Maximizing HD radio signals by maximizing isolation:
antenna options for simulcasting HD
by Henry Downs, Technical Director of RF Components and Subsystems Development for Dielectric Communications
As In-Band On-Channel (IBOC) HD Radio is becoming a reality for a growing number of radio stations nationwide, so also are the logistical challenges of integrating digital transmissions into an existing infrastructure. As with analog, the goal of any station is to ensure that the maximum amount of power produced by the digital transmitter is radiated from a suitable antenna into free space, where it can be picked up by the listening audience. There are two components involved in the maximization process, namely insertion loss and digital to analog isolation.
The insertion loss of any system is well understood and there are many proven techniques to minimize it, leaving broadcasters with the main challenge of isolating the analog and digital signals.
This challenge is compounded by the fact that the analog radiated power level is 100 times greater than that for the digital. In the case of insufficient isolation, not only is the digital power loss greatly increased, but the analog power which feeds back towards the digital transmitter will inevitably be sufficient to cause the digital transmitter to fold back.
The search for the most efficient antenna for simulcasting analog and HD radio has spanned the last decade and produced a number of options for broadcasters to pursue.
IBOC Combining Methods
The original proof of concept for IBOC radio utilized a 10-dB combiner on the ground that caused over 10% of the analog signal power and 90% of the digital to be dumped into a load. Conceptually a success, this method was extremely inefficient and not feasible as a long-term solution for most stations. Since that time, a number of manufacturers have invested resources in developing different combining methods and, to date, the most efficient method found is to combine the signals in free space by way of the antenna. The critical parameter for the antenna is to provide sufficient isolation to ensure a strong digital signal.
Separate Antenna Systems
Many stations have looked to work around the issue of poor isolation by utilizing separate antennas (or even separate towers) for their analog and digital broadcasts. By optimizing an antenna specifically to the low-power requirements of digital, the station minimizes the potential impact of an analog antenna's power loss from poor isolation.
Of course, this approach does have some drawbacks. In addition to the added equipment, this method also requires additional aperture on the tower. Since most stations do not own their own towers, this quickly translates into higher operating costs. Additionally, because the digital and analog antennas will be mounted to the tower at slightly different elevations, a station's coverage for its analog and digital transmissions will likely vary.
Interleaved and Reverse Polarized Antennas
Interleaved antennas have been in use for the past 30 years for two frequency-separated channels. Because of the unique structure of this device - in which two antennas are woven together - the two signals transmit from exactly the same spot on the tower, ensuring that a station is achieving equal coverage for each channel.
While this style of antenna eliminates the issue of purchasing separate tower space, the added weight of an interleaved antenna increases the windload and may require tower retrofitting to accommodate the new load. Perhaps an even more significant blow is the fact that for IBOC signals at the same frequency, the isolation is poor - typically no more than 12 to 14 dB.
In response to the poor isolation of traditional interleaved antennas, Dielectric developed the patent-pending HDR Plus, a reverse-polarized interleaved antenna. This product helps to maximize signal quality by allowing for a "lossless" combination of analog and digital signals. The system polarizes the digital antenna in a left-hand circular pattern while the analog portion continues to radiate in the traditional right-hand circular polarization.
The HDR Plus already has successfully demonstrated a minimum 42-dB isolation in real-world applications. This huge increase in isolation helps to lessen the costs of implementing a digital channel while also, in many cases, removing the need for an isolator, which has long been considered the weak link in a transmission system. Additionally, the lossless characteristic of this antenna allows stations to use a very-low-power digital transmitter and Type N cable, further minimizing the cost of adding the digital signal.
Master Antennas for Multi-Channel Stations
While solutions like the HDR Plus are sufficient for single stations or multi-channel stations with just two to three frequencies, most large multi-channel facilities require a master antenna and every aspect of the station's existing RF hardware has to be taken into account.
During the evolution of master antenna systems, in the analog-only era, manufacturers discovered that an asymmetrical dipole design could be utilized in conjunction with a short circuit on the load port of the 3-dB hybrid feed system to enhance the input match to the antenna.
When looking to adapt this style of antenna to accommodate digital broadcasting as well, the short-circuit was replaced with a low-power digital input. It was evident immediately that this redesign was not practical; the isolation between the digital and analog ports was as low as 12 dB. The asymmetrical design that was so successful for analog broadcasting was now allowing analog power to leak into the digital input.
The latest generation of FM master antennas - including Dielectric's HDFMVee - overcomes this problem by increasing the isolation to better than 30 dB. The HDFMVee's patent-pending methodology virtually eliminates cross-coupling within the radiator and also eliminates mutual coupling between the bays of the antenna array. In short, antennas like the HDFMVee allow the return loss (or VSWR) to be optimized for both analog and digital inputs across the complete 88-MHz to 108-MHz FM band.
This enhanced isolation provides a substantial advantage over other transmission methods because it allows the feed system for the digital signals to be reduced in size. In turn, this translates into lower weight and reduced wind load decreasing costs.
Capturing New Listeners
As stations develop their master plans for adding HD radio, it's crucial that methods for maximizing isolation are taken into account. Whether it's a small station broadcasting HD only a portion of the time or a multi-channel station that juggles analog and digital on a single master antenna, the universal factor determining the strength of an HD signal is antenna isolation. By maximizing isolation and thus minimizing power loss, a station can capture more of the growing HD radio market.
