Low-level digital combining for multi-station installations

Low-level digital combining for multi-station installations
Shively Labs
®
Low-Level Digital Combining for Multi-Station Installations
Existing Shively Model
Stations
Stations
6014 antennas and
1, 2, and 3
1, 2, and 3
Digital to
Figure 1
Model 2540 balanced
Analog to
Antenna
Modifications
at the Combiners
Antenna
combiners can be modiDirectional
Directional
fied to handle IBOC/HD
Coupler
Coupler
Radio, with several small
(0ptional)
(0ptional)
changes. Both the combiner and antenna can
be modified after the
equipment is installed.
The IBOC conversion
consists of:
• A “Digital Feed “ for
the antenna system.
• 1 or 2 runs of 1-5/8"
Model 2540
Combiner
Combiner
Combiner
heliax transmission
Balanced
Station 1
Station 2
Station 3
line for the digital
Combiner
feed.
• A 1-5/8" hybrid power
splitter for the digital
transmission lines, if
(replaces
(replaces
(replaces
dual feeds are dedummy
dummy
dummy
sired.
load)
load)
load)
• A 1-5/8" output
directional coupler
Isolator
Isolator
Isolator
for digital broadband
line.
Analog Input
Analog Input
Analog Input
• One isolator per
Station 1
Station 2
Station 2
station at the digital
input to the combiner.
Digital Input
Digital Input
Digital Input
Station 1
Station 1
Station 1
In experiments carried
out at the Entercom
Cougar Mountain facility in Seattle during the
summer of 2002, we showed that the balanced combiner system can isolate digital and analog feeds simultaneously. This is done by feeding the digital signal into the system through the input hybrid port normally
occupied by a small dummy load. In addition, each station requires a small isolator. Some stations may also
require an additional IBOC bandpass filter, but this will depend on the performance specifications of the
transmitter in use. The digital signal passes through the combiner system along the broadband line, 180° opposite the path of the analog signal, and exits through the broadband input of the module farthest from the
analog output. The system load normally occupying this port is removed. This arrangement is shown in Figure
1 above.
This process works well for any number or combination of stations. Further, running some stations with digital
signals and some without does not unbalance the system or cause interference.
Document No. tb-llcat (150320)
A Division of Howell Laboratories, Inc., P. O. Box 389, Bridgton, Maine 04009 USA
(207) 647-3327
1-888-SHIVELY
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Certified to ISO-9001
A 1-5/8" output direction(replaces
al coupler may be added
dummy load
Hybrids
at each hybrid)
to the digital broadband
line as shown, for use in
diagnostics and tests.
At the antenna, the
digital feed is identical to the analog feed,
but smaller because
Stations 1, 2, and 3
of the much reduced
Digital and
power level. The digital
Analog Output
signal is fed into the
bays through the hybrid
ports normally occupied
by small dummy loads,
as shown in Figure 2 at
right.
The average digital
power is 1% of the
average analog power.
For example, a 4-station
system running 10 kW
average analog TPO’s
(40 kW total) would
require approximately
400 watts of combined
digital power.
A single run of 7/8"
transmission line can
easily accommodate the
combined digital feeds
Existing Analog
Added
of almost any multi-staPower Divider
Low-Power
tion installation, but
and Distribution System
Digital
1-5/8" transmission line
Power
will probably be used at
Divider
and
most sites to minimize
Figure 2
Distribution
losses.
Modifications at the Antenna
System
For antennas designed
Stations
with dual feeds to allow
Stations
1, 2, and 3
the antenna to operate
1, 2, and 3
Analog to
Digital
to
independently on upAntenna
Antenna
per and lower halves, a
dual digital feed must
be used or the digital
system will not work if the antenna is operated in a half-antenna mode.
In a dual feed system, the digital broadband line is split into two feeds, using a hybrid power splitter just
like the one in the analog feed, but smaller.
The power dividers in the digital feed add a small amount of windload to the antenna system, but in general
the feed will occupy little space in the tower, because of the small size and bending radii of the feedlines.
Finally, the digital feed can easily be physically isolated from the analog feed to prevent loss of pressurization in one feed system if the other is damaged.
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