CA1216348A - Radio direction finder with enhanced null seeking - Google Patents

Abstract

ABSTRACT
A radio direction finder uses a pair of antennas (41,42) in which each antenna is skewed from a preferred null alignment (46) by a predetermined angle (0). The skewed antennas (41,42) receive signals at a level which is greater than the level that either antenna (41,42) would have if individually aligned in its null position (43).
Received signals detected by the antennas (41,42) are amplified in appropriate circuitry (51-54) and compared to one another at a comparator (55). When the antennas (41-42) are aligned so that their nulls are at an equal angle (0) from the direction (57) of a received signal, the comparator (55) indicates a minimum differential signal corresponding to the preferred null alignment (46) parallel to the received signal direction (57).
By detecting a signal strength with each antenna (41,42) aligned at an angle (0) from its null (43), the signal strength received by the antennas (41,42) is relatively high compared to signal strength at each antennas's (41,42) null alignment (43). The ability of the present invention to overcome noise further permits inexpensive direction finding techniques to be used with lower frequency radio signals.
(READS ON FIGURE 3)

Description

Disclosure 700-83-0010 RADIO DIRECTION FINDER WITH ENHANCED NULL SEEKING

BAC~GROUND OF THE INVENTION
This invention relates to radio direction finders of the null seeking type. More particularly, the invention relates to an antenna and receiver arrangement for enhancing accuracy in the null seeking function.
Radio navigation aids generally either interpret intelligence broadcast by radio navigation transmitters or determine the location of radio transmitters by measuring the signal strength of signals received from the transmitters through a directional receiving antenna whose directional alignment is rotated. The present invent-ion is concerned with the later type of navigation aid, called a radio direction finder (RDF). A typical radio direction finder uses a directional antenna which is rota-ted in order to obtain signal strength information. When the antenna is rotated slightly away from an optimum receiving position, radio signals below the ultrahigh frequency (UHF) band diminish only sliyhtly. Particularly in the lower radio frequencies, this slight difference is difficult to detect because such antennas have limited directional characteristics. At frequencies below lO MHz, radio receiving antennas usually include wound sections or "loading coils" used to shorten the physical length of resonant receiving elements. At lower frequencies, the coil sections are wrapped around ferrite cores in order to reduce the exterior dimensions of the antenna. For example, in a standard -3 ~

-2- Disclosure 700-~3-001 portable AM standard broadcast band (MW) radio receiver, a ferrite coil reduces the antenna size from over 100 feet to ~ithin the dimensions necessary to fit within the radio's enclosure.
It has been found that in radio direction finders operating at frequencies below the UHF band, the change in signal strength per increment of antenna direction rotation is more pronounced near the null points of reception than near the peak points of reception. This is conceptually the same as rotating an AM standard broadcast (M~J) radio until the signal is weakest. The radio station's direction can then be assumed to be such that the antenna must be turned 90 from the null in order to best receive the station. By basing an electronic "sighting" on the direction of a signal null, the signal-to-noise ratio is increased. This is because the background noise level remains fairly constant, while the detected signal strength at the null is, by definition, reduced. For this reason, there exists an ambiguity as to the true location of the null.
At remote locations, radio service is oFten limited, with radio signals in the very low frequency (VLF) spectrum being most easily received. This is particularly significant in Artic regions where radio navigation aids are scarce and magnetic compasses are useless.
For this reason, it would be advantageous to be able to use a radio direction finder which is able to operate within the VLF spectrum.
Present VLF navigation aids use phase relationship intelligence transmitted by a VLF station to provide positional information. An example is the intelligence transmitted by the Omega navigation aid system.

-3- Disclosure 700-83-0010 The present invention has, as its object, the enhancement of accuracy of radio direction finding using null-seeking techniques. It is furthermore important that such a radio direction finder be useful when homing on radio signals which do not transmit directional intelligence. It is further important that a radio direction finder in the lower frequency radio spectrums be constructed in a simple manner so that an accurate determination of direction can be made at a low cost.

