- 19 -
Noise that limits the ability to hear a weak signal on the lower bands is generally a mixture of local
ground wave and ionosphere propagated noise sources. Some installations suffer from a dominant
noise source located close to the antennas. Noise level differences between urban and rural locations
can be more than 30 dB during the daytime on 160 meters. Nighttime can bring a dramatic increase
in the overall noise level as noise propagates via the ionosphere from multiple distant sources. Since
the noise is external to the antenna, directivity can reduce noise intensity.
Consider these things about noise sources:
If noise is not evenly distributed, performance will depend on the gain difference between
the desired signal direction (azimuth and elevation) and gain in the direction of noise.
If very strong noise comes from the direction of a receiving antenna null, improvement in
S/N ratio can be as much as 30 dB or more
If noise predominantly arrives from the direction and angle of desired signals (assuming
polarization of signals and noise are the same) there will be no improvement in the signal-to-
noise ratio.
If the noise originates in the near-field of the antenna, everything becomes unpredictable. This is a
good case for placing receiving antennas as far from noise sources (such as power lines) as possible.
Proximity to Transmitting Antennas
Eight
DXE-RSEAV-1
Receive Antenna Active Vertical active elements, and your transmitting
antenna need only minimal physical separation to maintain safe power levels when the
DXE-
TVSU-1B
Time Variable Sequencer Unit is used. With 1500 watts output and a unity gain (0 dB)
antenna, the closest active element can be 1/10-wavelength from the transmitting antenna at the
lowest transmitting frequency. Doubling the protection distance quadruples safe power levels. See
Table 2
.
Band
Unity (0 dB) Gain 3 dB Gain (2x)
6 dB Gain (4x)
160m (1.8 MHz)
55 feet
110 feet
220 feet
80m (3.5 MHz)
28 feet
56 feet
112 feet
40m (7.0 MHz)
15 feet
30 feet
60 feet
Table 2 - Array Safety Distance Minimums at 1500 watts
Table 2
indicates minimum safe distances for the sequenced active array from transmitting antennas
with 0 dB, 3 dB and 6 dB gain (ERP) using a 1500 watt transmitter. Your actual system may vary
according to location and proximity to various objects. Your actual system may vary.
Safe distance
will vary depending on operating
frequency, antenna polarization and orientation, and
transmitting antenna pattern.
The Receive Eight Circle Antenna System should be separated from transmit antennas and other
metal structures by 1/2-wavelength (or more) at the lowest frequency of operation.
The
AVA-3
Active Matching Units on the
RSEAV-1
Receive Short Element Active Verticals
ground the antenna elements when power is turned off. The active antennas allow installations with
Содержание DXE-R8C-SYS-V3
Страница 31: ...31 NOTES...
