Support Center

Interference, Antennas, and Amplifiers

Last Updated: Sep 06, 2016 03:41PM EDT

What's the first thing to check if I am having trouble receiving WAMU's signal?

One of the easiest solutions to a reception problem, particularly if you are in the metropolitan area, is to move the radio to a different part of the room. Sometimes moving the radio only a few inches or feet will bring in a clear signal. Most portable and clock radios use either a telescopic whip or the AC power cord as an antenna. The whip can be extended, retracted, and rotated to change reception. The AC cord can be moved around and draped across objects for the best signal or wrapped up to reduce signal. If your clock radio has no visible antenna or connections for an antenna then you can assume that the radio is using the power cord for an antenna.

Does my radio need an antenna?

Stereo tuners and receivers require an external antenna (an antenna that is not a part of the radio). Many tabletop and clock radios have connections for an external antenna while allowing you to use the power cord should you not want to use an external antenna.

The antenna connects to either two screw terminals marked 300 Ohm or an "F" connector for coaxial (round) cable located on the back of the unit. A "twin-lead" wire dipole antenna is usually boxed with a new receiver or tuner. Many people attach this dipole antenna to their new set and then ball up the wire and stuff it behind the equipment. For strong stations this may work just fine. For many weak stations it will not work. If you don't have an antenna connected to your stereo, a dipole can be purchased or you can make one. The dipole is made of about five feet of flat antenna lead-in wire with the ends bared and twisted together. Another five feet of wire is attached to the center of the first piece of twin-lead at the middle of ONE of the wires by cutting it and baring the wire then twisting it together. If you want to make a dipole yourself specifically tuned for WAMU's frequency of 88.5 mHz, the length of the first mentioned piece of twin-lead should be 63 inches. The length of the center piece of wire can be as long as needed to reach the set, but no shorter than five feet.

The dipole has what is called a figure-of-eight reception pattern. It picks up signal on the broad sides of the antenna (and provides some antenna gain) and receives no signal on the ends. If the dipole is stretched out and rotated it is possible to point the antenna for best reception or to "null" a station that is interfering with the desired signal. The disadvantage of the dipole is finding a way to locate it in the room. If you are lucky, the direction of the desired station will correspond with one of the walls in the room and you can simply attach the stretched out twin-lead to the wall. Some people mount the dipole on a T-shaped frame made of wood and position it in the room for best reception. Others have mounted this same frame in the attic. It may be easier to buy a "rabbit ear" antenna like you would use for television reception. The rabbit ear is another form of dipole antenna. Each telescopic section should be adjusted for a length of 31.75 inches. The rabbit ear is often easier to place in the room or on the stereo and can still be rotated for best reception.

There are a number of indoor specialty antennas available for FM reception. Some work very well and some do not. Make sure that the antenna can be returned before purchasing it.

Why is antenna set-up important?

TV and FM reception is referred to as "line-of-sight." This means the transmitting and receiving antennas must be able to "see" each other. For reception outside of the metropolitan area, a roof or attic mounted antenna is recommended both for the increased height and for the additional antenna gain (increase in signal strength over a reference antenna) that comes with a directional antenna. Line-of-sight reception of WAMU should be good for 50 miles. Hills and mountains may extend this range (providing increased height) or reduce it (as an obstruction between the transmit and receive antennae). Directional antennas may also be mounted in the attic still providing increased height over lower level locations. A directional antenna can also help in reducing co-channel (a station on the same frequency as the desired station but located in a different direction) or adjacent channel stations that are interfering with the desired station. Many people who listen to a number of distant radio stations use a rotator to position the antenna correctly for each station. Directional antennas can be found at Radio Shack as well through many TV and antenna dealers (who will also have higher gain antennas). Be wary of the antenna claims of "reception from 110 miles" etc. Remember that unless you are looking down from the mountains with an unobstructed view or have a high tower for your antenna, curvature of the Earth prevents reception from that distance. Be more interested in whether the antenna has 3 db, 6 dB or 9 dB of gain. The more gain the better for distant stations but, the antenna is going to be larger for each increase in gain.

Will an amplifier help reception?

