FM direction finder
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08-06-2010, 11:18 PM

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FM Direction Finder has wide applications like navigation of ships, aircrafts, missiles, radar, tracking satellites and other astronomical radio sources. In the field of communications, certain requirements could be to cross check the intensity, direction and positioning of transmitters employed in a cluster of a GSM network . It might be used to locate the radio sources temporarily jamming any communication network. It is also used extensively in spectrum management for checking the growth of transmitters in specific regions like radio astronomical observatories, etc. Several radio direction finders have been built in the past. Different techniques have been employed in different instruments. These instruments could be broadly classified under vector-type or scalar-type of radio direction finders. The vector-types require both amplitude and phase information of the electromagnetic field at the antenna aperture, while the scalar-types require only amplitude information. In general, these instruments employ these measurements at various points in the three dimensional space.
The direction of radio signal is determined by applying some algorithms on these measured values. The direction finding could be either online or offline or both. The detected object may be a radio source or a passive device illuminated by electromagnetic radiations (like in RADAR). Majority of these instruments operate with small or medium bandwidths. With the increase of spectrum allotment to the communication channels and their growing numbers, the requirement of band coverage in radio direction finding has increased. Attempts have been made in recent years to broaden the frequency coverage.
In certain instruments based on the principle of radio interferometer, the intensity of the signal plays an important role in phase detection. If the signal to noise ratio is weak, the phase information might not be recovered correctly, especially when the source of the signal is amplitude modulated. On the other hand, the scalar-type radio direction finders might not be significantly accurate in pointing the direction, but might work at relatively low signal to noise ratios, and could also cover very wide range of frequencies. With the development of algorithms for categorically
analyzing the terrestrial spectrum, a requirement from the low frequency radio astronomy community grew for having a portable ultra wide band radio direction finder. This requirement was for cross verification of the direction of narrow and broad band radio sources. Based on the requirement, a scalar type of online radio direction finder was designed.
FM broadcast radio sends music and voice with higher fidelity than AM radio. In frequency modulation, amplitude variation at the microphone causes the transmitter frequency to fluctuate. Because the audio signal modulates the frequency and not the amplitude, an FM signal is not subject to static and interference in the same way as AM signals. Due to its need for a wider bandwidth, FM is transmitted in the Very High Frequency (VHF, 30 MHz to 300 MHz) radio spectrum. VHF radio waves act more like light, traveling in straight lines; hence the reception range is generally limited to about 50-100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.


Early direction finding technology required a movable directional loop antenna and a receiver. The arrival angle is measured by moving the antenna until a maximum or minimum signal strength is achieved. The simple loop antenna (Figure 3-1), or some derivative of it, can still be found today. One of the disadvantages of using this method is that the signal being measured is reduced, while noise from other directions is not. To overcome this problem, wave amplitude comparison (using two directional antennas with different orientations) was introduced.


It was not always feasible or desirable to physically move the antenna. As a result, some RDF technology used a fixed antenna that measured signal strength to provide an azimuth. One of the earliest of these systems used fixed crossed loops to feed a small orthogonal loop arrangement with a rotating loop inside of it. This was called a goniometry. Today, the term goniometry may refer to any type of mechanical or electrical cyclic sampling equipment.


The first cathode ray tube (CRT) direction finder used cross loops. Instead of using a goniometer, each loop was fed to a channel of a dual-channel receiver. The outputs from the receiver were applied to pairs of deflection plates within a special CRT. A clear signal produced a straight line, the angle of which gives the azimuth of the signal.

Until the end of World War II (WWII), the most common type of DF system was the crossed loop or Adcock antenna (Figure 3-3). This antenna system is still used in modified form for short-distance direction finding systems. The Adcock antenna used top horizontal members that were well shielded, to reduce polarization errors. These systems were small in relation to the wavelength of the received signals. They were therefore classified as narrow aperture direction finding (NADF) systems.
During WWII, the Wullenweber system heralded the era of wide aperture direction finding (WADF) systems . The Wulleweber has a circularly disposed antenna array (CDAA) up to 1,000 meters in diameter with a large number of elements. About a third of the elements are combined to form sum/difference beams. The beams are effectively rotated in azimuth by a rotating switch (goniometer) which connects and combines elements around the ring.


