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A 50 W, Solid-state C-band Amplifier for Outdoor Use

A cost-effective, 50 W booster amplifier that is lightweight and requires little maintenance is featured on this month's cover

April 1, 1998
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A 50 W, Solid-state C-band Amplifier for Outdoor Use

Wavesat Inc.
Montreal, Quebec, Canada

Satellite communications systems are expanding at an unprecedented rate and service providers require increasing numbers of satellite uplink stations to satisfy their customers’ needs. At the same time, cost control becomes a critical concern when a decentralized transmit/receive system is deployed. The system integrator is constantly faced with the seemingly contradictory demands of obtaining high reliability subsystems while minimizing capital outlay.

Both initial and operating costs contribute to the cost of ownership. Solid-state booster amplifiers offer a good price/performance ratio and are easy to incorporate as output stages for transmitters that require a low power amplifier. This cost-effective 50 W booster amplifier is lightweight and small enough to mount near the focus of an antenna and requires little or no maintenance.

The goal in designing this amplifier was to produce a unit as close as possible to the plug-and-play ideal where an amplifier block can be installed without affecting the system. This capability applies to both the electrical and mechanical designs. The housing is capable of supporting its own weight if flange mounting is provided at both ends and requires only a small bracket if a coaxial-style connector is desired at the input. The unit is intended for outdoor use and, therefore, is waterproof with a protected fan.

Description

The booster amplifier and its power supply are provided in a housing measuring approximately seven inches on a side, as shown in Figure 1 . The fan assembly is a smaller, oblong attachment off the transmission axis. The total cross-sectional area (shadowing the antenna) is less than 100 square inches, which allows the unit to be mounted directly behind the feed horn. The standard input for the unit is via a type WR-137 waveguide (flange type CPR-137) although several additional input options are available, including N and SMA connectors. These options may include custom mounting brackets if an amplifier equipped with coaxial connectors is required to be installed in a position other than horizontal. The output connector is a flange mounted on a type WR-137 waveguide. The onboard power supply presupposes a 48 V DC source supplied by the user. However, this external power source can be replaced by any DC source with some minor internal modifications. Indeed, if 12 V DC is available from the user, considerable savings in size and weight are possible. An AC supply can be accommodated with some increase in volume. Figure 2 shows the amplifier’s block diagram.

Electrically, the amplifier’s performance is defined between 5.85 and 6.425 GHz with an output power at the 1 dB compression point of 47 dBm. Its gain, shown in Figure 3 , is 15 dB nominally. However, this value can be adjusted by the supplier from 12 to 16 dB with a gain flatness of ±0.5 dB. Intermodulation products are relatively low in power, satisfying all but the most rigorous linearity specifications. Since the amplifier is an outdoor unit, it is not affected by rain or snow and features an operating temperature range of –40° to +70°C. A higher power 75 W version of this amplifier is available on relatively short notice. An internal monitoring option provides temperature and output power data, allowing the user to obtain real-time information on amplifier performance.

The RF section has been screened carefully from pernicious magnetic coupling and includes the usual filtering arrangements to suppress electromagnetic interference. With several circuits in close proximity, the possibilities of noise passing from one subsystem to another are increased greatly. Intelligent use of various attenuation components has all but eliminated signal leakage as a source of concern.

Mechanical Design Aspects

The unique heat-sinking arrangement was designed to deliver maximum longevity for the unit by ensuring that all internal components are kept well below dangerous operating temperatures. Regardless of wind direction, sufficient air flow prevents drizzle and light rain from entering the fan bay. In heavy rain, the incoming droplets are broken up to such a degree that they do not represent a hazard to the fan. The fan is a reliable ball-bearing type. For ease of maintenance, it is replaceable in the field in a matter of minutes.

The entire unit weighs approximately 7 kg (15.5 lb). Excess material has been eliminated from the housings and the design is optimized for aluminum alloy manufacture. For larger orders, a magnesium casting process can be used for lower weight and an even simpler mounting scenario. Alternative connector choices (in lieu of the standard MIL-C-5015 style, which, while reliable, is rather heavy) are available to reduce weight further. Ultimately, the amplifier’s design is versatile enough to provide a product that meets the user’s system requirements.

In addition, the unit can be designed with the inside accessible and mounting bosses provided if users desire to incorporate their own telemetry modules. Although the unit is not currently adapted for field change of such a module, it can be modified with little effort. A similar product line for Ku-band (14 to 14.5 GHz) applications is under development for use by very small aperture terminal station developers. Table 1 lists the main characteristics of the C-band amplifiers.

Table I
Key Specifications

 

Option I

Option II

Frequency Range (GHz)

5.850 to 6.425

5.850 to 6.425

Output Power (P1dB) (min) (dBm)

47.00

48.75

Linear Gain (dB)

15 Adjustable

14 Adjustable

Intermodulation Products (Two Tones at 5 MHz Spacing)

30 dB below tones, each tone 38 dBm

30 dB below tones, each tone 39 dBm

Input SWR (max)

1.4

1.4

Output SWR (max)

2.0

2.0

Power Monitoring at Output

Forward/Reverse

Forward/Reverse

Temperature Monitor

Analog 0 to 5V

Analog 0 to 5V

Nominal Power Supply

48 V, 275W

48V, 350W

Conclusion

This plug-and-play, 50 W, C-band, mast-mounted booster amplifier incorporates power amplification directly at the antenna focus, eliminating many problems associated with RF cable losses, crosstalk and receiver front-end overload. Its compact size and light weight allow for easy installation and its low initial cost and solid-state reliability minimize overall operating costs and system downtime.

Wavesat Inc.,
Montreal, Quebec, Canada
(514) 739-9918

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