You should also be aware that an amplifier (pre-amp at the antenna or distribution amp in the house) will not produce a better signal, only antenna gain will give you a better signal over noise. Amplifiers are used to make up for the signal losses in long runs of cable, such as from the roof or tower mounted antenna into the house, or from splitting the signal to feed multiple sets. The amplifier should always be inserted before the loss occurs.

How do I mount an outdoor antenna?

If you install an attic or outdoor antenna, be sure to follow good engineering practices by twisting the twin-lead (flat) cable and mounting on stand-offs or better yet, using coax (round) cable. Remember to always ground the antenna and the lead-in cable. Always use "splitters" or "couplers" for multi-set installations or joining several antennae to one lead-in cable. These practices ensure the rejection of unwanted and interfering signals being picked up by the cable (or sometimes retransmitted by YOUR antenna to the neighborhood) and afford protection against lightning.

What if I already have a good antenna installation?

Several things can cause interference. Interference in receiving the desired station is often caused through overloading of the radio's first amplifier by a different but nearby station. If you know that you are in an area where a radio or television transmitter is located, this may be your problem. There are several solutions for this problem. If the signal of the desired station is more than sufficient, then reducing the level of all signals into the radio, receiver, or tuner will help. On radios with telescopic antennas, you can make the antenna smaller by pushing the sections together. On clock radios, wrap up the power cord as short as possible. With stereo receivers and tuners you will need to install a "pad" between the antenna lead-in and the input connection to the set. Radio Shack makes fixed 6 dB pads for coax (round) cable . They also make an adjustable inline attenuator for coax connections that allows you to reduce the signal until the interference disappears. These pads and the attenuator can be adapted for 300 Ohm (flat) twin lead cable.

If the desired signal is weak (you're a long distance from the station) the best solution is to reduce the signal of the station causing the interference. This is done with a filter and is most easily accomplished on a tuner or receiver with external antenna connections. The cheapest filter is a quarter-wave open stub which you can make at home. If your antenna lead-in is the flat twin lead, start by cutting a length 33 inches long. Bare the wires at one end and attach it to the set's antenna terminals along with the lead-in from the antenna. Now, with the set tuned to 88.5mHz, start snipping off a 1/4 of an inch of the cable at a time until WAMU's signal improves. Stop cutting when the signal sounds good to you. You are tuning the stub filter to the interfering station's frequency. If you know that the interfering frequency is above 100mHz, you can start with 29 inches of cable and save some time in cutting.

A filter can also be made for antenna systems using coax (round) cable but is more involved. A coax filter would require a "TEE" connection (three connections; one for the antenna lead-in, one for a jumper cable to the set and one for the filter). A TEE adapter is not made for the "F" connector used for coax cable so an adapter would have to be made from separate adapters. This would consist of three "F" to "BNC" adapters and one "BNC" TEE. Then the coax is cut as described above for the flat cable, attached to the middle connector on the "BNC" TEE and snipped in 1/4 inch increments as described above.

Some interfering stations may be so strong (VERY close to you) that the open stub filter is not sufficient. There are filters that can be tuned for greater rejection at the offending frequency made by antenna manufacturers such as Winegard, Blonder-Tongue, Channel Master, and Jerrold. These filters however, are quite expensive.

What if my antenna is fine, I've eliminated the interference from other station signals and I am still getting interference?

Some interference is intermittent. This can be caused by a nearby amateur radio or CB transmitter. Many of the suggestions recommended in the FAQ above this one can improve reception and also filters are available from Radio Shack for amateur and CB bands.

Intermittent static interference is usually caused by electrical appliances (refrigerator, furnace blower motor, air-conditioner, etc). If this interference is coming into the radio via the AC power line (if it sounds OK running on batteries, this is the problem), an AC line interference filter (available at many stores that sell electronics) can be placed between the wall outlet and the power cord.

Multipath distortion is common in the metropolitan area. It is caused by signal reflections from buildings and is often heard in the car as ticks and pops as you drive around or, if you are driving more slowly, as small "areas" of distortion that you drive through. In the home multipath distortion may appear in one area of a room but not if the radio is moved to another part of the room. If your radio sounds distorted, particularly if the distortion comes and goes, one of the tips discussed in the first few paragraphs should clear up the problem.

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