This systems are of this type. In theory, the Doppler systems impose a phase modulation on the received signal by A further variation of the Wullenweber system is called moving the antenna in a circle. The phase of the the Quasi-Doppler or Pseudo-Doppler system modulated signal is a function of its direction. In actual . This variation has also been called a practice, a fixed CDAA is used. The receiver rapidly commutated-antenna direction finder (CADF). Most of samples each antenna around the ring by using a the older groundbased EAC tactical direction finding goniometer switch.


Interferometric System Wavefront analysis accepts all signals but attempts to recover the major ones. It attempts this by analyzing the Interferometric systems area completely different class of direction finding systems. The azimuth of an incoming wave is not deduced by rotating beams. It is taken from the phase measurements of signals, made on a number of spaced antennas. Unlike the beam-forming type of WADFs, interferometers accept all signals on the array. Two different approaches are used to process the results. Depending on the type of system, one or both of the complex voltages measured on the elements of the antenna array under wave interference conditions. Wavefront testing accepts only signals arriving from one direction or quasi uni-modal propagation (QUMP). QUMP is achieved by detecting a linear phase shift across the array with near equal amplitudes on all the elements. (This process is also called coincidence interferometry.) For further information on linear phase following may be used: shift and measuring wave amplitude, see TM 11-666.Wavefront analysis (WFA). Wavefront testing (WFT).


There are time-difference direction finders (TDDF) which measure the difference between the times of arrival of the radio wave at a number of receiving sites. A hyperbolic position line is obtained from the time difference between each pair of sites.


The method employed is similar to the stereophonic identification of sound's direction by human beings using their ears. Two antennas possessing identical electrical properties (Ant-1 and Ant-2) with polarizations aligned and directivities subtend an angle a, are positioned symmetrically with positioned in geometrical By identical electrical properties, it is meant that the antennas possess identical bandwidths and for every frequency, the radiation patterns, gains, polarizations and impedances are identical symmetry with a radio source. The antennas subtend equal angles with the source, and hence the powers received by these antennas from the source should be equal. A more detailed with radiation. Consider one of the four radio sources 'A', 'B', 'C' and 'D' to exist at a time and influence the antennas. If P1 and P2 are the powers received by the antennas Ant-1 and Ant-2 respectively from the radio source 'A', then P1 = P2. Similarly for the radio sources 'B', 'C' and 'D', the antenna output powers could be related as P1 > P2, P1 = P2 and P1 < P2 respectively. To be noted that the condition P1 = P2 occurs if the source is either 'A' or 'C'. This is because the radiation patterns cross each other at two points, viz. one using the front lobes and other involving the back lobes. If the intensities and distances of each of the sources from antenna system are assumed to be identical, then the absolute values of powers received from source 'A' would be more than those received from source 'B'. In other words, (P1A = P2A) > (P1C = P2C), where the subscripts 'A' and 'C' represent the radio sources. For any other position of the radio source P1 not equal to P2. The back lobes of the antennas could be further reduced by placing geometrically symmetric reflectors behind each antenna, such that (P1A = P2A) _ (P1C = P2C).
This could enable one to identify the position of the source (whether at front or at back). In the design, the angular separation a between the two antennas is chosen such that the gains of the major lobes at the points
of intersections are is less than 3 dB. The back reflector plates are attached behind each antenna such that the gain ratio of front to back intersection points is at least 5 dB or higher.


Figure 2 shows the block diagram of the system. Two electrically identical ultra wide band log periodic dual polarized antennas are mounted in an angle and backed by a common V-shaped reflector, such that the radiation pattern of either antenna is the mirror image of the other. The entire assembly of the antennas could be manually rotated between 0 and 360° for locating the radio source. A compass is attached to this assembly to indicate the direction of the source. The outputs from the antennas are amplified by two RF ultra wide band amplifiers^. The amplified signals are selected one at a time and fed to a spectrum analyzer. This is achieved using an SPDTRF switch controlled using a JK flip-flop. The clock to the JK flipflop (or the switching frequency) is controlled by a manually tunable square wave oscillator.
The spectrum analyzer is set to the required frequency range of observation. Table 1 lists the specifications of the devices and modules used.
Spectru Analyz
I Antenna Assembly Electronics Unit
Figure 3.2. Block diagram of the system


