THE BIRO TECHNICAL BULLETINS
APRIL 2004
REAL WORLD HDTV RECEPTION EXAMPLES
(Part 4 of 4)
In the December, 2003 issue of the Technical Bulletins the need for comprehensive technical information, the computer aided TV reception study was discussed.
In Part 2 of our HDTV tutorial, appearing in the January, 2004 issue of the
Bulletins, the subject was HDTV interference
identification.
In the February, 2004 issue of the Technical Bulletins, the need for an on-site HDTV reception verification testing was analyzed.
In Part 4 the last one of this series, HDTV reception theory will be substantiated by real world examples.
The Reception of Channel 33, Hartford and Channel 35, New Britain in
Groton, Ct.
The simultaneous operation of 995 analog and more than 1175 Digital TV stations created extremely crowded reception conditions in the 470 to 800 MHz UHF frequency range. Adjacent channel interference became the norm, not the exception.
The reception of Channel 33, Hartford and Channel 35, New Britain in Groton, Ct. is a real world example for adjacent channel interference.
Essential reception parameters of the two desired DTV stations:
| CHANNEL 33 | WFSB-DT | HARTFORD | 46.33 Mi | 200 kW output power | 946’ antenna height |
| CHANNEL 35 | WVIT-DT | NEW BRITAIN | 44.72 Mi | 200 kW output power | 1499’ antenna height |
The signal direction sheet below displays the
directions and distances of the desired stations. Their Azimuth Angles are close, but not exactly the same.
The first reception tests were conducted while Channel 34, New London a local DTV station, was not in operation. The photograph below, taken from the screen
of the TEKTRONIX Model 2710 spectrum analyzer, shows the received power spectrums.
The low amplitude Channel 33 power spectrum
is in the center of the screen. Channel 35, two divisions to the right, was
received with an almost 30 dB higher level.
At the time of testing Channel 33 operated with temporary facilities. The received Channel 33 picture quality was
better than observed on Channel 3, Hartford, its analog affiliate.
At noon, local Channel 34, New London, a medium power transmitter started operations. The New London station’s direction is 13º off the desired Channel 33, as shown on the signal direction sheet below.
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SIGNAL DIRECTION SHEET FOR DESIRED CHANNEL 33,
|
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In case of Channel 35 the relative angle to
the undesired is even less, only 6.6 degrees, as demonstrated below.
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SIGNAL DIRECTION SHEET FOR DESIRED CHANNEL 35,
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 |
The picture below documents drastically changed
interference conditions for Channel 33, as well as for Channel 35 after Channel
34 came on the air. The adjacent channel
interference on Channel 33 exceeded 48 dB, while on Channel 35 it became a less dominant
20 dB.
Interference
Protection
Is it feasible to carry the two DTV stations
on the system?
Originally, due to the distances involved, and
the proximity of the Channel 33 and Channel 35 directions, the application of
an 8’ diameter parabolic was considered, to receive both stations with the same
antenna. However, the extremely strong
Channel 34 adjacent channel interference made the application of a large parabolic
antenna impractical. The 20º main lobe
of its radiation pattern would not provide any adjacent channel protection against
the extremely strong local stations.
The verified interference dictated the application
of a phased-array of 12 element Channel 33 Yagis, with a horizontal spacing
of H = 44 ½” = 2.25 wavelength, to force a deep radiation pattern null into
he direction of the Channel 34 transmitter.
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TWO
CHANNEL 33 YAGIS, HORIZONTALLY STACKED. |
 |
The computer generated rectangular radiation
pattern above, developed for H = 44
½” spacing, shows the30 dB deep first
nulls at 13º on each side of the main lobe. Warning: the narrow radiation pattern
null requires extremely careful antenna orientation.
On Channel 35, the protection against the interference
with 6.6º angular difference dictated a horizontal spacing of H = 86 7/8”, which
is an unusually wide but still practical horizontal spacing.
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TWO
CHANNEL 35 YAGIS, HORIZONTALLY STACKED. |
 |
The radiation pattern of the larger horizontal
spacing phased-array developed five (5) major lobes! The central lobe is just slightly higher than
those on each side, requiring very careful antenna orientation. Not an ideal situation, not a 30 dB null,
but the best solution under given interference conditions.
Conclusion
Seven years ago the FCC declared that
by 2006 all television broadcast would be in High Definition, thus eliminating
the current VHF and UHF analog stations. As it stands today, the year 2010 looks as
a more realistic objective. Consequently,
analog and digital TV stations, operating side-by-side, will crowd the airwaves
for many more years.
There are instances when even provisionally installed TV receiving antennas produce perfect HDTV pictures at the CATV head-end. However, this “Ready, Fire, Aim” approach also leads to interference problems due to the crowded spectrum. To avoid the occurrences of less than satisfactory HDTV picture quality, it is prudent to follow these more involved guidelines:
For emergency engineering services:
Call: (609) 883-9866
E-mail: steven@biroengineering.com
Web site: www.biroengineering.com
Biro Engineering
P.O.BOX 2175
PRINCETON, N.J. 08543