SUMMARY OF THE INVENTION
In accordance with the present invention9 a pair of directional antennas are aligned so that their null points are directionally askew from one another. Signals from each antenna are amplified and the received signal strengths are then compared to one another. When-the signal strengths are equal, the antennas are considered to deviate from the null position by equal amounts, thus providing an indication of a relative direction of the null point.
Since directional sensing is obtained with the antennas askew from the null position, the signal-to-noise ratio of the received radio signal is increased over that obtained at the null position.
The use of a comparative output also provides an increase in the accuracy of signal strength measurements, even when the signal strength itself may flutter. The invention permits high accuracy in middle frequency and lower radio direction finding without using phase relationship intelligence from navigation aid transmitters~

3~
Specifically, the invention comprises a radio direction finder system which determines the direction of radio signals by determining the direction of a received signal null for -the system. The system includes a pair of substantially identical antennas each antenna having an antenna null alignment at which the antenna exhibits a minimum level, the two antennas being arranged so that their respective null alignments are at an acute angle to each other. Means are provided to select a signal to be received by -the antennas and means to separately detect the selected signals from each antenna, the means providing outputs corresponding to a signal level from each of the antennas with the outputs corresponding to each antenna. The outputs corresponding to each antenna being substantially equal for like signals reaching both antennas from like angles with respect to each antenna's null alignment. Means compare the outputs for indicating a possible system null indicated by the outputs being of equal magnitude wherein the radio direction finder uses the system null as a final reading of radio direction.

Accordins to a second aspect there is provided a method for determining a direction to a low frequency radio signal characterized by aligning a pair of substantially identical antennas having known null alignments, so that their respective null alignments are at acute angles to each other, selecting a signal to be received by the antennas, providing outputs of the selected signal corresponding to a signal gain from each antenna, comparing the outputs, establishing a possible system null direction when the outputs are at an equal magnitude, the system null being an indication Of signal direction.

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-4- Disclosure 700-83-0010 BRIEF DESCRIPTION OF THE DRAWINGS
. .
Figure 1 is a top view of a ferrite core antenna used in the present invention, superimposed over a graphic display of its directional characteristics;
5Figure 2 is a graphic representation of relative signal strength -of a signal received by the antenna of Figure 1 at different directions; and Figure 3 is a block diagram showing the configuration of a radio direction finder constructed in accordance with the present 10invention.

DETAILED DECRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment of the invention incorporates a pair of ferrite loop antennas 11, one of which is shown in a top plan view in Figure 1. The ferrite loop antenna 11 includes a ferrite loop 15portion 12 and a support portion 13. The support portion 13 is also used to connect the antenna 11 to an antenna lead-in cable (not shown).
The reception pattern of such an antenna is graphically represented by the dashed lines in Figure 1. The primary signal lobes 16 and 17 represent the antenna's gain for signals coming from various 20directions. Thus, if signals come from a direction of 90 from parallel to a center axis 19 of the loop 11, the loop 11 has a maximum resonance and a maximum gain. A similar maximum occurs in the opposite direction, that is, in the 270 direction. The antenna 11 exhibits the least gain in the 0 and 180 directions.

-5- Disclosure 700-83-0010 A signal coming From the maximum direction would be received with the antenna's maximum signal gain, represented on the chart by intersection point 21. It can be stated that such a signal is received from an angle of 90 from the antenna's center axis. If a signal originates from a difference of 10, for example, at an angle of 80, it would be received with an antenna gain, represented by intersection point 22, which is almost as great as that of a signal coming from the maximum signal strength direction (intersection point 21). Another signal coming from 20 from the maximum direction, for example, at 110, would also have a signal strength, represented by intersection point 23, which is likewise only slightly changed from the signal strength of the maximum and near maximum signals represented by intersection points 21 and 22. On the other hand, a signal originating from 10 from the null position would be received lS at a significan~iy reduced gain, represented by intersection point 24 for a signal coming from 10 from the null position. This signal strength is significantly different from the signal strengths for slightly different angles, such as a signal coming from 20 from the null, for example, from the 340 direction represented by 2~ intersection point 25.
Figure 2 shows the effect of received signal strength of the antenna of Figure 1 as the reception angle changes. As indicated by the top curved line 31, if maximum gain at 90 is indicated at the top of the graph, then as the direction of reception changes, the received signal exhibits a loss in strength until the null point is reached at either 0 or 180. On the other hand, signal noise, . -6- D i sc 1 osure 700-83-OOlO