Direction finders are normally equipped with vertically polarized antennas, making it impossible for them to perform accurate direction finding when they encounter signals with strictly horizontal polarization. For example, this is what happens in direction finding involving FM and TV transmitters which are commonly equipped with horizontally polarized antennas. Normally, of course, there is no need for direction finding with FM and TV transmitters since their locations are well known. However, in the case of illegal transmitters using horizontally polarized transmitting antennas, vertically polarized DF antennas and triangulation do not work. In these cases, DF antennas with vertical and horizontal polarization are needed. One obvious (but very poor) solution would be to simply rotate the vertically oriented dipole antenna elements by 90° so that they are horizontal. However, this results in an overly directional receiving characteristic. The DF accuracy and sensitivity would be inadequate in certain directions and it would not be possible to aurally monitor signals from those directions. Around the world, direction finders used for locating transmitters are typically equipped with a vertically polarized DF antenna. These DF antennas usually consist of multiple vertical dipole antennas arranged in a circular array.
Direction finders with vertically polarized antennas are not capable of accurately taking bearings on signals with strictly horizontal polarization. This is the case, for example, in DF applications involving FM and TV transmitters which are usually equipped with horizontally polarized transmitting antennas and mounted on high masts for better coverage. If the DF antenna is also located in an elevated position on a mast or on a roof, it will have more or less line-of-sight contact with the transmitting antenna Under these circumstances, erroneous results can be produced as the undistorted, horizontally polarized FM / TV signals reach the vertically polarized DF antenna. In addition to the direct wave, the DF antenna also receives reflected waves with a combination of vertical and horizontal polarization. Direction finders are normally better at measuring the vertical components of reflections than the directly received signal. This can produce extremely erroneous results due to the reflections. However, the poor DF quality usually provides a warning about this problem when it is present.
Fig.4.1 FM Finding Antenna
Direction finding often requires an antenna that is directional - that is, more sensitive in certain directions than in others. Many antenna designs exhibit this property. For example, a Yagi antenna has quite pronounced directionality, so the source of a transmission can be determined simply by pointing it in the direction where the maximum signal level is obtained. However, to establish direction to great accuracy requires much more sophisticated techniques.
The crossed-loops DF antenna atop the mast of a tug boat.A simple form of directional antenna is the loop aerial. This consists of an open loop of wire on an insulating former, or a metal ring that forms the antenna elements itself, where the diameter of the loop is a tenth of a wavelength or smaller at the target frequency. Such an antenna will be LEAST sensitive to signals that are normal to its face, and MOST responsive to those meeting edge-on, this due to the antenna sensing the difference between the voltages induced either side of it at any instant because of the phase output of the transmitting beacon. Turning the loop face on will not induce any current flow - think of the radio wave slipping through the loop. Simply turning the antenna to obtain minimum signal will establish two possible directions from which the signal could be emanating. The NULL is used, as small angular deflections of the loop aerial near its null positions produce larger changes in current than similar angular changes near the loops max positions. For this reason, a null position of the loop aerial is used. To resolve the two direction possibilities, a sense antenna is used, the sense aerial has no directional properties but has the same sensitivity as the loop aerial. By adding the steady signal from the sense aerial to the alternating signal from the loop signal as it rotates, there is now only one position as the loop rotates 360 Degs at which there is zero current. This acts as a phase ref point, allowing the correct null point to be identified, thus removing the 180 Deg ambiguity. A dipole antenna exhibits similar properties, and is the basis for the Yagi antenna, which is familiar as the common VHF or UHF television aerial. For much higher frequencies still, parabolic antennas can be used, which are highly directional, focusing received signals from a very narrow angle to a receiving element at the centre.

