Multipath is not a problem when hunting with SuperDF while mobile in motion. At 150 MHz, the multipath interference patterns in space have dimensions in the order of a few meters. At reasonable road speeds, many cycles of pattern are traveled through during the time constant of the SuperDF detection electronics. As the antenna moves through the complex multipath pattern, it takes a long-term average. SuperDF thus is able to provide a very stable bearing corresponding to the strongest path. When hunting mobile in motion, it is the antenna that is moving through the complex pattern of RF phase and amplitude (which is distributed through space from the multipath that is stationary).
When DFing an airborne transmitter from a fixed site, the multipath pattern is moving and the RDF antenna is stationary. In either the ground mobile or the aircraft case, the complex pattern and the antenna will move past each other.
There is a difference. In the case of the (moving) airborne transmitter, the multipath space pattern can have components which are moving slowly, and other components which are moving fast. Reflective objects close to the RDF antenna will produce components that are moving slowly, while reflections from distant objects will usually produce fast moving components of pattern. The reason is that for a distant moving source, the difference in path length changes slowly for nearby objects, but (usually) changes fast for distant reflective objects. A few minutes of sketching out different path scenarios on paper will demonstrate this.
The faster the pattern is moving, the better will the SuperDF work. Therefore, for the best results, the SuperDF should be located where there will be few reflections from close-by objects. Reflections from distant hills or buildings (usually) should not be problem. (There are certain geometries of flight paths and RF paths which will violate the above general rules, but they should happen rarely, if ever.)
George Russ Andrews President
George R. Andrews (Russ, K6BMG)
BMG Engineering, Inc.
9935 Garibaldi Avenue
Temple City, CA
91780, USA
Voice 1(626)285-6963
Fax 1(626)285-1684 (24 hour automatic)
America OnLine: Grandrews
Web: http://members.aol.com/bmgenginc
(31 Jan 1996)
Send E-mail to grandrews@aol.com.
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