A PORTABLE D/F UNIT

INTRODUCTION

This section describes a simplified Doppler D/F unit, suitable for portable operation, which employs only two antennas. It is intended to complement the mobile unit, which may not be suitable during the final phase of a hunt, where a vehicle D/F might be impractical. The user must provide a portable FM reciever, and a stereo headset.

At this time, ( late May 2000 ) a prototype of this unit has been completed. Bench checks have also been completed, but field testing is still in progress. When those ( field ) tests are completed, photos and comments on the tests will be added to the website. Results of the preliminary field tests are provided on this page, near the bottom.

During construction of the prototype, some flaws / mistakes were discovered in both the PC artwork, and in the original schematic diagram, and a few of the component values were adjusted. See the section called UPDATES for a history of these changes.


BACKGROUND

D/F’s similar to this one are often called TDOA’s, ( time difference of arrival ) which roughly describes the basic operating principle... see the section called [PULSES] for a more detailed explanation. No readout or display is usually employed with TDOA’s... homing is achieved by listening to the Doppler "tone", and rotating the ( two element ) antenna array until this tone is "nulled out"... with this achieved, the transmitter must lie directly before ( or directly behind ) the hunter. Some versions use ( various ) indicators to reveal left / right signal direction, whenever the array is rotated slightly "off null". These indicators serve to eliminate the front / rear bearing ambiguity. The unit described here also eliminates front / rear ambiguity, but in a different way...

In this unit, the two antennas are directly mounted on a stereo headset, ( one on each earpiece ) and the reciever audio is fed to a detector unit. The detector unit splits the audio into two paths, ( one for each earpiece ) and determines which antenna / earpiece is farthest from the transmitter. The audio in this earpiece is then attenuated by 20 decibels, but the audio in the opposite earpiece remains unchanged. When both antennas are equidistant from the transmitter, both earpieces operate at full volume, and the user "fine tunes" the signal direction by driving for a null tone.

Because the antennas are directly mounted on the headset, the hunter must merely "rock" his / her head from side to side, and listen to the ( acoustic ) results in the headset. This method is more intuitive, requires no visual interpetation, and keeps the hands and eyes free for other tasks. There is an adjustable ( left / right ) "deadband" zone, near the "null tone" position. Within this zone, both earpieces operate at full strength, and the user drives the antennas to achieve a null tone. If the headset is rotated outside this zone, the audio for the "far side" earpiece is attenuated.

Obviously, this unit could also be used with a separate antenna array... If you don’t want to be seen in public with antennas sticking out of your head, looking like an invader from Mars, you can look like Poseidon, instead. If you choose the latter, keep the distance between antennas less than 1/3 wavelength... and mark them in some way to indicate which antenna belongs on the left / right.


LIMITATIONS

Although this unit might seem to offer all the virtues of the mobile Doppler D/F, it has limitations... it is far more difficult to identify "high reflection" situations, and data accumulation is much slower. The mobile Doppler updates the indicated signal direction at all times, ( even while moving ) and reflections quickly become obvious, because the readout will "jump around". This unit usually requires that the user stop and rotate the headset from time to time, to obtain the same "feeling" about reflections. Still, for the last 100 yards of a hunt, portable equipment like this is often essential.


INITIAL FIELD TESTS

The initial ( very brief ) field test was conducted one afternoon, in a friend's front yard, and around the neighborhood, to a distance of about 2 blocks. The results were "mixed", but not really discouraging... the unit appeared to behave "electrically" as expected, but the presence of reflected signals made DF operation difficult. It became obvious that further tests will be required to gain familiarity with the unit, and to develope methods and procedures that can make it an effective DF tool. Changes to the antennas might prove useful, but all the electronic portions seemed to work properly.

At one point, reciever RF sensitivity was tested by running a 3 Watt mobile unit around the neighborhood, to a maximum range of about 1 mile... the DF reciever remained "full quieting" throughout this test, so RF sensitivity was OK. Earlier, a 100 Watt repeater located on a mountaintop at a range of 15 miles was heard inside a building, with some signal "dropouts" as the headset was moved... not bad, and certainly adequate to track down a transmitter during the last few hundred yards of a hunt.

