Transcript Test Poster Font Arial – pt 44

Senior Project – Electrical Engineering - 2008
Amateur Radio Repeater
Daniel Harkenrider
Advisor – Professor James Hedrick
Abstract
Preamplifier
There 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.
Different 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.
Noise source (left) and
preamplifier (right), connected
to spectrum analyzer (bottom).
Antenna
Courtesy of Jules Madey
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
In 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.
Results
Adding the preamplifier led to a significant improvement in
receiver sensitivity:
For 20 dB SINAD
Receiver sensitivity
Without preamplifier 5.9 mV
With preamplifier
0.8 mV
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.
Noise Source
Noise Source connected to
spectrum analyzer
Acknowledgements
Professor Hedrick
Jules Madey
Robin Stevenson