Antenna low-noise Q spoiling circuit

An input coupling circuit for detuning the Q of a high-Q ferrite rod antenna is disclosed. A high-impedence low-noise amplifier is used to amplify the output signal from the LC resonant tank of the rod antenna. A portion of the amplified signal is fed back directly into the magnetic circuit of the antenna tank. This negative feedback reduces the losses produced in the magnetic circuit due to the presence of nearby conductors. As a result of this reduction, an increase in the antenna sensitivity and a decrease in the cross-feed from other nearby antenna is produced.

BACKGROUND OF THE INVENTION 
This invention relates to antenna input detecting circuits. More 
particularly, it relates to a ferrite rod antenna input coupling circuit 
for detuning the Q of the antenna to achieve both a broadband frequency 
response and an increase in the antenna's sensitivity when positioned near 
conductive parts, i.e. metal supports, printed circuit copper lands, 
chassis, etc. 
It is known in the prior art that the Q of an antenna's input resonant tank 
circuit may be reduced (spoiled) by applying a portion of the detected 
antenna signal as negative feedback into the resonant tank circuit. By 
reducing the Q of the antenna resonant tank detecting circuit, a broadband 
frequency response is obtained. U.S. Pat. No. 2,787,704 illustrates the 
use of negative feedback to achieve a constant band-width input frequency 
response for a high-Q rod antenna. In this reference, the output from the 
rod antenna resonant tank circuit is buffered by a vacuum tube amplifier 
and a portion of the buffered signal applied through a feedback 
transformer into the resonant tank circuit. The secondary of the feedback 
transformer is connected in series with the tank inductor formed around 
the ferrite rod. Special tuning capacitors in the feedback circuit vary 
the feedback ratio of the feedback signal according to the frequency 
detected by the antenna to achieve a constant bandwidth regardless of the 
center frequency to which the antenna is tuned. 
Some of the advantages of active Q-spoiling are also disclosed and 
discussed by this reference. That is, the sensitivity of the antenna is 
not diminished even though the Q of the antenna is reduced. (As understood 
by those skilled in the art, the antenna sensitivity or signal-to-noise 
ratio of its output signal, which is expressed in micro-volts per meter, 
refers to the amount of external magnetic field required to increase the 
antenna output signal by a factor 1.414 or 3db over the noise level when 
no external fields are present). This is true because the high input 
impedance of the active amplifier does not appear as a resistive load to 
the resonant tank circuit. Resistive loading of a resonant tank reduces 
the Q but increases the noise in the antenna output signal. Another 
advantage discussed by this reference relates to the broadband frequency 
response resulting from the detuning of a high-Q circuit. The negative 
feedback voltage in series with the resonant tank voltage reduces the Q of 
the tank to effectively open up the frequency band to detect more 
frequencies which occur near the resonant center frequency. This enables 
the antenna to effectively respond to several relatively separated 
frequencies. 
One of the main advantages of a ferrite rod antenna is that it may be 
contained physically in quite a small volume. Therefore, in receivers 
which utilize rod antennas, it is inevitable that these antennas will be 
placed near other components, antenna's and metal parts. The resultant 
degradation of performance in the antenna's packaged configuration 
compared with that obtained when the rod antenna is isolated may amount to 
a having of the Q-factor of the antenna. In addition, the sensitivity of 
the antenna (signal-to-noise ratio of the detected output signal) is 
likewise reduced. When more than one rod antenna is placed in the same 
physical area, such as when two antennas are placed adjacent and 
orthogonal to one another, a problem of cross-talk between the antennas is 
created. This cross-talk between antennas produces undesirable signal 
responses in both antennas. In order to avoid this cross-talk problem, 
prior art receivers have resorted to elaborate configurations for the 
antennas. One such technique involves the use of four high-permeability 
ferromagnetic rods arranged in a square with the resonant tank coils for 
opposite rods interconnected to form a single effective rod antenna. For 
this technique, the physical configuration of the ferrite rods is critical 
in order for the antenna to maintain its desired frequency response and, 
at the same time, to minimize the cross-talk. 
A long range navigation system which employs a plurality of transmitters 
transmitting on discrete frequencies, such as the Omega Navigational 
System, requires a signal receiver whose antenna is capable of detecting 
each of the very low frequency (VLF) signals that are transmitted. In such 
a receiver, two orthogonally positioned ferrite rod antennas are required 
in order to produce a detected signal regardless of the orientation of the 
receiver. 
Accordingly, it would be desirable to provide a rod antenna input coupling 
circuit which enables the antenna to have a broadband frequency response 
while minimizing the cross-talk between adjacent rod antennas, and to have 
an increased sensitivity by reducing the antenna's internal losses due to 
the presence of nearby conductors. 
SUMMARY OF THE INVENTION 
In accordance with this invention, an antenna input coupling circuit for 
detuning the Q of a high-Q rod antenna resonant tank circuit is provided. 
