ENG central receive antennas

Regardless of what they are used for, antennas have a tendency to remain in place while the equipment at the other end of the transmission line changes several times over the life of the antenna. This has certainly been true in broadcast ENG, where the 2GHz relocation and the change to digital have suddenly put the spotlight on central receive antennas and their control systems, neither of which have changed substantially in the last decade.

While they continue to provide service in today's analog environment, the digital ENG environment of tomorrow is a new paradigm, with new rules. A surprising number of ENG receive sites have been in service for decades with the same antenna and LNA and have performed satisfactorily with occasional maintenance. Depending on their age and configuration, some of these existing antennas will require replacement, while others can, and will, be upgraded. With the first installation of new equipment about to begin, there is a fair amount of excitement about the new solutions that ENG antenna and controller upgrades and replacements will bring to broadcasters.

LNAs have improved

Beginning in 1999, when the first COFDM systems were being placed in service, a new set of operational problems began to unfold. When the AGC was peaked, the signal occasionally disappeared and a strong AGC was not always a predictor of good signal quality. These problems were traced to a need for new digital signal metrics and better LNAs.

A strong digital signal can cause a cascading set of problems, beginning with LNA overload. An analog signal may survive some amount of compression, but a digital signal cannot. In spite of robust error correction in COFDM systems, nonlinear amplifiers can cause bit errors beyond the demodulator's ability to correct them, which will cause decoding failure (e.g., fade to black). To an experienced analog ENG operator, it is completely counter-intuitive to think that a lower signal level will produce better results, which is exactly the case in this instance.

To combat this, newer LNAs have much higher third order intercept points (IP3) that raise the compression point to extremely high input levels. RadioWaves, for example, offers a 2GHz LNA with an IP3 of +37dBm.

With LNA linearity greatly improved, LNA gain control is the next issue — and another tool in avoiding overload. An ENG central receiver normally includes its own AGC system; however its effect is limited to the dynamic range of the receiver. Even though the LNA performance may be improved, too strong an incoming signal may still overload the receiver front end, unless some method of gain reduction can be easily accessed and adjusted.

Some real-world examples include NSI, which now offers its LNA with a continuously variable attenuator, and RadioWaves, which has an LNA that activates or deactivates amplifier stages in 5dB increments. Both MRC and Nucomm central receivers provide an AGC output voltage that may be used by the antenna controller to automate the gain adjustment process.

New filters and switching make the difference

The Nextel relocation philosophy is to have everyone in a given market upgrade or replace their equipment to meet the new 12MHz channel requirements and begin operation on the old band plan: “narrowed in place.” During this phase, stations have an opportunity to check out their new gear on familiar turf with regard to frequencies. At some predetermined time, all stations in that market will change to the new band plan. The success of this phase is dependant on being able to switch in the correct filter combinations at the right time and keep them that way.

Most ENG antennas have a band filter or channel filter in front of the LNA to avoid out of band or adjacent channel interference. These will have to be replaced to meet the new plan; however, the existing band and channel filter capability must remain available until the changeover takes place. In addition, new PCS and AWS band stop filters must be added to further attenuate strong signals in adjacent bands that may be present at ENG sites as Nextel and others move into their new spectrum.

If the antenna is not being completely replaced, antenna vendors agree that it is more cost-effective to replace the entire feed, including filters, LNA and switches, rather than replace individual components while strapped to a tower leg. This assures that a complete, factory-tested subassembly will be put into service from the beginning.

With the improvement in LNA and receiver dynamics, channel filters may or may not continue to be used as much in the future. Given the complexity of switching multiple sets of channel filters, and given the improved signal handling capability of LNAs, it may be worth considering standard bandpass filters at the antenna, with switched accessory channel filters in the same rack as the receiver.

Solving many problems

New and updated ENG antennas, central receivers and their associated digital demodulation equipment require considerably more control sophistication than previous generations. In addition to supporting traditional functions, like panning, azimuth presets, receiver frequency and LNA functions, the new breed of controller must also support additional filter and LNA switching, digital diagnostics, decoder control and digital signal acquisition tools.

Several companies have given considerable thought to the myriad of combinations and permutations that must be accommodated to support customers who will mix and match antennas and receivers to suit their individual needs. Some microwave vendors have stayed with parallel control for basic receiver functions to remain backwards compatible with older controllers, while moving to a serial interface for new digital demodulation equipment. Others feature a full serial interface but offer a parallel conversion option if required. This is one area that all station engineers should research carefully before the quote process gets underway.

The biggest changes are in digital signal acquisition and quality monitoring. As discussed earlier, the advent of COFDM brought with it a new set of problems with respect to aligning a D-ENG path for best video performance. Improvements in LNA and receiver technology do help, but are pieces of a larger puzzle.

Some microwave vendors have all developed their own unique set of digital signal metrics that can be displayed on ENG control screens. These metrics may include individual readouts of AGC, BER (pre- and post-Viterbi), MER and recovered SNR, plus a composite quality monitor that combines some or all of the individual metrics. Early users complained that the initial display implementations reacted too slowly to changing path conditions, but recent releases have solved the issues by using more real time information versus processed information.

In addition, several new tools have been added to augment the real time metrics. One is a spectrum analyzer display that monitors the ENG receiver, and the other is a histogram that provides a visual trend line with variable persistence.

Nucomm has integrated a spectrum analyzer display into its central receiver to allow a savvy operator to see the 2GHz band conditions as they exist on channel, as well as above and below the desired channel. MRC has announced a display option with similar capability in conjunction with the Troll Systems controller. Once a shot is set up, the spectrum can be constantly monitored to make sure that it stays clean as air-time approaches.

MRC's new histogram display provides a visual method of monitoring link confidence in an easy-to-interpret graph that combines multiple metrics on a weighted basis. The histogram sampling time can be easily adjusted to suit varying conditions.

For example, a 1-second refresh rate gives a news director the ability to see rapidly changing trends on an ENG path just before or during air-time. A longer sampling time can be used to provide insight into long-term trends for extended remotes. As time goes on, we can expect to see more developments in the way of digital tools.

When you are ready to gather quotes for your upgrades, it is clear that a greatly improved antenna system will be one of benefits of the 2GHz relocation process. Regardless of what you did in the past, it would wise to look at all of the new antennas, controllers, digital tools and interface options carefully before making a choice. Who knows how long these central receive antennas will have to function?

George Maier is the founder of Orion Broadcast Solutions.