In February, NPR Labs, the research project arm of NPR, studied the reports of reception difficulties with the Boston Acoustics Recepter HD Radio receiver. The research was initiated after reports that HD Radio reception inside homes and office buildings was less than satisfactory with some listeners. The complaints noted that hybrid analog/digital reception was adequate, but that digital reception had drop outs or was even nonexistent.
To investigate the matter, NPR Labs first measured the sensitivity of the radio itself and found it to be good. Because of this, NPR Labs turned to the antenna that is included with the radio as the source of the poor performance. The included antenna is an 18" long wire.
NPR Labs obtained active (amplified) and passive FM receive antennas for evaluation with HD Radio signals. These included a folded dipole, a compact amplified FM-only antenna, a compact amplified FM and AM antenna and a rabbit-ear FM antenna.
While a detailed report of the test will be released later this year, NPR Labs released its preliminary findings to help stations and consumers improve their HD Radio reception. Preliminary testing shows an advantage to using passive antennas, such as a folded dipole or rabbit-ear design, over low-cost active antennas. Figures 1 and 2 show the performance of 75Ω folded dipole antenna and an active FM-only antenna. The figures show the spectrum measured from 88MHz to 108MHz at the NPR headquarters in Washington, DC. Five stations are shown on the spectrum plots: WAMU 88.5MHz Washington, WETA 90.9MHz Washington, DC (transmitter is in Arlington, VA), WBJC 91.5MHz Baltimore, WASH 97.1MHz Washington, and WTOP 103.5MHz Washington.
The folded dipole results show that most FM station signals range between a signal level of 30dBm and 50dBm measured at spectrum analyzer input. WBJC in Baltimore has a weaker signal measured at 69dBm. The noise floor is below 90dBm.
The gain control of the active FM-only antenna was adjusted so that the level of FM signals near the middle of the band were about equal to the levels measured with the folded dipole, although the response of the antenna is not flat across the band. The lower channels are at least 10dB lower with the active antenna.
NPR Labs found that while the upper channels have a higher signal level, the noise floor is also increased. For example, the WTOP signal is about 20dB higher than it was with the folded dipole, but the 30dB increase in the noise floor actually decreases the WTOP S/N ratio by about 10dB. The S/N ratio for the lower channel stations is even worse. Similar results were found for other active antennas that sell for less than $70.
NPR Labs has recommended two passive antennas that provide improved reception on the Recepter. They are the C. Crane FM Reflect Antenna, which costs $24.95, and the Radio Shack Budget TV Antenna Model 151874, which costs $9.99.
A result of the NPR Labs test is that Boston Acoustics is now including a dipole antenna with the radio.
Source: NPR Labs IBOC Field Service Bulletin No. 02.20060216, Feb. 16, 2006