The top view, front view, back view and side view are respectively. The log periodic antennas have been constructed on glass epoxy PCB substrate and are mountedover the V-shaped aluminum reflector. 'H' and 'V' indicate the J At a given time, only one polarization (vertical or horizontal) of the antennas are selected manually horizontal and vertical polarization RF-connections for each antenna. These output connections are provided using chassis-mountable SMAtype 50 Ohm connectors. The compass has been mounted rigidly to the antenna system so that it rotates along with the assembly. A video camera stand has been used as the base of the antenna assembly. Theantenna system could be manually rotated and positioned anywhere between 0 and 360° in the azimuth over the stand.

For any operation, either the vertically or horizontally polarized antenna pair is selected. Figure 4 shows the complete experimental setup. For detecting the radio direction of any signal, the spectrum analyzer's span is set such that the signal is clearly visible on the screen. The operator should stand behind the antenna assembly so that the radio source whose direction is under determination is not blocked. On the spectrum analyzer's screen, the power level of the transmitted signal would be seen to toggle in two states at the oscillator frequency. These two levels indicate the outputs from the two amplifiers.
Fig. 6.1 experiment setup of direction finding
The complete experimental setup. require adjustments so as to keep both the toggling power levels within the screen. The toggling frequency could be controlled by manually tuning the oscillator such that they are distinctly visible to the human eye. The antenna assembly should be slowly rotated (in
azimuth) until the difference between the two toggling power levels get minimized (preferably zero). Since there are two possible angles at which the power levels equalize (shown in Figure 1), care should be taken to choose the angle at which the individual antennas deliver larger powers. Under this condition, the reading on the compass would directly indicate the direction of the incoming signal.


It is impossible, using amateur techniques, to pinpoint the whereabouts of a transmitter from a single receiving location. With a directional antenna you can determine the direction of a signal source, but not how far away it is. To find the distance, you can then travel in the determined direction until you discover the transmitter location. However, that technique can be time consuming and often does not work very well. A preferred technique is to take at least one additional direction measurement from a second receiving location. Then use a map of the area and plot the bearing or direction measurements as straight lines from points on the map representing the two locations. The approximate location of the transmitter will be indicated by the point where the two bearing lines cross. Even better results can be obtained by taking direction measurements from three locations and using the mapping technique just described. Because absolutely precise bearing measurements are difficult to obtain in practice, the three lines will almost always cross to form a triangle on the map, rather than at a single point. The transmitter will usually be located inside the area represented by the triangle. Additional information on the technique of triangulation and much more on RDF techniques may be found in recent editions of The ARRL Handbook.

Required for any DF system are a directive antenna and a device for detecting the radio signal. In amateur applications the signal detector is usually a transceiver and for convenience it will usually have a meter to indicate signal strength. Unmodified, commercially available portable or mobile receivers are generally quite satisfactory for signal detectors. At very close ranges a simple diode detector and dc microammeter may suffice for the detector. On the other hand, antennas used for RDF techniques are not generally the types used for normal two-way communications. Directivity is a prime requirement, and here the word directivity takes on a somewhat different meaning than is commonly applied to other amateur antennas. Normally we associate directivity with gain, and we think of the ideal antenna pattern as one having a long, thin main lobe. Such a pattern may be of value for coarse measurements in RDF work, but precise bearing measurements are not possible. There is always a spread of a few (or perhaps many) degrees on the nose of the lobe, where a shift of antenna bearing produces no detectable change in signal strength. In RDF measure-ments, it is desirable to correlate an exact bearing or compass direction with the position of the antenna. In order to do this as accurately as possible, an antenna exhibiting a null in its pattern is used. A null can be very sharp in directivity, to within a half degree or less.


