Amateur Radio Repeater Daniel Harkenrider ECE 499 Capstone

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 • Transmission must be by line of sight

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

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

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

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

Antenna § 11. 5 d. B gain § Installed on roof of Science & Engineering • increased antenna height • Line of sight

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

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 (d. B) • Provides a parameter for analyzing noise characteristics of component

Noise Source § Based around noise diode • noise power is proportional to the diode current. § Calibrated using existing calibrated noise source § Produces 15. 6 d. B ENR

Measurement and Calculation § § § 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 = log 10 (NON) – log 10 (NOFF) • NF (d. B) = 10 log 10 (10(ENR/10) / (10(Y/10) – 1))

Noise source calibration § Calibrated noise source (26. 2 d. B) § Add attenuators to source until power ratio for calibrated source = ratio for unknown source § ENRcalibrated – attenuation = ENRunknown 20. 0 V DC 25. 0 V DC Noise Source Calibrated Noise Source 3 d. B attenuator Attenuator Spectrum Analyzer NON/NOFF

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 § Ga. As FET used for low noise § Necessary to look at noise figure and change in receiver sensitivity

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 k. Hz tone Preamp Receiver Audio Output SINAD Meter

Receiver Sensitivity Results § Without preamplifier • 5. 9 u. V for 20 d. B SINAD § With preamplifier • 0. 8 u. V for 20 d. B SINAD

Preamplifier Noise Figure § NON = -104. 7 d. Bm § NOFF = -114. 0 d. Bm § Noise figure = 6. 84 d. B

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 d. B Test setup to measure SINAD Receiver sensitivity increased from 5. 9 u. V to 0. 8 u. V § Legal / regulatory • Obtained amateur radio license • CW ID

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

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