Antennas And Feedlines; Receiving Preamplifier (Pre-Amp); Intermodulation (Imd) Products - Mirage B 1018 G User Manual

4.6 ANTENNAS AND FEEDLINES

Any antenna and 50-ohm feedline can be used, providing it presents less than 2:1 SWR to this
amplifier. Linearity and splatter will be better controlled if SWR is as low as possible, with a 1:1
SWR ratio (unity SWR) being ideal.
Be sure to use proper lightning protection techniques when using this amplifier for home or fixed
operation.
5.0 RECEIVING PREAM PLIFIER (PRE-AMP)
This amplifier contains an internal receiving amplifier (also calle d a pre-amplifier or pre-amp)
that "boosts" weak signals. It has a noise figure of about 1 dB, and about 14 dB maximum gain.
Gain is selectable by internal jumpers located on the small PC board close to where the red and
black power leads enter the unit. These jumpers select approximately 14, 8, and 5 dB gain. As a
general rule, even the lowest gain setting will provide more than enough gain.
PREAMP
BOARD
˜ 5 dB gain
5.1 SIGNAL-TO-NOISE RATIO
At 150 MHz, the signal-to-noise ratio (SNR) of the received signal should easily be established at
the antenna. The required preamplifier noise figure varies greatly with the receiver bandwidth
and antenna type, as well as the location.
This preamplifier's noise figure is just over 1 dB, which is more than adequate for normal
antennas and normal bandwidths used in communications. Noise power is inversely proportional
to bandwidth, and the noise figure of this amplifier will allow CW signals of less than .03µV and
FM signals of less than .30µV to be detected with over 10 dB SNR.
A lower noise-figure amplifier is seldom (if ever) necessary for terrestrial communications. Since
this preamplifier will put almost any system well into ambient noise, no further improvement in
sensitivity will result from more gain or lower noise figure.

5.2 INTERMODULATION (IMD) PRODUCTS

IMD is the result of signals mixing in amplifiers or other less-than-perfect devices. IMD results
in signals appearing on entirely new frequencies. This problem can occur either external to the
PREAMP
BOARD
˜ 8 dB gain
12
PREAMP
BOARD
˜ 14 dB gain
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