Subject: [NEXTEL-1] 900MHz Trunking System Operators Pay a Premium for 'Vertical' Real Estate

From: Marcel <[email-masked]>
Date: Thu, 02 Jan 2003 21:04:22 -0500

900MHz Trunking System Operators Pay a Premium for 'Vertical' Real Estate

Using the Minimum Number of Tower and Rooftop
Antennas Pays Economic and Technical Dividends
By Decibel Products

As more and more 900MHz trunked radio systems are built, space for antennas
and coaxial transmission lines at tower and rooftop antenna sites may be
exhausted. The reason is because 900MHz trunked systems have the potential
to use more antennas than 800MHz trunked systems.

Transmitter Combining
Frequency spacing forces 900MHz systems to use more antennas. With
12.5kHz spacing between channels, cavity filters cannot be used to combine
multiple base station transmitters onto a single antenna. Thus, each transmitter
may require its own antenna so the system delivers maximum effective radiated
power (ERP). Alternatively, ferrite hybrid combiners are used to combine two or
more 900MHz transmitters to a single antenna.

Cavity filters work with 800MHz systems because channels in an individual
system are more widely separated. Cavity filter selectivity, or "Q", does not allow
their use with channel separations less than 250kHz. Cavity filters have low
insertion loss at their operating frequencies, so several 800MHz transmitters may
be combined onto one antenna without decreasing ERP much.

Increasing transmitter output power boosts ERP to compensate for the insertion
loss of any component that follows the transmitter. But components that follow
the transmitter must be robust enough to handle the higher power. And the power
race is on.

Previously, commercial base station transmitters were available in 35W, 60W,
75W, 100W and 125W models. Today, transmitters and after-market amplifiers
deliver 100W, 125W, 150W and 200W outputs. A 350W output transmitter is
expected.

High-power transmitters boost ERP, but improved transmitter combiners, coaxial
transmission lines, connectors and antenna systems are required to handle heat
created as a by-product.

Heat poses a greater challenge to 900MHz systems than 800MHz systems. The
900MHz systems' ferrite hybrid combiners convert 50% of the power fed through
them into heat. The 800MHz systems' cavity filters convert less of the power fed
through them into heat. The less power converted to heat, the more power
delivered to the antenna, and the greater the ERP for a given amount of
transmitter power.

A cavity filter designed to filter transmitter noise before it reaches receivers
usually is included in a 900MHz ferrite hybrid combiner design.

Receiver Multicoupling
Fortunately, with both 800MHz and 900MHz systems, each channel's receiver
may be coupled to a minimum number of antennas. Hybrid splitters and a
low-level, high-gain amplifier packaged into a module called a receiver
multicoupler, allow a minimum number of antennas to serve a maximum number
of receivers.

The receiver multicoupler amplifies low-level signals from the antenna and feeds
them to the receivers. Bandpass cavity filters, called window filters, attenuate
unwanted signals ahead of the multicoupler.

One more benefit: The same receiver multicoupler may be used for 800MHz and
900MHz systems at the same site if cavity preselector filters are selected
properly.

Tower Top Amplifiers
A tower top receiver amplifier may be used to offset coaxial line loss and filter
insertion loss. The tower top amplifier includes an amplifier and window filter,
coaxial relays and lightning protection.

Gain delivered by a tower top amplifier may improve talkback signals from
mobiles and portables greatly. But the amplifier increases the system noise floor.
Thus, using a tower top amplifier involves limitations and trade-offs. Figure 1
shows a common transmitter and receiver antenna system that includes an
optional tower top amplifier with a window filter.

How Many Antennas?
Choosing how many antennas and thus how many ferrite hybrid combiners to use
depends on an evaluation of cost and system performance.

Where tower and rooftop space is limited, lease costs may escalate each time
the lease is renewed or each time equipment is added. Controlling the amount of
space required and therefore the lease cost may be the overriding factor. That
would indicate a minimum number of antennas or multiple antennas in radomes.
For a minimum number of antennas, high-capacity ferrite hybrid combiners are
required, reducing ERP.

At sites with low lease costs arid plentiful tower or rooftop space, a maximum
number of antennas can be used to reduce the need for higher transmitter power
to overcome ferrite hybrid combiner losses. Such a configuration may deliver the
best coverage.

Figure 2 shows systems that use ferrite hybrid combiners. The systems in Figure
1 and Figure 2 could be modified to use a duplexer to combine the receive
subsystem to one of the transmit antennas. Figure 3 block diagram represents
such a modification.

Comparing ERP figures helps you to decide which configuration fits your
requirement.

