin
• Voltage spikes on sensor input cause the
  microcontroller to generate interrupts resulting
  in inaccurate turn counts
• Solution
• Disabled interrupts on the sensor input
• Used debouncing to eliminate false triggering
• Results in accurate counts

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Accuracy Tests

• Saved positions in 4 buttons
• 15 iterations per button
• Results
• Controller successfully returned antenna within
  /- 2 turns

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Projected Costs
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What Works

• Voltage regulation operates within /- 5
• Accurately counts inductor turns
• Controller can save and retrieve positions
• Recalls positions within /- 1 inductor turn
• Controller operates successfully exposed to 5
  watts of RF through antenna

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Unverified Tests

• Temperature operation not verified since
  prototype is on breadboard
• Full power RF testing unknown

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Senior Design II Goals

• PCB layout and testing
• Packaging for use in an automobile
• High power RF testing

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Acknowledgements

• Mr. Martin F. Jue
• Dr. Patrick J. Donohoe
• Dr. Raymond Winton
• Mr. Harry Wong
• Mr. Joaquin Bonilla

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Questions?