booster for rf radio signal
jonas ribeiro
The components and operation of a one-way Signal Booster shares many aspects that are common to a two-way Signal Booster, therefore a good understanding of the one-way Signal Booster is important to understand two-way Signal Boosters.
TX RX Systems "OLC" type designs are used in this presentation as they are the most common in use today.
A one way OLC type Signal Booster is organized as follows;
The operating frequency band and bandwidth of the Signal Booster is
determined by the characteristics of the bandpass filters (cavities)
used. These characteristics are engineered to meet the specific
applications requirement. Bandwidth values generally range from 250 KHz
to 20 MHz and typically 2 to 10 MHz.
The amplifiers will amplify all signals within the bandpass, therefore it is important to reduce the bandwidth to the minimum practical bandwidth for optimum operation. Note that 'practical' also relates to size and cost.
In all TX RX Signal Boosters the amplifiers are special ultra-linear amplifiers with a preset gain value. Common OLC Signal Booster models are available with gains ranging up to about 80 dB. The maximum output power of the output amplifier is set below the amplifiers 1 dB compression point ( +35 dBm) to prevent damage to the amplifier and minimize undesirable RF output signal components. This results in a typical operational output of about + 25 dBm for one or two channels input, depending on the specific model used.
The output power level is monitored by the OLC circuit and a gain adjusting control voltage is fed back to an electronic attenuator at the preamplifier input. The dynamic (output level control) range of the OLC circuit is typically 30 to 40 dB depending on the specific model used.
Since the amplifier gain and OLC control dynamic range are fixed values, coaxial pads should be inserted before the electronic attenuator when the input signal levels are high enough to constantly cause attenuation at the electronic attenuator. The addition of coaxial pads, if required, restores the full dynamic operating range of the Signal Booster and makes the output signal levels more consistent and coverage more predictable. Fixed value coaxial attenuator pads are preferred over variable attenuators due to the infrequent need to alter the settings, the higher cost of variable attenuators and the lesser possibility of noise being generated within the attenuator itself.
The use of coaxial pads instead of amplifier gain adjustments allows the amplifiers to operate within the best possible operational range, which is often compromised in circuits using current or voltage adjustments to control the gain. The amount of relative attenuation is easily observed by measuring the DC voltage applied to the electronic attenuator control input. The factory provides a verified chart of control voltages with each Signal Booster shipped.
Decoupled RF test points are also provided in standard versions of TX RX Systems Signal Boosters to permit performance testing or alignment while the Signal Booster is in operation. Non-disruptive testing is a common feature in most TX RX Systems products.
As no input-to-output frequency conversion occurs in a Signal Booster, the RF output must not be close coupled to the RF input to prevent feedback saturation of the Signal Booster. The isolation within the Signal Booster amplifier itself is well over any possible gain of the Signal Booster and does not present a concern so long as the factory connections and shielding is maintained. The amount of external output to input isolation requirement is usually not a problem since the input and output are normally located on different sides of the signal blocking obstacle (wall, ground, etc.).
If insufficient isolation exists, antenna placements, input pads and/or amplifier stage gain reductions must be implemented.
When a radiating cable, or "leaky coax", is used at the input and output ports of a Signal Booster there is little potential of feedback due to the high coupling losses of such cables.
A good rule of thumb is a minimum output-to-input path loss equal to the maximum operating gain of the Signal Booster plus 10 dB. For example, a Signal booster with 55 dB overall maximum gain should have minimum of -65 dB output-to-input isolation.
OUTPUT SIGNAL LEVEL:
The non-channelized Signal Booster will amplify all signals within the input filters bandpass therefore the effective output power per single channel will change as more channels are amplified.
In other words, the maximum output power level is shared by each input channel. The exact proportion of 'sharing' is determined by the number of channels AND the power level of each channel in relation to all the other input channels. A chart illustrating the effect of multiple channel amplification is included in the Addendum.
When using a non-channelized Signal Booster where many undesirable channels may be present at the input also, the effect of those channels may be reduced by several means;
- Use a directional antenna to direct the signals to the desired repeater site. In some cases, a corner reflector may be used to increase rejection of unwanted signals from the sides of the path. This increases the desired signal level and greatly reduces the power loss effects on the desired channel at the output of the Signal Booster caused by undesired signals.
- Use more selective bandpass filters in the signal path coming from the antenna. Some filters may have to be placed outside of the Signal Booster or in a larger cabinet because of their physical size.
- If the above steps are inadequate, then channelized, Class A, signal boosters should be considered.
Note that, in many cases, any additional filters only have to protect the INPUT from the 'outside world' because only 'desirable' signals normally exist inside the area to be covered.
INPUT SIGNAL LEVEL:
The input signal level is perhaps the greatest item of confusion between
conventional repeater stations and Signal Boosters.
A conventional repeater is designed to provide radio coverage for great
distances and requires only a few microvolts of signal input.
A Signal Booster is designed to "fill-in" an area that would normally
have a very good signal level and therefore operates best at higher
input signal levels.
A nominal input signal level of -60 dBm can be used as a guideline input signal level for most standard Signal Booster configurations, although Signal Boosters will operate at much lower levels with the acceptance of reduced output signal levels.
As examples:
A -60 dBm input level applied to a Signal Booster with 75 dB gain would have a maximum output level of +15 dBm.
A -85 dBm input level to the same Signal Booster would provide a -10 dBm output signal level.
Sensitivity and quietening level specifications are not applicable to Class B Signal Boosters due to the relatively high level of the input signals and the lack of any frequency conversion or demodulation circuits.