Site Configurations
Here are some typical 10-channel system configurations for sites with:
Little space or high lease cost:

Two five-channel ferrite hybrid combiners.
Two transmit cavity filters.
Two transmit antennas.
One receive antenna.
One active receiver multicoupler subsystem.
Three coaxial transmission lines.

Slightly more space or lower lease cost:

Two four-channel ferrite hybrid combiners.
One two-channel ferrite hybrid combiner.
Three transmit cavity filters.
Three transmit antennas.
One receive antenna.
One receiver multicoupler subsystem.
Four coaxial transmission lines.
The output of this system is unbalanced, meaning ERP may differ among
the channels. (See Figure 4.) Reducing some of the transmitters' power
levels should overcome this problem. \

Moderate space and/or low lease cost:

Five two-channel ferrite hybrid combiners.
Five transmit cavity filters.
Five transmit antennas.
One receive antenna.
One active receiver multicoupling subsystem.
Six coaxial transmission lines.

Plenty of space and extremely low lease cost:

Ten transmit antennas.
Ten transmit cavity filters.
One receive antenna.
One active receiver multicoupling subsystem.
Eleven coaxial transmission lines. (See Figure 5)

Sometimes, the FCC allocates more than 10 channels at a site. For example,
one operator may have a 10-channel system and another operator may have a
10-channel system, both at the same site. Channels on one system may be no
closer than 250kHz to channels on the other. Figure 6 shows how, low-loss
cavities may be used in expanding the number of channels.

A 10-channel low-loss, combiner may be used as part of a multiple 900MHz
combiner if channels are separated by a minimum of 250KHz. In such a
configuration a minimum number of antennas are required, and losses per
channel are lower than they would be with ferrite hybrid combiners.

High Power Considerations
As antenna sites fill, options for using multiple sites to cover a wide area become
more limited. Instead, extending coverage by raising ERP may be the only
practical option.

At 900MHz, trunked radio, paging and link systems may transmit an ERP of
1,000W per channel. To reach that level, a combination of equipment must be
used, including high-power transmitters and coaxial cable, cavity filters and
combiners with the lowest possible losses. But high power causes problems for
the combiner because the filters generate heat.

High-Q cavities tuned to a resonant frequency have low insertion loss and
generate little heat. They operate at low ambient temperatures as long as the
room temperature is reasonable.

On the other hand, hybrid combiners have little area for heat dissipation but
generate a lot of heat in their 50-ohm load resistor. The resistor takes 50% of the
transmitter power fed into the combiner. If a transmitter broadcasts for a long
interval at high power, the load resistor temperature may rise well above 100
degrees C, its maximum operating temperature.

To transfer the heat, a large heat sink with large surface fins mounted vertically is
attached to the resistor. Furthermore, a fan and air-cooling ducts may be required
to force air between the fins and to carry heat away. If the fan fails, the load may
overheat and cause damage to the isolator, and possibly, the transmitter.

Opportunities
Many antenna sites already have in place 800MHz conventional and trunked radio
systems, 900MHz paging systems and 900MHz point-to-point link systems.
Adding 900MHz trunked radio equipment to such a site that has limited space
may be accomplished by adding ferrite hybrid combiners.

The five-channel ferrite hybrid combiner occupies only 18 inches of vertical rack
space. The same rack space contains transmitter channel isolators and lowpass,
second order harmonic filters. A companion panel, 18 inches high, contains the
50-ohm load resistors.

Some of the paging and link transmitters can be combined to antennas shared
with the 900 MHz trunked system by feeding them through a properly tuned
hybrid coupler. The 900 MHz trunked system channels may be combined to
antennas shared with existing 800 MHz conventional and trunked channels as
long as proper cavity filters and phasing cables are added. These possibilities
require the use of broadband antennas.

Sharing antennas with existing 800MHz and 900MHz paging, link, trunked, and
conventional systems uses site resources as efficiently as possible.

If the 900MHz trunked system is to be expanded slowly, such as during a
three-year period, unused transmitter combiner and receiver multicoupler ports
may be used by the other services. As the 900 MHz trunked system expands,
paging and link channels would be fitted with their own transmit antennas and
ferrite hybrid combiners or cavity filters.

With this approach, the paging and link frequencies will experience a higher
insertion loss than they would if they were fitted with their own cavity filters or
simple antenna systems. Losses associated with combing them to the 900MHz
antennas are on the order of 8dB.

The receiver subsystem may be shared by all services when proper filtering is
provided. The receive antenna must be broadband to allow all frequencies to pass
with maximum possible gain. The receiver filtering can be designed to require no
new antennas or filtering when the 900MHz trunked system exp

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