Multipath Interference

Review CCR's guitar intro to "Born on the Bayou," or the 1960's release of "Crimson and Clover" for a very cool "vibrato" aka "tremolo" effect. This neat-o technique is pervasive in rock, blues, Country and sometimes Jazz, music recorded after the late 1950's. It is caused by modulating the audio signal with a sub-audible frequency.

But, that "tremolo" sound effect is decidedly uncool for radio reception, and is often caused by multipath interference.

And we hear it often on the ham bands.

A bifurcated, multipath signal arriving out of phase at or near the same amplitude and polarity will cause a null. For physical reasons, the null is often rhythmic, with fades in slow or rapid periods. Skywaves and groundwaves interfere often in this manner. Anyone listening late at night to distant MF and HF broadcasts has heard this effect.

Airborne craft reflecting MF and HF signals late at night, and at any time for VHF and UHF, cause this, and are recognizable by a slow, to rapid, to slow null periodicity as the aircraft passes through the region.

Motorists in urban areas experience this problem as "picket fencing" on VHF FM reception as their vehicles pass reflective structures and other vehicles. In stop-and-go traffic the signal can disappear completely. In such a case, easing ones vehicle forward a few feet (approximately a meter) YMMV will restore reception of the signal.

Long and short path signal collisions can cause nulls for near-antipodal locations. "Antipodes" are diametrically opposed points on a spherical surface. Except for an island or two, the entire antipodal region for North America lies on ocean waters. Therefore, with the exception of vessels at sea and small populations, antipodal path interference would not impact QSOs with North America.

Solution

Omni-directional antennas are particularly vulnerable to multipath interference. Antennas with good front to back ratio will reduce this problem in the same way they will for long and short path "echoes." Two or more phased receive antennas can be used to reduce multipath interference. It is claimed that two or more "active" receive antennas can be very effective for noise cancellation and multipath interference, and therefore could be an attractive option for a QTH within limited space or other antenna-restrictive environments.