Expensive and Inexpensive Ways to Improve the Quality of FM Reception


Author: Wayne Harris (Click HERE to view more articles by Wayne Harris.)
Originally appeared in the September/October 1989 issue of Car Stereo Review magazine.


In the past twenty years or so, the auto sound enthusiast has experienced a quantum leap in the fidelity obtained from his or her sound system. During this period, head units have evolved progressively from primitive to today's state-of-the-art technical marvels.

Initially, AM radio was the only technology available. It's limited bandwidth, poor frequency response characteristics, and susceptibility to natural as well as man-made noises made for a poor means of transmitting high-fidelity sound. This, in conjunction with the spirit of free enterprise, paved the way for developing a better medium for broadcasting sound.

With the introduction of FM radio came a significant improvement in the fidelity of auto sound systems. Better frequency response, less noise, greater dynamic range, and stereo broadcasting contributed to make FM an almost overnight success. It wasn't long before most automotive sound systems boasted head units with both AM and FM reception capabilities. At this point, the demand for better quality sound began to snowball.

Innovation after innovation and breakthrough after breakthrough were soon made evident in the auto sound aftermarket. Clearly, a flurry of engineering, research and development, and marketing effort was being expended in tremendous proportions. Eight track tape players were introduced only to see their demise a few short years later with the introduction of the cassette tape player. Manufacturers were only too happy to oblige consumers with better and better sound. In short order, Dolby, Dolby B, and Dolby C were added to an ever growing list of features sported by most AM/FM Cassette decks.

The most significant advancement in high-fidelity auto sound source units came in 1984 with the advent of the automotive compact disc player. Random track selection, extremely high dynamic range, superior signal to noise ratio, flat frequency response, and low distortion are some of the features that made the CD player an audiophile's dream come true.

Since then, this trend towards space-age technology has continued at an ever increasing pace. It is interesting to note that in almost every instance, breakthrough improvements in sound quality have been accomplished by changing the medium in which sound is reproduced. Unfortunately, the AM/FM tuner portion of the source unit has been locked into the same particular format for almost twenty years.

Even though tuner design has been riding the same wave of technological advancement as that of the CD player, the same basic reception problems plague radio now as it did two decades ago. This is because the problems associated with high quality radio reception are located not internally, but external to the receiver itself.

Interference, both natural and man-made, is the primary cause of degradation. Since it is usually impossible to remove these potential sources of interference, it is vital that we take advantage of any means at our disposal in order to reduce interference and enhance our reception as much as possible.

The single most important component affecting reception quality is the antenna. This device, which is simple in concept yet complex in operation, intercepts incoming electromagnetic radiation that has been transmitted by the radio station. As the radiation comes into contact with the antenna, minute amounts of electrical current are induced into the antenna and the coaxial cable that links the antenna to the tuner. The tuner then amplifies these current variations and extracts the usable sound information.

For the most part, there are only two varieties of antennas in common use by vehicle manufacturers at present. These are the whip variety and the windshield variety.

The whip variety is usually fashioned in the shape of a rod about 31 inches long. However, telescopic and motorized antennas also fit into this category as they have the same basic shape when fully extended. The ideal mounting location for a whip antenna is in the center of the roof. A more popular mounting location that gives only slightly less performance is on the fender of the car or along the windshield's "A" pillar.

The windshield variety is another popular type of antenna in widespread use today. These antennas, which usually consist of two fine strands of wire, are embedded between the various laminations in the windshield itself.

In addition to stock antennas, several different aftermarket manufacturers also produce antenna products. These can range from an exact OEM replica to something that is fairly exotic. Usually, the consumer has the option of purchasing these aftermarket antennas either with or without a built in pre-amplifier.

