ON-SITE DIGITAL TV SIGNAL SURVEYS
Computer aided DTV reception studies are great, but the computer
cannot be programmed for electrical (AC) interference, generated by high
voltage power lines, or RF interference, caused by mobile radios, harmonics of
broadcast stations, vehicular traffic, etc. Nor can the computer predict the presence or absence of reflections
(ghostings), a real HDTV picture quality killer.
The professionally conducted on-site DTV reception verification testing will deliver conclusive test results, assisting the
survey engineer in making proper antenna tower height and antenna-array
recommendations.
An ideal verification survey should use a 150’ high
telescopic mast, a rotor-mounted 6’ diameter parabolic antenna on the top of
the mast, conducting a full week on-site testing to experience inclement
weather conditions, with hourly readings, 12 hours a day. Of course, this is just a dream. No company management would authorize such an
expensive project.
The Biro DTV reception verification survey uses a bucket
truck, with rotor-mounted test antenna.. elevated to
35 ft above ground.
Separate antennas are employed
for surveying UHF High-Band and low-band DTV stations.
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LOW-BAND ANTENNA |
HIGH-BAND
ANTENNA |
The most desirable time for conducting a DTV reception
survey is at mid-day, particularly 1:00 to 3:00 PM, when the signal path from
the distant UHF station may be broken by heat turbulence and ascending warm
layers. This does not mean that surveys
conducted on foggy or rainy days are of no value, but early afternoon hours represent the most
favorable testing time under any conditions. Almost all distant UHF stations exhibit variable fades between 1:00 and
5:00 PM, with maximum depth of fade occurring around 3:00 PM.
A well conducted DTV reception verification survey requires
the following equipment:
- Rotor-mounted
high-gain, high directivity antenna (Radio Shack antennas no need to
apply), separate antennas for UHF and VHF stations.
- Low-loss
coaxial cable, connecting the antenna with the instrumentation.
- High
quality spectrum analyzer, digital camera with close-up lens.
- Large
screen LCD TV monitor.
- Computer
Run and Signal Direction Sheets, to alert the survey engineer about low
output power, low transmit antenna height stations and potential
adjacent/co-channel interference difficulties.
Co-channel and
adjacent channel interference identification is highly desired objective of
the on-site signal survey.
The FCC sold off the top 100 MHz of the UHF band for 20 billion
dollars, forcing Channel 52 to Channel 69 stations to relocate into the 470 to
700 MHz frequency range . When a lot of TV stations are squeezed into a
limited frequency range, the trouble is just around the corner. The net result: increasing number of
co-channel and adjacent channel interference conflicts.
The photograph above shows a serious adjacent channel
interference situation, The desired Channel 31 power spectrum, in
the center of the screen, was found at a level 15 dB below the undesired adjacent Chanel 32.
The carriage of Channel 31 DTV could not be
recommended. When the undesired DTV
station’s amplitude is 15 dB higher than the desired, there is no guarantee that even a
phased-array will provide satisfactory protection.
SIGNIFICANT SAVINGS
Let’s make it perfectly clear, an on-site DTV signal survey
is not sponsored by the Ford Foundation. However, the cost benefits of the survey, such
as an antenna tower shorter than recommended by the antenna manufacturer’s
salesmen, or parabolic antennas providing simultaneous reception of co-located HDTV stations, could more than compensate
for the costs of the verification survey. And last but not least, in the event of major problems with the proposed
site, it is much better to discover the problems before signing a lease, making a costly deposit on the property or installing
an unreasonably tall antenna tower.
Biro Engineering
POBox 2175
Princeton, NJ, 08543-2175 |
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