How to give radio signals a boost
Getting the desired radio signals into and out of buildings is a challenge that concerns every radio-system operator and user. Critical communications can be disrupted by radio-signal attenuation caused by the building structure. It is important to provide communication between the outside world and the building interior, as well as communication between floors or through walls within the same building.
Figure 1 shows a UHF repeater operating at an output (transmit) frequency of 455 MHz and an input (receive) frequency of 460 MHz. The handheld radio transmit/receive frequencies are reversed from the repeater, that is, the handheld unit transmits at 460 MHz and receives at 455 MHz.
Suppose that the repeater signal is attenuated by the building structure, allowing the input signal level at the handheld unit to be -130 dBm. If the handheld unit has a sensitivity of -115 dBm, the handheld unit will not be able to hear the repeater. Furthermore, if the handheld unit can’t hear the repeater, it certainly will not be able to talk back to the repeater, because the power output of the handheld unit is much less than that of the repeater. Therefore, in order to establish communication between the repeater and the handheld unit inside the building, some means of amplifying — or “boosting” — the signal level between the repeater and the handheld unit must be employed.
Figure 2 shows the simplest possible solution. Here, an antenna is installed on top of the building and connected to a second antenna installed inside the building. This forms a simple “passive” repeater. In some cases, to solve a specific communication problem, this is all that is needed. However, most cases of in-building coverage are more complicated.
To take this to the next logical step, an amplifier is added to the mix as shown in Figure 3. This unit receives the signal from the antenna atop the building, amplifies it and passes it on to another antenna located inside the building. This increases the signal level from the repeater to the inside of the building. However, this does not improve the communication from the handheld unit to the repeater, because the amplifier only amplifies in one direction.
In order to provide stronger signals in both directions, a bidirectional amplifier (BDA) is needed. Figure 4 shows a bidirectional amplifier used to boost signals into and out of the building between the handheld unit and the base station (repeater). Figure 5 shows a simplified block diagram of the BDA. The upper section of the BDA passes and amplifies Frequency 1 [F1] at 455 MHz and rejects Frequency 2 [F2] at 460 MHz. The lower section of the BDA passes F2 at 460 MHz and rejects F1 at 455 MHz.
The bandpass filters (BPF) must provide enough isolation at the reject frequency to prevent the amplifier from oscillating. The isolation provided by the input and output bandpass filters should exceed the gain of the amplifier by 10-15 dB. This should ensure stable amplifier operation. Oscillating BDAs can create severe interference and can cause someone to waste considerable time and effort tracking down the source of the problem.
The BDA in Figure 5 doesn’t show it, but many Class B broadband BDAs incorporate an OLC [output level control] circuit, which controls the gain of the amplifier with a widely varying input signal level. Not all BDAs will have an OLC circuit.
The setup in Figure 4 might work fine for a one-story building with a large open space. However, in high-rise buildings, this design would be inadequate to distribute the signal through many floors all the way into the basement. Consequently, a signal-distribution system is required to get the signal in and out of each floor.
One way to distribute the signal throughout the building is by using discrete antennas placed at strategic locations. In very tall buildings, long runs of coaxial cable will be required. Since coaxial cable has a given amount of loss per unit length, usually specified as so many dB/100ft., more than one BDA will be required to get sufficient signal coupling into and out of the many floors of a large building.
Figure 6 shows a possible arrangement for radio coverage in a tall building. This setup establishes communications between handheld units in the building and the outside communication system. The directional antenna installed atop the building is aimed at the “donor” site. The in-building units are linked to this site through the various combinations of leaky coax and indoor antennas throughout the building. The outside directional antenna is connected to the input of the head-end BDA. The head-end BDA is a high gain amplifier with very low distortion characteristics. The gain of the downlink side of the head-end BDA may be less than the gain of the uplink side. The downlink side brings signals into the building from the donor site while the uplink side carries signals from the building to the donor site. The power output of the handheld unit inside the building is much less than the power of the transmitter at the donor site. Thus, the gain of the head-end BDA should be greater in the uplink direction.
Referring to Figure 6, the handheld radio is transmitting. The signal is picked up by the inside antenna indicated on the drawing. Let’s say that the combined attenuation between the antenna and the input to the head-end BDA is 35 dB. This would include all coaxial cable losses, RF tap and two-way splitter losses. Further, assume that the operating frequency is 450 MHz and the handheld radio has an output power of 2W or +33 dBm. The loss of the helical spring antenna is approximately 10 dB compared to a half-wave dipole. If the path loss between the handheld radio antenna and the indoor antenna is 30 dB, the signal level placed on the cable at the antenna will be -7 dBm (33 dBm – 40 dB). The losses between the indoor antenna and the input to the head-end BDA add up to 35 dB, making the signal level at the input to the head-end BDA approximately -42dBm. The output of the head-end BDA is +10 dBm. With the directional antenna installed atop the high building, the in-building handheld radios are linked to the donor repeater. Communication between two inside handheld radios will also go through the external donor system.
Radiating cable is used to cover certain areas of large buildings. It also is used in tunnel applications. A radiating cable has slots in the shield allowing RF to escape or “leak” out, hence the name leaky cable. Conversely, radio waves from a transmitter operating near the cable will enter the cable through these slots. Coupling loss between the cable and the receiver or transmitter is quite high. Still, this loss is tolerable, because the amplifier boosts the signal level enough to get it to the donor site. In some cases, parallel runs of radiating cable are used to minimize the distance between the radio and the cable.
Additional amplifiers called line BDAs are connected in the line in order to keep the signal level up. For example, the design may require that a line BDA be placed at the point where the attenuation between the head-end BDA or the last line BDA has reached 25 dB. The line BDA would have a gain of 25 dB in order to overcome the losses up to that point.
Designing an in-building system requires careful planning, knowledge of the building and knowledge of the products that are available. Testing the system after installation can be as simple as walking through the building with a handheld radio and using the familiar “can you hear me now?” technique. However, a more scientific and quantitative analysis is done with a signal generator connected to the input of the head-end BDA. The generator is modulated at ±3 kHz deviation and the output level is set to the anticipated level from the donor site. Then, with a SINAD meter connected to a handheld radio, the radio is walked throughout the building, with the variance on the SINAD meter noted. In order to test for a margin of reliability, set the signal generator output level to 15 dB or 20dB below the anticipated level from the donor site.
There is much more to in-building radio coverage than can be presented here. Companies that manufacture or sell in-building communication products have application engineers who are eager to help you solve your in-building communication problems.
Until next time — stay tuned!
Harold Kinley’s newest book, The Radioman’s Manual of RF Devices, is available from Noble Publishing. Visit www.noblepub.com to order.
“Repeater Amplifier Systems: Principles and Applications”
Ernesto A. Alcivar, ©1994 by TX/RX Systems, Inc.
“In Building Tricks”
Dennis J. Burt, ©1994 by Intertec Publishing Corporation
“Introduction to Signal Boosters”
Jack Daniel, ©1996 – 2001 by the Jack Daniel Company