Are you ready for an HD Radio Power Increase?
The digital sideband power for HD Radio may get a boost. Are you prepared for it?
In April of this year, the Corporation for Public Broadcasting engaged NPR Labs to undertake a further study of the potential interference effects of higher-power HD Radio transmissions on the analog FM radio service. The results of this study are expected by the end of this summer, prior to the NAB Radio Show that will be held in Philadelphia this year. Although I did not attend the 2009 NAB Show myself, several of my colleagues who did all formed the impression that some amount of increase in digital power is inevitable.
I'll look at the practical means by which a station can increase its digital power level. If your station is waiting until the dust settles prior to committing to the purchase and installation of an HD Radio transmission system, hopefully you can gain some insight in to what will be necessary when you do make the decision to go forward. Let's first review the common means used up until this point to get HD Radio transmissions on the air.
Probably the most common method of creating the hybrid HD Radio signal is by way of a high-power, -10dB, four-port injector installed between the output of the analog transmitter and the main antenna. The analog transmitter's power output needed to be increased by 10 percent to make up for losses through the injector; the HD Radio transmitter had an output that was 10 times the TPO needed to get the correct HD ERP, because the coupling of the high-power injector is -10dB (in other words, only 1/10 of the power inserted on that port of the injector, or -10dB, actually makes its way out of the injector and in to the main transmission line). The other 90 percent of the digital transmitter power was terminated in a waste load and dissipated as heat.
This method proved relatively easy, as long as the analog transmitter was sized adequately, and the facility could provide the extra ac power and air conditioning capacity. Because the analog and digital made use of the same antenna, the -20dB ratio between the analog signal and the digital signal was consistent for coverage throughout the listening area.
Another way to use the same antenna for digital and analog was to procure a transmitter that was a common amplifier — one that had the analog and digital signals common in its output. This of course required buying a new transmitter, and was somewhat limited in the amount of power it could supply — at least early on. The transmitter technology for HD Radio has moved along well in the last four years.
If a facility had a licensed auxiliary antenna that had a HAAT of at least 70 percent (and no more than 100 percent) of the main antenna, while being within three arc-seconds in both longitude and latitude, a station could use it to transmit the HD Radio signal. This ended up being a convenient method for some stations. I have used this method, and early on we were concerned that the -20dB ratio of digital to analog would not be consistent in the field due to the use of different antennas. In practice it did not turn out to be an issue though.
Finally, if the station had a high-power analog transmitter that lacked 10 percent headroom in its output capability, but you were determined to use the same antenna to transmit both signals, then Split-level combining was sometimes a way to go. A combined amplifier analog and digital transmitter was combined with an analog transmitter to develop the correct amount of digital RF and analog RF for a common antenna feed.
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