Hello Michael,
It is no surprise that someone trying to use a single bay for IBOC and a 10-bay for analog would have problems with the 10 dB increase. Not understanding the physics of gain antennas is the reason.
I could not give accurate figures, but something between 2–3 dB is the typical capture ratio of a decent analog FM receiver. Ten dB down would work fine with HD as long as the characteristics of the antennas are thought of before the increase, not after.
The problem is very simply that the two antennas chosen for the “Looming Danger of Digital Host Interference” article (RW Engineering Extra, Feb. 18) are absolutely the wrong antennas for the job, no matter where the antennas are on the tower.
There was a presentation to the Tennessee Association of Broadcasters in January by Bob Surette at Shively Labs that outlined this exact problem very well. The PowerPoint presentation is available for download at tiny.cc/Shively. An overview can be found at sbenashville.org/pages/tab-2009.php in the “Afternoon Sessions” section.
HD Radio at 10 dB down from the carrier power will work fine; it does have its challenges and they can be overcome. HD using high-level combining is not going to work at the higher power levels. Low-level combining and possibly for those already running IBOC with separate antennas it might be possible to use a very clean amplifier with current IBOC hardware.
A single bay for HD Radio along with a 10-bay antenna for analog is asking for trouble on many different levels. Depending on physical isolation, intermod products are possible. There will be many areas where the IBOC RF will be well above the analog signal.
To me, the article was well done, but it illustrated a problem with not knowing the physics and related patterns of the antennas.
David R. Wilson, KU4B
Engineer
The Cromwell Group
Nashville, Tenn.
