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Amateur Radio Repeater

Amateur Radio Repeater. Daniel Harkenrider ECE 499 – Capstone Design Advisor: Professor Hedrick March 1, 2008. What is a Repeater?. Receives a low-power signal and retransmits at a offset frequency Higher power and greater sensitivity allow for extended range Especially necessary for UHF

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Amateur Radio Repeater

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  1. Amateur Radio Repeater Daniel Harkenrider ECE 499 – Capstone Design Advisor: Professor Hedrick March 1, 2008

  2. What is a Repeater? • Receives a low-power signal and retransmits at a offset frequency • Higher power and greater sensitivity allow for extended range • Especially necessary for UHF • Transmission must be by line of sight

  3. Reasons for Project • Interest in communications • Study elements of RF communications • Circuits • Transmission range • Receiver design factors • Sensitivity • Noise

  4. Uses of Repeater • Study of RF communications • Propagation, noise • Relay amateur radio transmissions • Modify for digital data transmission • Emergency services

  5. Factors affecting Radio Range • Antenna height • Especially for line of sight propagation • Antenna gain • Receiver sensitivity • Transmitter power • Loss in transmission line

  6. Repeater design objectives and Criteria • Tasks to make repeater operational • Antenna in place • CW ID installed and operational • Design improvements • Range – does not cover entire campus • Increase to minimum 10 mile radius • Improve receiver sensitivity

  7. Antenna • 11.5 dB gain • Installed on roof of Science & Engineering • increased antenna height • Line of sight

  8. Receiver Sensitivity • Minimum signal power detected • Signal to noise ratio (SNR) • Amplification can boost weak signal, but also noise • How to maximize SNR in receiver

  9. Noise • From transmission or receiver components • In receiver: thermal, shot, etc. • Noise Factor: Each stage of receiver adds noise, SNR decreases • F = (SNRi/SNRo) • Noise Figure = 10 log F (dB) • Provides a parameter for analyzing noise characteristics of component

  10. Noise Source • Based around noise diode • noise power is proportional to the diode current. • Calibrated using existing calibrated noise source • Produces 15.6 dB ENR

  11. MeasurementandCalculation • Noise power measured with spectrum analyzer • Noise power (noise source on): NON • Noise power (source off): NOFF • Excess noise ratio (ENR): (NON – NOFF ) / NOFF • Calculate noise factor NF • Y = NON / NOFF = log10 (NON) – log10 (NOFF) • NF (dB) = 10 log10 (10(ENR/10) / (10(Y/10) – 1))

  12. Noise source calibration • Calibrated noise source (26.2 dB) • Add attenuators to source until power ratio for calibrated source = ratio for unknown source • ENRcalibrated – attenuation = ENRunknown Noise Source NON/NOFF 20.0V DC 3 dB attenuator Spectrum Analyzer Calibrated Noise Source NON/NOFF 25.0V DC Attenuator Spectrum Analyzer

  13. Preamplifier • Improve signal to noise ratio of receiver • Amplify signal close to receiver front end • Signal power increases relative to internal noise • Operates in 70 cm band • GaAs FET used for low noise • Necessary to look at noise figure and change in receiver sensitivity

  14. SINAD measurement • Signal-to-noise plus distortion • Gives best measure of sensitivity • At SINAD meter, signal is filtered from noise, and the power levels are compared Signal generator 447.55 MHz 1 kHz tone Preamp Receiver Audio Output SINAD Meter

  15. Receiver Sensitivity Results • Without preamplifier • 5.9uV for 20 dB SINAD • With preamplifier • 0.8uV for 20 dB SINAD

  16. Preamplifier Noise Figure • NON = -104.7 dBm • NOFF = -114.0 dBm • Noise figure = 6.84 dB

  17. Completed • Increased Range: Preliminary testing 12 miles • Antenna placement • Low-noise, high gain preamplifier installed • Receiver design • Constructed test setup to measure noise figure • NF of preamplifier = 6.84 dB • Test setup to measure SINAD • Receiver sensitivity increased from 5.9uV to 0.8uV • Legal / regulatory • Obtained amateur radio license • CW ID

  18. Remaining tasks • Measure loss in feed line from transmitter to antenna • More complete measure of range of repeater • Adjust preamplifier to reduce noise figure

  19. Acknowledgements • Professor Hedrick • Jules Madey • Robin Stevenson, EE 2006 • Slide 3:The ARRL Handbook. American Radio Relay League, 1997. p. 23.4 • Slide 15: Jules Madey

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