indicated by line 33, rernains relatively constant. While the slope of signal strength line 31 changes significantly near the null points, the noise level remains high, resulting in a high signal-to-noise ratio. This creates an ambiguity on the graph between points A and B, here shown at 2 l/2 on either side of the 0 null point. In operation, the antenna shown in Figure l does not exhibit the perfect reception pattern shown in that figure. Most significantly, side lobes and other ambiguities are present. These effects increase rather than decrease the ambiguity of reception, particularly at the null points. For this reason, such ambiguities can be treated as part of the noise level indicated by line 33 in Figure 2.
By measuring a signal at an angle of lO frorn the null position, the slope of the signal strength line 3l is still significantly high, as indicated at points D and C in Figure 2. Point C corresponds to intersection point 24 in Figure l. Referring to Figure 2, at points C and D, the signal strength, indicated at line 3l, is significantly greater than the noise level, indicated at line 33. In the case shown, the signal-to-noise ratio is approximately ten decibels.
In obtaining radio reception over long ranges, signal strengths tend to vary with time. For this reason, the relative signal strengths of signals received at points ~D and C can be assumed to be equal only under certain circumstances, usually at the same time or within a short period of time. The present invention contemplates radio direction finding by obtaining signals at reception angles, such 3~

-7- Disclosure 700-83-0010 as the angles of points C and D, which are slightly deviated from a null point.
Referring to Fiyure 3, in the preferred embodiment of the invention, a pair of ferrite loop antennas 41,42 are disclosed so that their center axes, represented by dot-dashed lines 43, are separat.ed by an angle 20. While the null for each antenna falls along its respective center axis 43, a system null, represented by arrow 46, occurs in a direction separated by an angle 0 from each center axis 43. To the extent that each antenna 41,42 has an equal gain capability and a symmetrical gain pattern, a signal coming from the direction indicated by arrow 46 should produce the same gain in each antenna 41,42. The signals from antennas 41 and 42 are provided to preamplifiers 51 and 52, respectively and, from the preamplifiers 51 and 52, to receivers 53 and 54 respectively. The preamplifiers 51 and 52 and receivers 53 and 54 are adjusted so that a signal coming from the direction of arrow 46 produces an equivalent gain in the outputs of both receivers 53,54. The outputs of the receivers 53,54 are provided to a comparator 55. Therefore, when the receivers 53,54 receive a signal coming from the direction indicated by arrow 46, the comparator 55 would indicate an equivalent output from both receivers 53,54.
Arrow 46 represents the pre-selected null for the system and is fixed with respect to axes 43 of the antennas 41 and 42. Antennas 41 and 42 can be adjusted so that ~ is either increased or decreased in accordance with signal strength and signal-to-noise ratios of the system. Referring again to Figure 2, if 0 is set at 10 the ;3~

-8- Disclosure 700-~33-0010 signals processed through receivers 53 and 54 would occur at points D
and C on the graph. If 0 is changed to, for example, to 2 1/2 then the signals received by antennas 41 and 42 and processed by receivers 53 and 54 would occur at points A and B on the graph. A
preferred angle for the antennas 41,42 is 0 = 10. This angle 0 could range from 0 = 2.5 to 0 = 30. 0 = 5 to 0 = 20 is a preferred range for 0. It is also possible to operate the device as a conventional direction finder with the angle for the antennas 41,42 down to 0 = 0 by using signal strength measurements. The comparator would, of cowrse, not function at such angles.
The direction finder of Figure 3 will provide a zero difference output from the comparator 55 at four directions. Two o-F these indications will be nulls, indicated by double arrow 57 which is, of course, parallel to arrow 46. The two other directions will represent equivalent peaks which may be nearly perpendicular to the null directions 57. The null directions 57 `can be distinguished from the peaks by merely reading signal levels on one of the receivers 53 or 54. It should be noted that, by using two amplifiers and a comparator, the received signal can fluctuate with little effect on directional sensing.
It is possible to achieve similar results in a system using a single receiver (not shown). In that case, the receiver is switched from one antenna, such as antenna 41, to a second antenna, such as antenna 42. The outputs obtained from the two antennas are then compared. While the use of a single amplifier creates a real time difference between the reception of the signals from the two antennas, ;J,3~

