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IBOC Field Service Bulletin No. 02.20060216
Issued: February 16, 2006
Subject: Indoor FM HD Radio Antenna Performance
Equipment: Boston Acoustic Recepter Table Radios
Type: Anomaly
Symptom: HD Radio reception inside homes and office buildings with the new
Boston Acoustics Recepter® table model radios has been disappointing for some
listeners. In some cases, analog reception of hybrid stations has been adequate,
but the HD Radio reception has dropped in and out, or has been non-existent.
NPR Labs’ measurement of this model shows very good sensitivity at the
antenna input, which suggests that the supplied 18-inch wire antenna is a major
fault. A better indoor FM antenna is needed to improve HD Radio reception.
Recommendations: NPR Labs obtained the following samples of active
(amplified) and passive FM receive antennas for evaluation with HD Radio
signals:
• Folded dipole
• Compact amplified FM-only antenna
• Compact amplified FM/AM antenna
• Rabbit-ear FM antenna
Testing is underway and a full report will be available later this year. However,
the need for improved HD Radio reception indoors prompts NPR Labs to release
this bulletin to help guide stations and consumers in choosing antennas that are
effective in improving reception.
Preliminary testing shows a clear advantage to passive antennas, such as folded
dipole and rabbit-ear types, over low-cost active antennas. The figures below
show an example of the performance difference between a 75-ohm folded dipole
antenna and an active FM-only antenna. These figures show the spectrum
measured from 88 to 108 MHz at the NPR headquarters building in downtown
Washington DC. The antennas were placed on a large empty cardboard (nonconductive)
box approximately 5 feet from a north-facing 5th floor window. The
spectrum plots are marked with five sample stations:
WAMU 88.5 MHz Washington DC
WETA 90.9 MHz Washington DC (Arlington VA transmitter)
WBJC 91.5 MHz Baltimore MD
WASH 97.1 MHz Washington DC
WTOP 103.5 MHz Washington DC.
The spectrum of the folded dipole shows most FM station signals ranging
between -30 dBm and -50 dBm (measured with a 50-ohm spectrum analyzer
input). WBJC, a Baltimore station, is shown at approximately -69 dBm. The
noise floor, which is a combination of analyzer internal noise and low-level FM
signals is below -90 dBm.
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 approximately equal to the levels
measured with the folded dipole; this occurred at a rotation about 1/3 clockwise
from minimum. It is apparent that gain of the antenna unit is not flat across the
band; the Reserved Band (88-92 MHz) stations are at least 10 dB lower with the
active antenna. The WTOP signal is approximately 20 dB higher than it was with
the folded dipole. However, this signal increase is accompanied by a noise floor
increase of nearly 30 dB so the net signal-to-noise ratio is decreased by
approximately 10 dB. The signal-to-noise ratio for the Reserved Band stations is
even worse (note that weak WBJC is almost lost in the noise). This performance
was typical of other amplified antennas tested that sell for under $70.
The source of noise in amplified FM antennas is likely to be 3rd order and 5th
order intermodulation products generated by the internal amplifier. Adjustment of
the gain control lower will reduce the IM product levels, but also reduces the
signal levels below that of the sample dipole antenna.
Suggestions of passive antennas that were found to perform well are:
• C. Crane “FM Reflect Antenna”, $24.95 (www.shop.npr.org)
• Radio Shack “Budget TV Antenna Model 15-1874”, $9.99
(www.radioshack.com)
Folded Dipole Antenna Active FM-Only Antenna
NPR Labs will provide updates to this field service bulletin as field information
warrants.
Please contact [email protected] for further information concerning this NPR
Labs FSB. Refer to Field Service Bulletin No. 02
Radio Shack Budget
TV Antenna Model
CCrane FM Reflect
Antenna
Issued: February 16, 2006
Subject: Indoor FM HD Radio Antenna Performance
Equipment: Boston Acoustic Recepter Table Radios
Type: Anomaly
Symptom: HD Radio reception inside homes and office buildings with the new
Boston Acoustics Recepter® table model radios has been disappointing for some
listeners. In some cases, analog reception of hybrid stations has been adequate,
but the HD Radio reception has dropped in and out, or has been non-existent.
NPR Labs’ measurement of this model shows very good sensitivity at the
antenna input, which suggests that the supplied 18-inch wire antenna is a major
fault. A better indoor FM antenna is needed to improve HD Radio reception.
Recommendations: NPR Labs obtained the following samples of active
(amplified) and passive FM receive antennas for evaluation with HD Radio
signals:
• Folded dipole
• Compact amplified FM-only antenna
• Compact amplified FM/AM antenna
• Rabbit-ear FM antenna
Testing is underway and a full report will be available later this year. However,
the need for improved HD Radio reception indoors prompts NPR Labs to release
this bulletin to help guide stations and consumers in choosing antennas that are
effective in improving reception.
Preliminary testing shows a clear advantage to passive antennas, such as folded
dipole and rabbit-ear types, over low-cost active antennas. The figures below
show an example of the performance difference between a 75-ohm folded dipole
antenna and an active FM-only antenna. These figures show the spectrum
measured from 88 to 108 MHz at the NPR headquarters building in downtown
Washington DC. The antennas were placed on a large empty cardboard (nonconductive)
box approximately 5 feet from a north-facing 5th floor window. The
spectrum plots are marked with five sample stations:
WAMU 88.5 MHz Washington DC
WETA 90.9 MHz Washington DC (Arlington VA transmitter)
WBJC 91.5 MHz Baltimore MD
WASH 97.1 MHz Washington DC
WTOP 103.5 MHz Washington DC.
The spectrum of the folded dipole shows most FM station signals ranging
between -30 dBm and -50 dBm (measured with a 50-ohm spectrum analyzer
input). WBJC, a Baltimore station, is shown at approximately -69 dBm. The
noise floor, which is a combination of analyzer internal noise and low-level FM
signals is below -90 dBm.
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 approximately equal to the levels
measured with the folded dipole; this occurred at a rotation about 1/3 clockwise
from minimum. It is apparent that gain of the antenna unit is not flat across the
band; the Reserved Band (88-92 MHz) stations are at least 10 dB lower with the
active antenna. The WTOP signal is approximately 20 dB higher than it was with
the folded dipole. However, this signal increase is accompanied by a noise floor
increase of nearly 30 dB so the net signal-to-noise ratio is decreased by
approximately 10 dB. The signal-to-noise ratio for the Reserved Band stations is
even worse (note that weak WBJC is almost lost in the noise). This performance
was typical of other amplified antennas tested that sell for under $70.
The source of noise in amplified FM antennas is likely to be 3rd order and 5th
order intermodulation products generated by the internal amplifier. Adjustment of
the gain control lower will reduce the IM product levels, but also reduces the
signal levels below that of the sample dipole antenna.
Suggestions of passive antennas that were found to perform well are:
• C. Crane “FM Reflect Antenna”, $24.95 (www.shop.npr.org)
• Radio Shack “Budget TV Antenna Model 15-1874”, $9.99
(www.radioshack.com)
Folded Dipole Antenna Active FM-Only Antenna
NPR Labs will provide updates to this field service bulletin as field information
warrants.
Please contact [email protected] for further information concerning this NPR
Labs FSB. Refer to Field Service Bulletin No. 02
Radio Shack Budget
TV Antenna Model
CCrane FM Reflect
Antenna