Radio transmitters for air and sea navigation are known as beacons and are the radio equivalent to a lighthouse. The transmitter sends a Morse Code transmission on a Long wave (150 - 400 Khz) or Medium wave (AM) (520 - 1720 Khz) frequency incorporating the station's identifier that is used to confirm the station and its operational status. Since these radio signals are broadcast in all directions (omnidirectional) during the day, the signal itself does not include direction information, and these beacons are therefore referred to as non-directional beacons, or NDBs.
As the commercial medium wave (AM) broadcast band lies within the frequency capability of most DF units, these stations and their transmitters can also be used for navigational fixes. While these commercial radio stations can be useful due to their high power and location near major cities, there may be several miles between the location of the station and its transmitter, which can reduce the accuracy of the 'fix' when approaching the broadcast city. A second factor is that some AM radio stations are omnidirectional during the day, and switch to a reduced power, directional signal at night.
DF was once the primary form of aircraft and marine navigation. Strings of beacons formed "airways" from airport to airport, while marine NDBs provided navigational assistance to small watercraft approaching a landfall. In the 1950s the aviation NDBs were augmented by the VOR system, in which the direction to the beacon can be extracted from the signal itself, hence the distinction with non-directional beacons. Use of marine NDBs was largerly supplanted in North America by the development of LORAN in the 1970s.
Today many NDBs have been decommissioned in favor of faster and far more accurate GPS navigational systems. However the low cost of ADF and RDF systems, and the continued existence of AM broadcast stations (as well as navigational beacons in countries outside North America) has allowed these devices to continue to function, primarily for use in small boats, as an adjunct or backup to GPS.
Fig. 8.1 FM Direction Finder


The radiation patterns of the two antennas for every frequency do not produce identical gain at the zero axis of the antenna system. Thus the actual direction of the incoming signal might contain some error. These errors might be corrected manually from the radiation patterns. TV and GSM transmitters detected at the GMRT site. The measured directions and the actual directions have been compared and the percentage errors are calculated. The use of radio for direction-finding purposes (RDF) is almost as old as its application for communications. Radio amateurs have learned RDF techniques and found much satisfaction by participating in hidden-transmitter hunts. Other hams have discovered RDF through an interest in boating or aviation, where radio direction finding is used for navigation and emergency location systems. In many countries of the world, the hunting of hidden amateur transmitters takes on the atmosphere of a sport, as participants wearing jogging togs or track suits dash toward the area where they believe the transmitter is located. The sport is variously known as fox hunting, bunny hunting, ARDF (Amateur Radio direction finding) or simply transmitter hunting. In North America, most hunting of hidden transmitters is conducted from automobiles, although hunts on foot are gaining popularity. There are less pleasant RDF applications as well,such as tracking down noise sources or illegal operators from unidentified stations. Jammers of repeaters, traffic nets and other amateur operations can be located with RDF equipment. Or sometimes a stolen amateur rig will
be operated by a person who is not familiar with Amateur Radio, and by being lured into making repeated transmissions, the operator unsuspectingly permits himself to be located with RDF equipment. The ability of certain RDF antennas to reject signals from selected directions has also been used to advantage in reducing noise and interference. Through APRS, radio navigation is becoming a popular application of RDF. The locating of downed aircraft is another, and one in which amateurs often lend their skills. Indeed, there are many useful applications for RDF. Although sophisticated and complex equipment pushing the state of the art has been developed for use by governments and commercial enterprises, relatively simple equipment can be built at home to offer the Radio Amateur an opportunity to RDF. This chapter deals with antennas suitable for that purpose.


1. FAA (2003). Pilot's Handbook of Aeronautical Knowledge. US Dept. of Transportation.
2. Civil Aviation Safety Authority. "[ Operational Notes on Non-Directional Beacons (NDB) and Associated Automatic Direction Finding (ADF)]" (PDF). Retrieved August1, 2008.
3. Bob Tait (2008). CPL Navigation. Bob Tait's Aviation Theory School.
4. Smith, D.J. (2005). Air Band Radio Handbook (8th Edition). Sutton Publishing. p. 104-105. ISBN 0-7509-3783-1.
5. {Keen R, Wireless Direction Finding(8th Edition), 1947, Iliffe, London
6. deRosa, L.A. (1979). "Direction Finding". in J.A. Biyd, D.B. Harris, D.D. King & H.W. Welch, Jr.. Electronic Countermeasures. Los Altos, CA: Peninsula Publishing.
7. J. Hereford and B. Edgerly (2000). "457 kHz Electromagnetism and the Future of Avalanche ([dead link] _ Scholar search). International Snow Science Workshop (ISSW 2000). bcaccessdocuments/457andFuture_000.pdf.
8. Titterington, B.; Williams, D. and Dean, D. (2007). Radio Orienteering - The ARDF Handbook. Radio Society of Great Britain. ISBN 9781-9050-8627-6.
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