NOAA All Hazards Weather Radio

RECEIVER RECALLS
For information on Weather Radio receiver recalls, go to the U.S. Consumer Product Safety Commission
(CPSC) web site and choose "Radios (Weather)" in the product Type list.
 

The continuous weather radio broadcasts of the National Weather Service range between the frequencies of 162.400 megahertz (MHz) and 162.550 MHz. These frequencies are in a band referred to as Very High Frequency (VHF) that extends from about 50 MHz to around 175 MHz. The frequencies used by commercial FM radio stations occupy the lower portion of this band. Also the middle channels of VHF television broadcast in this band. Because of the physical characteristics and behavior of VHF broadcast signals, it is not uncommon to find owners of weather radios, like owners of commercial FM or TV receivers, experiencing difficulty receiving a suitable broadcast even though they are within or near the normal range of the transmitter.

The typical service area or range of weather radio stations can best be approximated by a circle having a radius of 40 miles from the transmitting antenna. Technically, the National Weather Service (NWS) attempts to provide an unobstructed signal level of 8 microvolts to receiving antennas 6 feet above ground at 95% of all points within a radius of 40 miles from the transmitter. This is not always possible because of local terrain features such as valleys, mountains, large buildings, etc. Another factor which could reduce the signal level is where the transmitting antenna is mounted on the side of the tower (i.e. the top is already occupied by another antenna). The signal is then reflected off the face of the tower creating a radio shadow, much the same as a light shadow, on the opposite side of the tower. Depending on the width of the tower face, antenna height, and transmitter power, the tower shadow can be quite significant. The NWS office operation the weather radio station can tell prospective weather radio users if and where such a shadow exists. You should also refer to the computer generated broadcast coverage maps above for each station. The table of coverage maps will give you a better idea of what reception should be like in your location for a given broadcast station.

Weather radio receivers, both crystal-controlled and tunable, capable of picking up the weather radio broadcasts, are available in a variety of styles and prices. The price is usually proportional to the sensitivity and quality of the receiver. Below are listed the technical specifications to look for when purchasing a radio. The specifications can be compared to those found in the radio operator's book that accompanies each radio. The individual radio specifications can usually be found near the front or back of the booklet. This comparison will help the purchaser make a wise and informed choice of radio. For further clarification or assistance, a purchaser may wish to consult with a local electronic supplier or technician. The NWS recommends the public purchase receivers with a 0.5 to 1.0 microvolt sensitivity for 20 decibels (dB) quieting, a selectivity of 45 to 70 dB down at +/- 25 kilohertz (kHz), tunable or switchable to all frequencies, a warning alarm feature, dual power source of AC/battery with automatic switchover to battery during commercial AC power outages, and a collapsible or fixed indoor telescoping antenna with a provision for an external antenna input connection.

If the owner has a weather radio meeting the general specifications outlined above in proper working order or a lessor quality radio and still cannot receive an acceptable broadcast, the cause can usually be found to be: A) too weak a signal due to distance from the transmitting antenna or loss of signal penetrating into a building. B) a geographical or man-made obstruction C) interference from another station on the same frequency (co-channel interference).

It must be remembered the signal gets weaker the further one gets from the transmitter. The strength of these signals on the average becomes quite weak at distances greater than 35 to 45 miles. Also, when the receiving antenna is inside a building (not in an open area) some of the signal will be absorbed and/or reflected by the materials which make up the building resulting in further loss. Buildings constructed of steel and masonry products usually attenuate or weaken the signal more than wood or frame construction. Locations below ground are the worst locations for receiving such a broadcast.

The methods described later are frequently not effective at distances greater than 50-60 miles. Distances on this order and greater are beyond the distance for which broadcast programming is intended or provided.

Geographical obstructions that cause reception problems are usually mountains, a large ridge or hills located between the transmitting and receiving antennas. Man-made obstructions may be classified as large buildings or other structures between the transmitting and receiving antennas resulting in weak or complete loss or signal. Tower shadows would fall into this classification. VHF radio transmissions do not bend to any significant degree and generally follow a line of sight path from the transmitting antenna to the receiving antenna.

Co-channel interference is caused when the receiver lies in or near the fringe area of two different stations transmitting on the same frequency. If two broadcasts are of nearly equal signal strength at the receiving antenna, both stations will be detected by the receiver either causing one to be heard for a few seconds or minutes followed by the other or both mixing and just being garbled.

For the situations just described, moving the radio to another location can frequently improve reception. For example, in fringe areas, placing the radio close to a window on the side of the building from which the broadcast comes will often substantially improve reception. Sometimes placing it near any window will work. In practically all situations described above, the use of a properly matched outside antenna will improve reception of the broadcast.