The antennas consisted of a pair of 8 inch wire elements, made with wire cut from a clothshanger, and bent slightly outward to clear the rest of the earpiece structure. The antennas were tilted about 20 to 30 degrees from vertical, as a result, and the top of the antenna elements extended about 5 inches above the top of my head.

The left / right earpiece muting worked as expected, and the BALANCE and DEADBAND adjustment trimpots operated properly... the left / right DEADBAND width exhibited sensitivity to the setting of the reciever volume control, as expected.... higher volume control settings made the DEADBAND zone narrower, and ( in the extreme case) the left / right DEADBAND width dropped to zero, so that audio was always attenuated in at least one earpiece.

DEADBAND sensitivity to audio level changes was fairly smooth, and proved to be a useful way to "adjust" the DEADBAND width, during the test.... once the initial adjustment of the DEADBAND trimpot was completed, it was found that audio level changes of 15 to 20 db would change the DEADBAND size from 360 degrees to zero, in a fairly smooth and useful manner.

The TDOA tone was clear and obvious in the reciever audio, and achieved a complete null as the headset was rotated. The maximum level of the TDOA tone was small enough to allow "copy" of the background FM ( voice ) modulation, while the DF operated. Left / right earpiece switching was generally symmetric, except in situations rich with reflections.

As the headset was rotated and each earpiece switched from full volume to low volume, a small zone was found (at the switching threshold "boundary") where the threshold detectors "chattered" in/out of attenuation, causing a "buzz" in the earpiece, whose frequency equaled the antenna switching rate. This zone was very small, and actually seemed to be useful, by accentuating the fact that the threshold was being crossed.

As expected, DF operation appeared to be most reliable when a clear path to the transmitter was available, and the unit worked properly down to a distance of about 5 to 10 feet from the transmitter. Even so, areas near the transmitter could be found where results were inconclusive, or even contradictory. Walking along, left / right earpiece switching could be heard, even though the bearing to the transmitter was constant. This might have been due to a DEADBAND zone that was adjusted to be excessively narrow, but there was a lot of activity going on at the moment, and I did not investigate it closely.

I suspect most of this behavior could be attributed to the (very crude) antenna system, which had only an "accidental" ground plane, provided by the co-ax shields... more antenna work might improve this performance, but it might also be necessary to simply gain more experience with the "behavior" of the DF.

In any event, it did not behave like a foolproof DF "appliance", and further experience and/or modifications will be required, to make it truly useful... probably both, but mostly experience, I suspect.


FURTHER FIELD TESTS

A second ( more extensive ) set of field tests revealed that the antennas cannot be effective if they are directly mounted on each earpiece, as was originally intended... The unit performed properly with an external antenna array, and with a ( temporary ) antenna array, ( mounted on the top of the headband ) but not with the antennas mounted on the earpieces.... It appears that the "shadow" of one antenna ( created by my head ) causes the RF signal on the "far side" antenna to come from a reflected source, instead of directly from the transmitter.... both antennas need a clear, unobstructed view of the horizon, in all directions, all the time.

The "temporary" antenna array consisted of a circular ground plane, ( diameter = 8 inches ) with two small antennas ( 3 inches high ) separated by a distance of only 3 inches. I had to increase the gain of the ERROR amplifier to compensate for the small antenna separation. This was done by increasing the values of R6 and R8 from 100K to 1.0 MEG. As before, the actual DF circuit performed properly, but the antennas posed the problems.

I intend to perform further tests with a much smaller ground plane, ( and antennas mounted on top of the headband ) to determine if the original concept is still viable... ( i.e. head - mounted antennas ) If it is, I also intend to enhance the orignal design by the addition of a PIC chip. It will generate an audio tone whose pitch is proportional to the size of the ERROR signal, and this audio will be mixed with the other audio sources. That will take a while, and the PC board layout will have to change.

Stay tuned...


[HOME PAGE][NEXT PAGE]