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Doppler Direction Finder

In the world of radio operators, there are several ways of determining where, and in which direction, a transmission comes from.

In the case of distant signals, traditional methods, such as directional antennas of 2 or more elements, are a good choice when trying to position the bow.

When we approach the transmission area (300 or 400 meters), high signal becomes poor because it bounces off different objects, such as trees, buildings, electric lines, etc.

When we approach the last meters (100 meters or closer), measuring (signal strength) becomes impossible.

We should then use passive or active RF attenuators in order to reduce signals as much as possible so that we are able to distinguish which is the right direction.

We can also use directional antennas with field intensity meters; however, our hidden friend often changes the transmission powers, so, even though we think he is close, we can completely lose him in the next transmission.

There also exist more complex antennas which can detect the “zero signal”. A combination of dipole antennas with inverted phases is a good choice; they are more reliable than directional antennas.

Another example of this type of antenna is the old-fashioned and not-highly-praised “ARO” or halo antenna, which has to be positioned at right angles to the transmission direction, so that we can get low signal. Another good choice…

However, both of them have, in lower degree, the same faults as the first ones. In the last meters, experience and slyness are more valuable than antennas; they also signal in all directions.

And now last years´ star appears: ELECTRONICS.

The simplest and more effective choice is the pair of dipoles switched by a small oscillator in an approximate frequency of 300 and 800 Hz.

This oscillation gives audio a harmonious sound. When the dipoles are put facing each other, at the same distance as the transmitter, they receive the signal at the same phase point so that noise disappears.

When we shift the dipoles in any direction, the phase point will vary and that phase difference will become audible through the speaker.

The main advantage of this method is that it can perfectly distinguish the direction, whatever the distance which separates us from the transmission source may be or whatever the power with which it transmits may be; we are not looking for signal variations, we are looking for sound variation.

In the market, there are kits for working with this system; the most elaborate ones have a pair of LEDs which point in which direction we should move (right or left).

But not everything is good news. Sound variation caused by modulations, constant tones, etc. can cause wrong electronic tagging due to coincidences, in the sound frequencies, with the oscillator which switches the antennas.

Another important issue is that these systems only signal right or left. Distinguishing between forward and backward comes with practice: when we detect the right direction of a transmission, such direction can either be forward or backward.

It is also interesting to mention the nerves caused by that moment, which can turn something very simple into something very complex.

Now it is time to deal with a more interesting issue. . . And that issue is “the Doppler”.

SPUTNIK was developed. It was named that way because of the shape of the equipment placed on the vehicle roof.

We placed 8 dipoles (on the vehicle roof) so as to get more precision and sensitivity. Then we crowned the design with a filter for the phase-indicating signal which turned out to be extremely good due to its simplicity and effectiveness.

Voices, tones, weird sounds and the Leds showed us where we should go.

A wonderful team which brought us joy, happiness and triumphs we never though we could achieved in only six unforgettable months.

But we still had a problem.

We could not always reach the transmitter or the fox´s hiding place.

In this part of the story, Benjamin comes on stage, a hand-held Doppler called “Enterprise" due to its resemblance to the TV series navy.

Both the Doppler placed on the vehicle roof and the hand-held Doppler gave us what only a few people had.
Now that we know it works, we decided to bring it to light so that everybody could use it.

Finally I would like to add some comments:

First, it is not a car automatic pilot which will take us directly to the desired place while we drink coffee and eat cookies.

Second, the spread of this kind of equipment will turn hunting into real car races. Be wise. Even though it is a competition, we should not be rude with the opponent. It is, above all, an entertainment.

Third, we would like to thank those who helped us in the development of the equipments, as well as thanking our hunting mates for the kind treatment we received.

 AND NOW ..... BUILD IT !!!!!

Service Translator : Natalia Pineda

Mario Sacco - LU6DPP - Servisystem





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Amateur Radio Direction Finding Web Ring
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