Senior Project – Electrical Engineering - 2008

Amateur Radio RepeaterDaniel Harkenrider

Advisor – Professor James Hedrick

AbstractThere are a number of factors which affect wireless radio-frequency communications, including antenna height and gain, transmitter power, and receiver sensitivity. Several of these concerns needed to be addressed in order to make the Union College amateur radio repeater operational. An antenna had to be acquired and installed in a high enough location to transmit and receive signals adequately. The sensitivity of the repeater’s receiver had to be improved, and this was done by adding a preamplifier to the receiver front end. A noise generator was constructed and used in order to test the performance of the receiver with and without the preamplifier. Results from the modified receiver show a significant improvement in sensitivity from the original.

Repeater Background

A radio repeater is a station which receives a signal from one radio and retransmits it to another at a slightly offset frequency. Having a more powerful station to retransmit a signal provides a greater total range. The Union College repeater operates in the 70 cm (420 to 450 MHz) frequency band.

Noise SourceIn order to test the noise level of the receiver without actually transmitting a signal, it was necessary to construct a calibrated noise source. This device is based around a noise diode, which produces noise to input to the receiver. When the noise levels with the noise source turned on and off have been measured, the excess noise ratio of the noise source can be determined. When a noise source with a known excess noise ratio is connected to a device, the noise figure of the device can also be determined, giving the relative decline in signal to noise ratio across that device.

PreamplifierDifferent types of noise can affect the sensitivity of a radio receiver. Some occur during the transmission of a signal. Others, of which the most common is thermal noise, occur within the circuitry of the receiver. One way to limit the effect of these internal sources of noise is to place an amplifier close to the receiver front end. This device, known as a preamplifier, serves to amplify the received signal before it passes through the rest of the receiver. This will improve the signal-to-noise ratio of the receiver and make it possible to receive weaker signals. Receiver sensitivity can be measured with and without the preamplifier in the circuit, using a SINAD meter. If the preamplifier is fulfilling its role, then there should be an improvement in sensitivity when it is added. The noise figure of the preamplifier can also be measured. The lower the noise figure, the more effective the preamplifier will be in improving receiver sensitivity. Conversely, a preamplifier with a high noise figure may worsen the sensitivity of the receiver.

Antenna

The radio system required a new antenna. Since a transmitter in the UHF range uses line-of-sight propagation, it is necessary that the antenna should be located well above ground level to give it the greatest possible coverage. A gain antenna with 11.5 dB gain was installed on the roof of Science & Engineering. This will help to improve the performance of the radio.

Noise source (left) and preamplifier (right), connected to spectrum analyzer (bottom). Courtesy of Jules Madey

Noise Source Noise Source connected to

spectrum analyzer

For 20 dB SINAD Receiver sensitivity

Without preamplifier 5.9 mV

With preamplifier 0.8 mV

ResultsAdding the preamplifier led to a significant improvement in receiver sensitivity:

Calculation showed the noise figure of the preamplifier to be 6.84 dB, which is comparable to some commercially available low noise amplifiers.

Conclusions

The main improvements needed by the repeater system have been provided. With an antenna in place, it is able to receive and send signals. The addition of a preamplifier to the receiver has improved its sensitivity. The repeater is now fully operational. In the future, modifications could be made to the preamplifier in order to further improve its performance. Testing should also be done to determine the maximum range of the repeater.

AcknowledgementsProfessor Hedrick Robin StevensonJules Madey