NOISE:
The noise figures of the amplifiers are normally of no concern except
when many high gain Signal Boosters are placed in series. This is
minimized by reducing the gains of the Signal Boosters and losses
between the Signal Boosters. The first amplifier is the dominant
contributor to the noise figure any design using amplifiers in series.
The factory will provide assistance in determining specific noise
figures for complex systems using several Signal Boosters in series, as
the resultant noise figure will be much less than the simple addition of
the individual Signal Boosters noise figures.
Note that the "system noise figure" specified by TX RX Systems is the worse possible overall value, with the maximum gain of all stages applied.
POWER SOURCES:
Signal Boosters are normally configured to use a 120 or 240 AC power source.
Optionally, they may be operated from a DC source, which requires a DC voltage converter if the DC source is less than 24 volts.
Non-disruptive automatic transfer battery backup options are available.
If power is not available at the Signal Boosters physical location, the Signal Booster may be powered by inserting a DC supply voltage on the coaxial cable at a point where the cable is near a power source. This requires the addition of optional coupling devices that are available from TX RX Systems and an external DC power supply.
ACCESSORIES:
Additional supporting devices, such as power splitters, antenna
decouplers, indoor antennas, terminating loads for radiating cables,
etc. are also available from TX RX Systems to complete almost any
possible application of Signal Boosters.
TWO WAY (Bi-Directional, Boadband) CLASS B SIGNAL BOOSTERS:
Two-way Signal Boosters are basically two, one-way signal booster sections in the same cabinet with one section operating in one frequency band and direction and the other section operating on another frequency band in the opposite direction.
All the previous one-way Signal Booster criteria applies to two-way Signal Boosters.
Two way Signal Boosters use the bandpass filters in a manner similar to a duplexer to maintain separation and isolation between the two frequency bands while using a single coaxial port for the RF signals on each side of the amplifier sections.
In frequency bands below 450 MHz, the variable and small amount of separation between transmit and receive frequencies frequently requires larger filters and cabinets. These are engineered, quoted and specified on a case by case basis by the factory.
Naturally, Signal Boosters cannot amplify a simplex frequency signal in both directions.
TX RX Systems has FCC type accepted Signal Booster models in the VHF , UHF and
800 - 960 MHz bands.
Antennas, Radiating Cables and other Radiators:
The 'external' antenna used for the path between the Signal Booster and the distant base/repeater station should be a directional, gain antenna which will improve the signal path and reduce undesired signal that may occur in other directions. Parabolic and corner reflector type antennas often have better rejection of unwanted signals that appear to the side of the antenna.
Non-radiating cables should be low loss, well shielded coaxial cables fpr best results.
The amount of radiation and signal levels inside the area served by Signal Boosters can be controlled by selection of radiating coaxial cables (i.e. "leaky coax") or antennas as radiator elements. In some cases both may be used.
The choices of radiators is based on several factors;
- The topography of the coverage area. (Long and narrow areas versus large open areas).
- Costs of radiators and installation.
- Maintaining input levels to the Signal Boosters within the OLC control range.
Radiating cables provide coverage that can be easily controlled and is especially applicable to tunnels, stairwells, passageways, etc. The disadvantages of using radiating cables is the cost of the cable, its additional installation concerns and the high amount of coupling losses. The primary advantages of using radiating cables inside passageways and tunnels is due to the fact that the distance between the cable and portable radios does not vary greatly and the high coupling losses are usually acceptable because the portable is always near the cable.
Antennas provide the minimum coupling loss and often lower installed cost. Generally, unity gain omni-directional antennas are preferable.
More than one antenna may be coupled into a non-radiating cable to distribute the signal over a large area. Multiple antennas should be placed such, or decoupled, so that the signal levels coming into the Signal Booster are as uniform as practical.
Decouplers, 'taps' and 'splitters' are used to connect antennas and cables to the main feed lines. These devices are avaiable in many types and values, including multi-port and multi-band models with coupling ranges from 3 to 40 dB per port. These, along with simple coaxial RF pads, allow the system designer to distribute the signals equally throughout an area.
Internal gain antennas can be a disadvantage when the radio users roam from side to the center of the antennas beam, causing great signal level variations.
Isolation between the Signal Boosters output-to-input ports may become more difficult to achieve when gain antennas are placed near outside windows, doorways, etc.
An example of a system using both types of radiators: Radiating cable is used inside a stairwell that leads to a large open underground parking garage. The end of the radiating cable is in the garage and connected to a antenna placed high on one side of the garage. The high antenna placement prevents a radio from being close to the antenna to minimize signal level variations to the Signal Booster.
Multipath effects:
Some system engineers initially express concern over the possibility of
conflicting signals coming from both the direct path and the Signal
Booster output. In practice however, the multipath effects when using a
Signal Booster has been found to be less than that normally experienced
in any mobile radio system.
The output of the Signal Booster is usually much greater than that found
in the direct path, allowing inherent FM capture effects to minimize
any interaction at the receivers when the radio is operated inside the
coverage area provided by the Signal Booster.
There is always the possibility that the differential between the two
signals will not be adequate for FM capture at some location at the
outer edge of the Signal Boosters coverage. The most likely place for
this to occur is when the radio user is near a doorway or an outside
facing window.
Other factors also aid in reducing multipath resulting from Signal Boosters because of the nature of the users and the application itself; (1) Signal Boosters are usually used to improve signals in areas where portable radios (as opposed to mobiles) are primarily used. (2) Portable radio users instinctively make minor adjustments in their radio positions to get the clearest reception. This is also the location of lowest multipath.
Signal Booster multipath is similar to that inherent in any mobile/portable radio system used in an urban area and often with a much smaller area of potential equal level (non-capturing) multipath signals.