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Thinking “inside the box” for a transmission-line alternative

It’s standard in many RF designs to place a low-noise amplifier (LNA) as close to the antenna as possible to boost received signal strength before sending the weak received signal to the rest of the front-end circuitry. It’s done to minimize degradation of signal-to-noise ratio (SNR). The reason is simple: to overcome signal attenuation in the coaxial cable between the antenna and the front-end and minimize the impact of thermal and other noise added by the transmission path from LNA to remotely located front-end.

A typical LNA at the antenna will provide 10 to 20 dB of gain with a noise figure under 1 dB; for extreme applications such as deep-space receivers, the LNA and signal path may be supercooled to reduce self-generated thermal noise to near zero. Placing the LNA at the antenna feed is used by large dish antennas as well as consumer products such as the fixed-orientation home-TV receiver dish—the DISH Network in the United States—or even Internet access via very small aperture terminal (VSAT) antennas, which have a diameter of under 3 meters and are mounted on rooftops or sides of houses (Figure 1). The LNA is powered by DC sent up the same coaxial cable which carries the amplified RF signal down from amplifier; the entire LNA assembly is often called a low-noise block (LNB).

Figure 1 The use of a head-end LNA with power delivered via the antenna coaxial cable is standard practice in consumer satellite dishes. Source: SatGist/Pearce Communication

So far, this all seems straightforward enough, but it’s a very different story when you have a steerable dish which is tens of meters wide and captures incredibly low-power signals from deep-space probes. In those cases, even a fractional dB of signal loss or added noise are costly, while the LNA subject to temperature extremes adds other stresses. In addition, accessing the front-end circuity is difficult and awkward.

To overcome the issue, a relatively new approach has been devised. It’s been used on several large antennas at the NASA Goldstone antenna site facility. It’s also in use on the 34-meter DSS-53 antenna at the Deep Space Network’s Madrid complex, which was formally activated in February 2022 (Figure 2). It is designed for Ka-band receive (31.8–32.2 GHz) and transmit (34.2–34.7 GHz) operation.

Figure 2 The 34-meter DSS-53 antenna of the Deep Space Network at the Madrid site was activated in February 2022 for Ka-band downlink and uplink operation. Source: SciTechDaily

The retrofittable beam waveguide (BWG) antenna uses a special “feed” arrangement with five precision RF mirrors to reflect radio signals along a tube from the antenna to a below-ground room (Figure 3). These are much more than just waveguides, as they actually bounce the energy as light would reflect from a mirror, rather than just confine and direct it. This design allows sensitive electronics to be in a climate-controlled equipment room instead of outdoors. The arrangement is not a “straight line” mirrored part, but instead uses the RF mirrors to make several right-angle turns.

Figure 3 A series of mirrors reflects the RF signal from the feedhorn down to the amplifier in a convenient, sheltered setting; the mirror path works in the uplink direction as well. Source: ResearchGate

This is actually a “bidirectional” story in addition to the LNA aspects. The same antenna is used with an 80-kW transmitter, and the transmit and receive function are obviously aligned. Thus, significant simplifications are possible in the design of high-power water-cooled transmitters and low-noise cryogenic amplifiers since these systems do not have to tilt as in normally fed dual-reflector antennas. The configuration also simplifies maintenance and modification of the equipment as new technologies are developed.

This type of design is not implemented without extensive simulation and test, of course. Before committing, a BWG test structure was installed in a microwave anechoic chamber 6 meters wide, 6 meters high and 18 meters long to verify the proof of concept (Figure 4).

Figure 4 This one-quarter scale-model test arrangement was used to validate the BWG concept and check the simulations. Source: Jet Propulsion Laboratory

For the test, one-quarter-scale paraboloid mirrors—compared to those used in the full-scale 34-meter antenna—were machined from solid aluminum blocks and used in one-, two-, and three-mirror test configurations (Figure 5).

Figure 5 The BWG mirrors are machined from blocks of aluminum and have the necessary paraboloid curved surface for tightly-focused reflection. Source: Jet Propulsion Laboratory

Since the mirror-tube approach offers many potential benefits compared to other approaches, its designers actually prepared two independent designs in the R&D antenna. First, a “bypass” design places the BWG outside the existing elevation, so it can be retrofitted to, or used in parallel with the existing systems. Second, a “center-only” design for new systems places the BWG through the center of the dish inside the elevation bearing. So, it doesn’t allow the older-style feed arrangement.

If you assume that these large dish antennas represent mature technologies and the only areas of improvement are in the precision-positioning systems and RF electronics, this BWG technique shows that there is still room for significant innovation. You can read all the technical details in “Chapter 7: The 34-Meter Research and Development Beam-Waveguide Antenna” and “Chapter 8: The 34-Meter Beam-Waveguide Operational Antennas” in the Jet Propulsion Laboratory (JPL) Deep Space Communications and Navigation Series (DESCANSO) book available for free. The entire book series is a fascinating read on leading-edge design to meet severely challenging scenarios.

What do you think of this arrangement? Can you see a smaller-scale version being adapted to 5G and other links where exposed electronics—even in non-mechanically steerable antennas—is a problem due to need for heating, cooling, and access?

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