SR MSB 2001-01
The Importance of Educating the Public Regarding NOAA Weather Radio
Timothy W. Troutman and Lawrence J. Vannozzi, NWS/SRH/MSB
Following the Palm Sunday tornado outbreak (NOAA, 1994), Vice President Gore's initiative helped expand the NOAA Weather Radio (NWR) network in the southeast United States by about 30 additional transmitter sites. The increased use of weather radios revealed signal reception problems, especially in mobile home parks and metal structures. This paper will highlight recent tests involving NWR signal strength in metal buildings and mobile homes, and suggest methods to improve broadcast availability in these areas. It also highlights new NWR public education efforts aimed at increasing awareness of reception problems and appropriate corrective measures.
2. House and Structural Attenuation
From an engineering perspective, regardless of how strong a NWR transmitter signal may be, any one of the following types of interference may block its reception in a given residence or commercial building (Kandell, 2000):
1. Reflection of the signal off of the building's structural elements, causing out-of-phase multipath signal cancellation.
2. Signal interference from a secondary radiating source, such as cable TV or electrical service to the building.
3. Signal blockage due to the construction features, such as metal wall studs, metallic sun screen tinting on windows and the receiver location with respect to those features.
In order to verify that signal attenuation (reduction in strength) might be caused by home construction, two tests were recently completed by the Florida Division of Emergency Management on the basis of several complaints received from residents in the state. Measurements were taken with a calibrated Motorola Model R-2600 service monitor, the IC-8500 receiver used for the project field radial measurements, as well as the Radio Shack 12-249 NWR receiver. The first signal test was completed in Broward County community of Coral Springs, Florida, near the home from which reception complaints had originated. The home is 21 miles from the Miami NWR transmitter site. The initial step was to monitor the Miami NWR transmitter site to determine the signal strength in the immediate vicinity of that residence. In all cases, the signal was found to be strong (greater than 5 micro volts), with no evidence of fading or blockage.
Evaluators from the Florida Division of Emergency Management visited the 4,500-foot residence and found similar sized homes nearby. No unusual structures existed in the neighborhood which would cause local blockage of the NWR signal to the home. It was discovered the weather radio used at the residence was mounted on a patio wall close to the home's swimming pool. Findings from the site's signal test indicated that the NWR signal was lost directly under the radio's location. The signal reappeared at 5 micro volts approximately six inches on either side of the location of the NWR. As the signal test continued in either direction, 16 inches right or left of the radio, the signal was again lost.
It was concluded from the signal test that metal wall studs grounded through the home's construction design likely completely blocked the signal to the NWR receiver. The homeowner had actually solved his reception problem in the time since he filed the complaint to the National Weather Service. He had attached a marine-type mobile whip antenna to the external antenna port of the NWR. He had also mounted it outside on top of the metal screen enclosure surrounding the swimming pool. This test demonstrates how micro-environments within a structure can drastically affect NWR reception.
3. Manufactured Home Test
Because a large segment of the Florida population lives in manufactured homes, Florida's Division of Emergency Management completed a second NWR signal test to determine what type of signal reception a NWR receiver exhibited inside a manufactured home with aluminum or vinyl siding. Kandel (2000) indicated that a geometric structure built of solid metal or metal mesh and attached to the ground, has the ability to block almost all radio signals from penetrating the structure. This is known as a "faraday shield." Electronics manufacturing firms use faraday shields called screen rooms to adjust sensitive communications equipment, thereby eliminating interference from outside electrical sources. A manufactured home with aluminum siding will contain many of the physical characteristics of a faraday shield, with the exception of windows and flooring. This type of shield may limit a NWR receiver from receiving an accurate signal, especially if the receiver is improperly placed in a manufactured home.
This second signal test was performed in Palm Beach County's Lake Worth Village mobile home park. The home selected for the test was a single wide unit, with aluminum siding and an attached patio/carport. The home was located only 9.9 miles southwest of the West Palm Beach NWR transmitter. Signal tests were completed inside the manufactured home, outside the home in close proximity to the structure on all four sides of the structure, and away from the structure on the adjoining road.
