Build a diversity antenna and improve the performance of any car stereo! (Cover Story)

Eliminate multipath noise from your FM radio as you drive with this diversity circuit that switches antennas.

WHEN LISTENING TO AN FM STATION in your car, have you ever noticed the sudden onset of noise--pops, clicks and hum--that lasts for just a short time? Maybe you stopped at a traffic light, heard the interference, but found that it disappeared when you drove away--less than a car length.

This audio annoyance could be caused by local sources of noise, or it could be caused by multipath--the convergence of FM signals at your car's antenna that arrived by taking different paths from the FM transmitting antenna. The interference is commonly called "picket fencing" because it comes and goes as you drive, much as your view changes as you walk by a picket fence.

You could be stopped under overhead power lines or near neon lights, motors or relays that could introduce noise into the FM receivers of even the latest model, high-priced cars. However, if you look around and find no obvious sources of electrical noise, consider the possibility that the interference is caused by multipath.

As FM radio waves travel from the transmitting antenna to your receiving antenna, they can take many different paths: Some travel directly to your antenna; others take more devious routes as they bounce off buildings, hills or mountains.

Figure 1 illustrates multipath radio waves converging on your car's antenna with different phase relationships as a result of traveling over paths of different lengths. This same phenomenon can cause "ghosts" in television reception. It is most noticeable with radio waves that have wavelengths of 3 meters or less.

The strength (and quality) of the received FM signal is the resultant of the phase and amplitude of all the waves at the same frequency which arrive at your antenna from the station tuned in. Radio waves of the same frequency that are out-of-phase, as shown in Fig. 2, can cancel each other in certain locations and blank out the received signal, regardless of the FM station's transmitter power.

However, the effects of multipath are more likely to show up as partial cancellation of the received signal accompanied by extraneous noise. If you keep driving, you will soon pass out of this "noisy" region. Fluctuations in signal strength might occur just a quarter wavelength apart.

The FM broadcast band covers the radio-frequency spectrum from 88 to 108 MHz. Thus at the approximate midband frequency of 100 kHz, wavelength is 3 meters or about 3 1/4 yards. That's why moving your car only a few feet can take it out of the noise region.

By contrast, the amplitude-modulated (AM) broadcast band, covers the much lower frequency spectrum of 540 to 1600 kHz. Thus at 1000 kHz, an AM signal has a wavelength of 300 meters--100 times the length of the FM signal. That's why AM reception is unaffected by multipath.

Diversity systems

In the FM reception situation described, if instead of moving the car out of the noisy region, a second antenna were positioned at least 30 inches away from the first, reception could be restored. Unfortunately, connecting two antennas simultaneously to a single car radio will not solve the problem.

The signals from the antenna in the noisy region would combine with the signals from the antenna "in-the-clear," and reception would not improve. The answer to this dilemma is find a means of switching automatically to the one of two antennas situated in the most favorable receiving position.

There is nothing new about the concept of switching antennas to improve reception. One method called diversity reception was developed in the early days of radio to counter the effects of "fading" in shortwave reception. Shortwave or high-frequency (HF) signals, are capable of traveling thousands of miles by "bouncing" off ionized layers 100 kilometers or higher in ionosphere. They were once the best method for long-range communication, and fading could break that communications link.

In those early high-frequency diversity systems, two separate antennas positioned several miles apart fed two separate receiver sections. Electronic circuits compared the relative strengths of the two received signals, and automatically selected the strongest for further amplification and reception. The selection was performed by automatic gain control (AGC). The output DC level was proportional to the strength of the signal being received.

The same circuitry could improve mobile FM reception, but two complete receivers would be required--obviously impractical and expensive. Moreover, opening a standard automotive receiver case to add circuitry could pose a problem due to space and power limitations. The circuit described in this article solves that problem. …