These antennas have a rather small aperture at rest. Thus they are very sensitive to local variations in the multipath phase front condition present where they are located.
The SuperDF samples RF phase at two (nearby) points in space. When multipath is present using the SuperDF will result in detecting the distorted phase front. The bearing will likely be incorrect, as shown by arrows B & C of the Figure 9. If you are lucky, the phase front measured will matched the strongest path, as shown by arrow D.
Often Bearings taken with SuperDF and Dopplers can be quite good from tops of hills looking down into the valley where the transmitter is located. In these cases the direct path is very strong compared to what other paths that might be present.
In multipath situations you will not be able to obtain a complete nulling out of the RDF tone as you take your bearing. If you can hear the RDF tone at its weakest point, there is some multipath present. If the multipath is severe enough as you swing the SuperDF around the compass headings to take a bearing, you will find headings where the tone suddenly takes on a rather harsh or raspy sound. Thus by listening to the quality of the tone and the depth of the tone null, you can make a judgment of the amount of multipath present.
When on foot, multipath situations can be compensated for by taking many bearings from locations that are rather close together (every foot or so) and noting the two most extremely divergent headings. Snoop around looking for the most extreme bearings. The best bearing is at the middle of the two extremes.
A Synthetic Aperture Realized
Multipath becomes virtually a non-problem when DFing is mobile-in-motion. Referring to Figure 9 again, note that the average phase front is exactly the phase front of the strongest path only, as indicated by arrow A. Thus if you are moving fast enough the SuperDF will take a long enough average of the various phase fronts seen so that it indicates the direction of the strongest path. The SuperDF is sampling the phase front about 800 times per second. At 30 miles per hour (44 feet / sec) this is a sample about every 0.66 inches! The slow response mode of SuperDF has a time constant of about 1 second. So in this example, we are taking about 800 samples over a distance of about 44 feet. This distance is likely to be many cycles of the sinuous (distorted) phase front. Thus the bearing taken while moving will be much better than when standing still. In general, the higher the operating frequency and the faster the road speed, the better the average, and the better the bearing.
SuperDF provides a very stable bearing corresponding to the strongest path. But remember, the bearing obtained while moving is that of the strongest path, which is not necessarily the map heading to the transmitter. This would happen if the direct path is shadowed by a hill or other large obstruction, while there is a relatively strong signal scattered to your location from another large hill.
SuperDF must be rotated by hand, which is a complication in relation to a Doppler.
It is extremely sensitive. See the article which explains SuperDF Sensitivity. Does not require an attenuator nor an S meter. Can be used quite easily on foot. Receiver does not overload. Hunts best while in motion. Can hunt either vertical or horizontal signals (by changing the mounting). Can measure the angle of elevation of the signal when used hand held. Very wide frequency coverage. While mobile in motion it supplies a very stable and very accurate bearing on the strongest path present. Excellent accuracy, better than the other antennas listed in this article.
Very fast response time compared to a beam. Ignores even the wildest signal strength fluctuations.
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
(7 Feb 1996)
Send E-mail to grandrews@aol.com.
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