In simplest terms, an antenna pre-amplifier takes the current variations in the antenna and amplifies them prior to the tuner. The effect of this pre-amplification process can produce results ranging from good to bad. On the good side, a high-quality pre-amplified antenna can extend the usable range of an FM tuner. Weak signals become stronger and thus are of a higher quality. On the bad side, a pre-amplifier amplifies everything. This includes broadcast stations, natural noise, man-made noise, everything. In addition, a pre-amplified antenna can have a detrimental effect on reception by reducing the tuner's selectivity or ability to single out a specific station.

Another form of antenna pre-amplifier is the antenna booster. Usually, these devices come in the form of a "black-box" that plugs in between the existing antenna and tuner. As a general rule, a booster doesn't work as well as an antenna preamplifier since the actual antenna and booster circuitry are not optimized for each other.

Based on my experience, I wouldn't recommend a pre-amplified antenna or antenna booster for large metropolitan areas. Rural or fringe areas are another matter. A good, high quality pre-amplified antenna or booster just might be the ticket for converting a marginal signal into one that is acceptable.

Another option available to the consumer is to replace the standard tuner with one that supports "Diversity Tuning" techniques. A system utilizing diversity tuning consists of a specialized tuner and two antennas located on opposite sides of the vehicle. When listening to the radio, a special electronic circuit monitors the signal strength at both antennas and electronically selects between the stronger of the two. The speed at which this switching process occurs is so rapid that the listener does not even perceive that it has taken place.

Systems that incorporate diversity tuning have excellent multipath rejection. Multipath is that swishing sound you hear when driving around large buildings, structures, or mountains. It is created by the reflections of the radio signal reaching the tuner's antenna a brief instant after the direct path of the radio signal. These multiple path (multipath) signals can vary from an in phase condition to one that is completely out of phase at the antenna. When the signals are out of phase, the quality of the reception is very poor. By using two antennas, it is extremely unlikely that multipath signals will be out of phase at both antennas simultaneously, thus assuring good reception at one of the antennas at all times.

The installation of any of these antenna systems can also determine, to a large extent, the quality of the signal obtained. Some important factors to consider are the location of the antenna and the routing of the coaxial cable.

As previously mentioned, an antenna can be mounted almost anywhere on the vehicle. My recommendation is to stay away from the engine if possible. The electrical discharge of the spark plugs and other engine components create a phenomenon known as Radio Frequency Interference (RFI).RFI can have adverse effects on radio reception in several ways. First, it can actually interfere with reception by radiating it's own signal on or close to a desired channel. This is especially true for the AM band. Another problem created by strong RFI is called de-sensing. When this occurs, the noise source radiates such a powerful signal that it actually overloads the electronics in the front-end of the tuner. This results in a reduction in tuner sensitivity, selectivity, and therefore quality.

If you are using a high-powered amplifier in your auto sound system, you will want to keep your antenna away from it as well. These amplifiers generally utilize a switching power supply to create the high voltages required for amplification. Since these supplies produce a high frequency / large amplitude square wave rich in harmonics, they often emit RFI that can degrade from the performance of the tuner.

The above installation strategies apply to routing the coaxial cable as well. Keep it away from potential noise sources such as the engine, onboard computer, and power amplifiers. Also, don't run the coax adjacent to any power wires if at all possible. These wires could act as a transmitting antenna for the RFI generated by the engine and power amps. In addition, I would recommend wrapping the connection where the coax plugs into the tuner's receptacle with electrical tape. This will prevent the outer metal shell of the plug from touching the chassis of the vehicle and creating a potential ground loop.

Speaking of grounds, the chassis of the vehicle provides the "ground plane" for the antenna. Because of this, a good antenna ground is absolutely essential. During installation, make sure that the metal is scraped clean of any paint or corrosion at the location of installation. Once the installation is complete, coat any exposed bare metal with grease or silicone glue to prevent oxidation and corrosion.

By investing just a little time and money, you can have a substantial effect on the quality of reception you experience. And although the medium used for the transmission of radio has remained basically the same for the past couple of decades, your tuner doesn't have to sound like it did way back when.