The illustrations included on this page describe several types of antennas and devices that could be used to enhance a weather radio broadcast. Some general provisions to keep in mind are, (a) the range and clarity of reception is dependent upon the antenna height (the higher the better) and its orientation (toward the desired station), (b) the use of a sensitive and quality receiver with an antenna assures more reliable and clearer reception, (c) weather radio transmits a vertically polarized signal, therefore a vertically polarized receiving antenna (elements up and down perpendicular to the ground) that will receive signals in the 130-175 MHz range will give best reception, (d) at distances up to 30 to 40 miles, a 1/4 wavelength antenna element (18") is suggested; for distances greater than 40 miles a 1/2 wavelength antenna (36") should be used.

The simplest antenna to construct is called a 1/2 wavelength folded dipole antenna and is shown as number (1) near the end of this page. The antenna is constructed from a piece of flat type parallel TV antenna lead-in. About 1" of insulation is stripped from the two wires at both ends of a 36" piece of lead-in wire. The ends are twisted together and soldered. Midway between the ends, one wire is cut and the insulation stripped back a sufficient length (1") to attach the leads from a balun. A balun is a device that matches the type of antenna or lead-in to the input of the receiver. Once the connections are made the exposed wires should be taped. A home made balun is shown in (2) and an inexpensive commercial model is shown in (3). The finished antenna should be mounted so the two twisted ends are straight up and down as shown in (1). It could be attached to an inside window sill on a window on the side of the building from which the signal is transmitted; or better, it should be placed outside, possibly next to the window or some other convenient location to easily reach the radio in its most desired location. The lead from the antenna, if it is the parallel flat TV type, should be twisted once for each foot of lead and extended at a right angle from the antenna for approximately 18 inches prior to any vertical run.

Another simple antenna to use in improving the reception of the broadcast is to connect the radio to an existing outside TV antenna. This procedure is illustrated in (4). This method is usually very effective when the TV antenna is constantly pointed in the direction of the desired weather radio station. Also, this type antenna will help when another station on the same frequency is interfering with the desired station (co-channel interference). The weather radio frequencies are between TV channels 6 and 7. Therefore, any TV antenna designed for all VHF TV channel reception will work very well. Even though the TV antenna is horizontally polarized (elements parallel with the ground), there is usually enough vertical signal due to scattering and reflection of the signal that it will significantly enhance the received signal. A device called a band separator or splitter (5) is attached to the end of the TV lead with one output connected to the TV and the other to a balun (2/3) then to the weather radio.

For serious problems in receiving weather radio broadcasts, more sophisticated receiving antenna systems will likely be necessary. When a home made 1/2 wave length antenna may not be suitable and an outside TV antenna is not available, a commercially constructed ground plane antenna (6) with an 18" or 36" vertical shaft is recommended where directionality is unimportant. This type of radio receives equally well from all directions.

When directionality is important due to a very weak signal strength, co-channel interference, or the desired weather radio station being in a different direction from the most often watched TV station, a specially built antenna (7) or a separate all VHF channel TV antenna can be attached to an existing TV mast (8) and permanently pointed in the direction of the desired weather radio station. To further improve the signal, the antenna should be mounted so the elements of the antenna used for the weather radio are aligned vertically to the ground. The lead-in can be combined with the existing TV lead-in and split (5) once inside the building. It would be better, however, for TV/weather radio reception if the lead-ins are brought separately from the antenna to the radio or TV.

For severe cases of co-channel interference or extremely weak signal areas, a corner reflector (9) will generally be the only one to provide suitable reception. The antennas described here are of the type, which can usually be installed by most people with a minimum amount of effort, tools, and experience. As a general rule, it is better to use 50-70 ohm coaxial cable for lead-ins from the antenna to the radios. This type of cable is far less likely to pickup interference and will offer less resistance to the signal picked up at the antenna. For further information regarding antennas available in the local areas and specific instructions for connecting such antennas, a radio dealer or electronic show should be consulted. They can assist the purchaser to select the best antenna system to match the specific radio and to solve reception problems.

The costs of the various methods and devices are listed below and are only estimates of maximum prices, excluding the cost of labor should professional installation be required. (1) $7-10, (2) $3, (3) $5, (4) $30, (5) $3, (6) $30, (7) $50, (8) $30, (9) $175.

As a word of caution, extreme care should be exercised when working with outside antenna systems. Antennas are usually most stable when mounted along the side of a building then extended above the roofline. Sloping roof surfaces, ladders, and roof edges are very dangerous making serious falls possible. Also, support masts or antennas can easily come in contact with the above ground electrical lines causing potentially fatal shock. If there is any doubt about one's ability to make an outside antenna installation, professional or experienced help should be obtained.