-9- Disclosure 700-83-0010 such an arrangement has the advantage of providing the same amplifier gain for the signals from both antennas, because the same amplifier is used. It is, therefore, anticipated that a wide variety of modifications may be made to the preferred embodiment of the invention.
The preferred inventive system is operable with signals transmitted from Omega system transmitters, although intelligence transmitted with the Omega transmissions is not used. The system is also operable with VLF stations. It is possible to provide a similar system which is operable at other frequencies. Accordingly, the lo invention should be read as limited only by the claims.

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A radio direction finder system which determines the direction of radio signals by determining the direction of a received signal null for the system characterized by:
a pair of substantially identical antennas, each antenna having an antenna null alignment at which the antenna exhibits a minimum level, the two antennas being arranged so that their respective null alignments are at an acute angle to each other;
means to select a signal to be received by the antennas;
means to separately detect the selected signals from each antenna, the means providing outputs corresponding to a signal level from each of the said antennas with the outputs corresponding to each antenna, the outputs corresponding to each antenna being substantially equal for like signals reaching both antennas from like angles with respect to each antenna's null alignment; and means to compare the outputs for indicating a possible system null indicated by the outputs being of equal magnitude, wherein said radio direction finder uses said system null as a final reading of radio direction.

2. Radio direction finder as described in Claim 1, further characterized by:
the antennas being loop antennas.

3. Radio direction finder as described in Claim 1, further characterized by:
the antennas being ferrite loop antennas.

4. Radio direction finder as described in Claim, further characterized by:
the radio signals including signals in the frequency band used for transmitting Omega radio navigation signals.

5. Radio direction finder as described in Claim 1, further characterized by:
the means to separately detect including a single receiver for converting signals from one of the antennas to an output corresponding to the signal gain from that antenna and switch means to switch the receiver to receive signals from the other antenna.

6. Radio direction finder as described in Claim 1, further characterized by:
the means to separately detect including a pair of receivers, each of the receivers being connected to one of the antennas so that the outputs are provided simultaneously.

7. Radio direction finder as described in Claim 1, further characterized by:
the antennas being adjustable in position so that said acute angle can be varied.

8. Radio direction finder as described in Claim 1, further characterized by:
each antenna being aligned such that the antenna's null alignment with respect to the system null is between 2.5° and 30°.

9. Radio direction finder as described in Claim 1, further characterized by:
each antenna's null alignment with respect to the system null being between 5° and 20°.

10. Radio direction finder as described in Claim 1, further characterized by:
each antenna being adjustable in position so that said acute angle can be varied to less than 30°.

11. Radio direction finder as described in Claim 1, further characterized by:
the means to detect the selected signals including separate preamplifier means.

12. Radio direction finder as described in Claim 6, further characterized by:
the means to detect the selected signals including separate preamplifier means.

13. Method for determining a direction to a low frequency radio signal, characterized by:
aligning a pair of substantially identical antennas having known null alignments, so that their respective null alignments are at acute angles to each other;
selecting a signal to be received by the antennas;
providing outputs of the selected signal corresponding to a signal gain from each antenna;

comparing the outputs;
establishing a possible system null direction when the outputs are at an equal magnitude, said system null being an indication of signal direction.

14. Method as described in Claim 13, further characterized by:
simultaneously amplifying the signals from the antennas in order to provide outputs corresponding to the signal level at each antenna at the same moment in time.