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Spring 2004 Issue |
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Get Better Performance From
By Joe Moell,* KØOV |
 This DFjr antenna array has been modified with a NØQBF one-piece ABS plastic crossarm to replace the stock aluminum arms. As in the original arms, there are holes for placement of the whips for the 2-meter, 125-cm, and 70-cm bands. Separate quarter-wavelength whip sets for each band ensure that the antenna elements are near resonance. (KØOV photo) |
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Instant transmitter hunting . . . just add a receiver! Well, it’s not quite that simple, but Doppler sets are indeed a quick way to get started in VHF and UHF mobile radio direction finding (RDF). Because they attach to ordinary mobile or hand-held FM transceivers and require no rotating parts or holes to be drilled, you can join the hunt in minutes, even in a rental car. Doppler RDF sets for ham radio are easily identifiable by their array of three to eight vertical antennas. Four of them in a square pattern is the most common arrangement (photo A). A switching circuit connects them to the narrowband FM receiver one at a time in rapid sequence to simulate a single whip moving on a circular track. Due to the Doppler Effect, this apparent motion of the receiving antenna applies periodic frequency modulation to all incoming signals. The display unit detects the phase of this imposed FM with respect to the antenna switching sequence. This reveals the direction of the incoming signal relative to the vehicle heading. Because the simulated rotation rate is about 30,000 RPM, Dopplers can capture bearings on signals lasting only a fraction of a second.
If you need to track very weak signals or
signals with no carrier, such as single-sideband voice, forget about using
a Doppler. It is also not a good choice if you will be doing your tracking
on foot or going after signals below 50 MHz. If you need to do RDF
remotely from a mountaintop radio site or go after horizontally polarized
signals, there are more effective methods to consider. However, if you
want to jump into your vehicle and quickly find a strong VHF-FM or UHF-FM
hidden transmitter, spurious signal, or jamming station, then a Doppler
set may be just the right tool for you. In honor of the 200th anniversary of Christian Doppler’s birth, I began a multi-part “Homing In” series on Doppler RDF technology last year (in the February, March, May, July, and August 2003 issues of 73 Magazine). My goals were to increase hams’ understanding of how Doppler RDF sets work, to dispel some of the myths about them, and to help hams optimize their Doppler installations for best performance in competitions, enforcement, and search-and-rescue applications. I began with a biography of this important Austrian physicist, who had keen insight into how waves are perceived when source and observer are in relative motion. I showed that Doppler RDF sets do indeed follow the principles and equations propounded by Christian Doppler, even though practical units simulate antenna movement electronically by switching a ring of stationary elements.
In part 2, I showed why the fundamental
frequency of the induced Doppler tone is always the same as the array
rotation rate in revolutions (or cycles) per second. That tone must fall
within the audio passband of the receiver. Tone amplitude (FM deviation)
must be high enough to provide good signal-to-noise ratio for the
zero-crossing detector, but not so high as to be corrupted by phase
distortion in the receiver’s discriminator and audio circuits. Click here to return to this month's highlights Click here to subscribe to VHF and read more about Homing In
_________________ © Copyright 2004, CQ Communications, Inc. All rights reserved. This material may not be reproduced or republished, including posting to a website, in part or in whole, by any means, without the express written permission of the publisher, CQ Communications, Inc. Hyperlinks to this page are permitted.
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