Calibrating Your Doppler Set

Before you take your new Doppler set out on a hidden transmitter hunt, you'll need to set the calibration.  You should also verify that display indications are correct for signals from all directions, proving that the antenna control lines are wired correctly.  Re-calibration should be done each time you change receivers, vehicles, or ham bands, but it is not required when you QSY within a band.

The intuitive way to check calibration would be to take a quick walk around the vehicle with a transmitting hand-held, watching the LED display to see if it follows along.  But this is not a reliable method.  A perfectly good antenna system is likely to give bad results, for two reasons.  First, the display electronics and switcher diode currents may be upset by the intense RF field from the HT.  Second, a Doppler array is designed to work with a "planar" wavefront in the "far field," to use some terms that $200-an-hour antenna engineers like to toss around.  Put more simply, the wavefront coming off your HT's "duckie" is a circle that expands outward.  It's just like the ring of ripples you get when you toss a rock into a still pond.  When the transmitter is very close to the receiver (in the "near field"), the part of the wavefront that strikes the receiving antenna has a lot of curvature to it.  When it is many wavelengths away ("far field"), the wavefront circle has become so big that the segment reaching the receiver has very little curvature and appears to be planar.

Another factor to consider is that Doppler accuracy can be degraded by proximity to anything that disrupts the planar characteristic of the incoming wavefront.  Other communications antennas on your car may "pull" the Doppler indication in their direction.  The effect is most detrimental when whips are in front of the Doppler array, or when you have a rotatable beam mounted on the car.  I'll write more on this effect in a later installment.

For the most accurate calibration, you need a steady signal that's in a known direction, as you travel on a street to average the local multipath effects.  Some hams have a friend drive in front of them at the same speed, a couple of blocks ahead.  That works, but it's not convenient for frequent or prolonged testing.

The "standard course" that I use here in Fullerton is a city street that runs radially from a two-meter repeater at an aerospace facility.  While the repeater is active, I tune to the output and drive directly toward it on a half-mile section of this street that is about 2 miles away.  To verify indication of other directions, I drive circles in an open parking lot next to this street.

The repeater is mostly line-of-sight from this test course, but there are urban features to provide some signal bounces (multipath).  I can readily compare multipath performance between antenna systems and display models.  I can also make checks before T-hunts to make sure that the calibration is still good and that the antenna system is working properly.  One day I observed that the display did not track the repeater as usual.  It just bounced around in one quadrant of the display, no matter which way the car was traveling.  I checked the DC whip voltages and noticed that one was different from the others.  Sure enough, a resistor lead in one antenna base was poorly soldered (oops!) and had lost contact, keeping the two PIN diodes in series with that whip from conducting.

If the Doppler under test has serial bearing output that can be connected to a computer, the accuracy and multipath performance comparisons can be documented.  In an upcoming column, I'll tell you about a ham who has done just that, with very interesting results.

Text and photos © 2003 and 2025 Joseph D. Moell. All rights reserved.

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This page updated 24 April 2025