4. Manufactured Home Test Results
Inside the home, measurements with the Radio Shack NWR receiver were taken along a wall containing two windows separated by 36 in. Signal levels in and near the home were determined to be as shown in the following table:
|Location of Receiver/Antenna||Signal Strength (micro volts)|
|1. Outside of Mobile Home|
|a. on road adjacent to mobile home||60-65|
|b. more than 6" away from outer wall||60-65|
|c. less than 6" from outer wall, 16" off ground||2 (noisy)|
|2. Inside Mobile Home|
|a. more than 6" from outer wall||2-5|
|b. less than 6" from outer wall||0-2|
|c. prototype J-Pole external antenna hanging in window||5-8|
Table 1: Signal strength in various locations in and near mobile home
The test demonstrated that the manufactured home is indeed a strong attenuator of the NWR radio signal, especially when the NWR receiver is mounted on the wall and when a built-in telescoping antenna is in close proximity to the wall. The test results were found to be very similar to the Coral Springs test that was completed in the home with metal wall studs. This second test also importantly showed that when the NWR receiver was placed near the wall between the two windows and about five feet above the floor, the signal was not apparent. The worst case scenario inside the home occurred when the radio was placed on the floor and the pull-up antenna was opposite the house electrical wiring. In this situation, a 2.5 micro volt noise level (possibly 60 cycle hum) was indicated. This noise completely prevented any NWR signal from reaching the receiver. However, a very strong signal occurred when the J-Pole antenna was plugged into the NWR receiver and hung in the window only eighteen inches away from the NWR receiver.
5. User Training
The acceptable NWR receiver reception standard is five micro volts. Due to this sensitivity, the manufactured home owner must be carefully instructed as to the placement of the NWR receiver. To ensure the NWR signal can be adequately received, it is recommended that an external antenna be hung in a window, preferably facing the direction of the local NWR transmitter site. Public education and user training is crucial to the successful use of NWRs by the general public. A tri-fold pamphlet which explains the pitfalls of improper location and installation of NWRs is being developed by the Florida Division of Emergency Management.
6. Prototype External Antenna
Based upon the field test findings from Florida's Division of Emergency Management, an external auxiliary antenna was developed in conjunction with RF Products, Inc. of Rockledge, Florida. The antenna was designed with both effectiveness and expense in mind. The end result is a three dB gain, inexpensive, flexible, light-weight antenna, which can be hung in a window location in an inconspicuous manner to greatly improve NWR signal reception. This particular antenna, Figure 1, will be distributed to 12,000 schools, businesses and other government agencies as part of the FWIN (Florida Warning and Information Network) project to ensure that these locations receive a reliable NWR signal (Hagan 1999).
The tests involving NWR were completed by the Florida Division of Emergency Management in response to complaints from the public regarding limited NWR reception in Florida. It is hoped this study's preliminary results will alert WFOs, emergency managers, and other local officials to the potential for poor NWR reception in mobile homes and metal buildings, as well as what corrective measures to take. The use of light-weight external antennas and the correct placement of NWRs should be stressed during NWS outreach activities and public education efforts. Obviously it is very important for the public to purchase a NWR, but they must be educated about proper unit placement and/or antenna availability. These education efforts should inform customers, reduce complaints regarding NWR reception, and lead to a safer public.
The authors would like to thank T.L. Farrow, SRH NWR Engineer for his technical expertise regarding NOAA Weather Radio and Judson Ladd, SRH Meteorological Services Branch Chief, for his thorough review.
Kandel, J.I., 2000: NOAA Weather Radio Coverage for the State of Florida- Phase Two Report to the Division of Emergency Management. pp.1-21.
Hagan, Chuck, 2000: The Florida Warning and Information Network. Florida Hazardous Weather Awareness- A Preparedness Guide Including Safety Information for Schools. pp. 28.
NOAA, 1994: Southeastern United States Palm Sunday Tornado Outbreak of March 27, 1994. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Silver Spring, Maryland. pp. 1-54.
Figure 1. Example of a J-Pole Antenna used in mobile homes and metal structures in Florida to increase NWR signal strength.