These antennas have a rather small aperture at rest. Thus they are very sensitive to local variations in the phase front condition present where they are located. (Yes, the Doppler is a phase measuring device: it is not a true Doppler shift device.) As you move very slowly through space, they will indicate a rather wide shift in bearing. When stopped, they are very likely to give an incorrect bearing.
A Synthetic Aperture Realized
The Doppler RDF antenna creates a synthetic aperture antenna in two ways.
First, there is some filtering the electronics does. Most Dopplers I have seen do not have a long time constant to do averaging over a long distance, because that would greatly slow down the response time (that is, how long it takes for a new signal to produce a stable display.) I have seen this in my own Doppler. Too much time constant make the unit take "forever" to come to rest, so it becomes difficult to get a bearing on a short transmission.
With a short time constant for integrating (adding up the readings taken while moving) the synthetic aperture is not very long, but it is significantly better than when standing still. Therefore, the Doppler gives much better bearings when moving at a good road speed.
The second effect is really the work of the human mind. My Doppler has 32 LEDs in a ring. As I travel through a multipath environment, I will typically see a "swath" of light from a group of adjacent LEDs. This might be anything from 2 to 8 LEDs (22.5 to 90 degrees), depending on how bad the multipath is. The human brain is able to answer the question, "Where is the center of the swath of light," and use that as the bearing. Thus the brain "creates" the synthetic aperture.
Doppler is an automatic system. It does not require rotation of the antenna hardware. Does not require an attenuator nor an S meter. Receiver does not overload. Hunts best while in motion. Very wide frequency coverage. Ignores even the wildest signal strength fluctuations.
Typical Dopplers are hard of hearing. They fail the hunter at weak signal levels while beams or SuperDF are still able to hunt easily. Can't be easily taken on foot.
Can't hunt horizontal signals well. It becomes susceptible to scattered of reradiated signals, often seeing them better than the direct horizontal signal. This is because the secondary signals are likely to have vertical components, which the Doppler sees well, while it pretty much ignores the direct horizontal signal.
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)
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