Detuning of the Q of the antenna resonant tank by the present invention 
increases the antenna sensitivity by desensitizing the antenna to the 
presence of nearby conductors. A parallel inductor-capacitor resonant tank 
circuit is used to produce an antenna detect signal in response to the 
magnetic field generated in the ferrite rod by the electro-magnetic field 
detected by the antenna. A high input impedence amplifier is used to 
amplify the antenna detect signal. Associated with the ferrite material of 
the rod antenna is a second feedback inductor which is magnetically 
coupled to the magnetic field present in the resonant tank inductor. A 
feedback resistor is connected between the output of the amplifier and the 
feedback inductor to apply a portion of the amplifier output signal back 
to the magnetic circuit of the resonant tank. The signal fed back is in a 
negative sense. The necessary phase inversion to achieve negative feedback 
may be accomplished by the arrangement of the turns of the feedback 
inductor around the ferrite rod material or it may be produced by 
inversion through the amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 which is a circuit diagram of the preferred embodiment 
of the present invention, a high permeability ferrite rod M is shown 
magnetically coupled to inductors L1 and L2. That is, the magnetic flux in 
rod M passes through both inductors. Both inductors L1 and L2 consist of a 
number of turns wrapped around the ferrite rod M. Connected in parallel to 
inductor L1 is a parallel combination of capacitors C1 and C2. Capacitors 
C1, C2 and inductor L1 comprise the rod antenna LC resonant tank circuit 
3. The magnetic field produced in the ferrite material M by the 
electromagnetic field detected by the antenna generates the antenna detect 
signal. The antenna detect signal is inputted to a negative feedback means 
consisting of amplifier 4 and coupling means 5. The negative feedback 
means functions to produce the antenna output signal 2 by amplifying the 
antenna detect signal 1 and to feed a portion of the amplified signal back 
into the magnetic circuit of the resonant tank 3. 
For the preferred embodiment, amplifier 4 consists of a field effect 
transistor Q1 coupled to an operational amplifier A1. The high-impedance 
low-noise field effect transistor Q1 is used both to provide amplification 
and to present a high-impedence load to the antenna detect signal 1. The 
amplified signal from Q1 is capacitively coupled through capacitor C3 to 
the inverting input of operational amplifier A1. The series circuit 
consisting of resistor R3, capacitor C4 and resistor R4 is connected to 
the non-inverting input of amplifier A1. This circuit provides an 
identical input impedence circuit to the non-inverting input as is present 
on the inverting input in order to balance the input currents to amplifier 
A1. Operational amplifier A1 operates as an inverting amplifier whose 
closed loop gain is determined by the ratio of R5 to R1. A voltage divider 
network consisting of resistors R6 and R7 is used to apply a portion of 
the amplified antenna detect signal 2 back into the field effect 
transistor amplifier circuit via capacitor C5. As a result, the closed 
loop gain of the amplifier 4 is controlled by the ratio of resistors R6 
and R7. For the preferred embodiment, the close loop gain of amplifier is 
approximately 40 db. 
Also connected to the output of amplifier 4 is a coupling means 5 
consisting of a feedback resistor Rf and inductor L2. As previously 
mentioned, inductor L2 is coupled to the magnetic material M so that it 
responds to the same magnetic field as inductor L1. The turns of inductor 
L2 are applied to the ferrite rod M in the same sense as inductor L1. That 
is, the voltage applied to inductor L2 will produce a voltage on L1 that 
is in phase with the voltage on L2. Because of the inversion that is 
present in amplifier 4, the magnetic field produced by the current in L2 
will be 180.degree. out of phase with the antenna detect signal 1. The 
magnitude of feedback resistor Rf controls the amount of current applied 
to inductor L2 thereby controlling the amount of signal that is fed back 
into the antenna's magnetic circuit. 
While the preferred embodiment shows an inverting amplifier 4 to produce 
the necessary phase inversion to achieve negative feedback, it is obvious 
to a person of ordinary skill in the art that other ways are possible to 
achieve negative feedback, such as the use of a non-inverting amplifier 4 
with a reversal in the sense of inductor L2 relative to inductor L1. 
Additionally, it is obvious that other ways of controlling the amount of 
signal fed back other than by the use of the feedback resistor in series 
with inductor L1 are possible. 
As explained above, the sensitivity of an antenna is defined to be the 
amount of external magnetic field, expressed in micro-volts/meter, needed 
to increase the antenna output signal by a factor of 3 db. This 
sensitivity may be measured by isolating the antenna from all external 
magnetic fields and nearby conductors and measuring the noise level 
present on the output of amplifier 4. Having measured this voltage, the 
antenna is subjected to a magnetic field of the appropriate frequency to 
produce an antenna output response signal. The level of magnetic field is 
increased until the measured voltage on the output of amplifier 4 
increases by a factor of 3 db. At this point, the amount of required 
signal to produce the desired antenna output signal above the level of 
noise when the antenna was isolated is the measure of the antenna's 
sensitivity. If the antenna resonant tank is unloaded and the antenna is 
isolated as described above, the sensitivity of the antenna will be the 
same as when the antenna is isolated and there is Q spoiling (loaded). 
However, if the antenna is placed in the proximity of electrical 
conductors, and the sensitivity measurement is made for both the unloaded 
and the Q spoiled condition according to the present invention, an 
improvement in the antenna sensitivity will be measured. That is, the 
losses in the ferrite rod due to the proximity of conductive parts is 
reduced when negative feedback of the amplified antenna detect signal 2 is 
fed back into the magnetic circuit of the rod antenna. Nearby conductors 
respond to the magnetic field of the rod antenna by producing eddy 
currents in their conductive surfaces. These eddy currents, in turn, 
represent magnetic losses in the magnetic circuit. The resultant 
degradation of performance compared with that obtained when the rod 
antenna is isolated may amount to a having of the Q factor. Because the 
feedback signal of the present invention is applied to the magnetic 
circuit in a negative sense, the magnitude of the induced eddy currents in 
nearby conductors is significantly reduced--as much as 30:1. The eddy 
current induced losses in the magnetic material M appears in the antenna 
output signal as noise. Because the antenna is desensitized to the 
presence of the nearby conductors and to the presence of other nearby 
antennas, the noise level is reduced. Accordingly, the amount of external 
magnetic signal needed to produce the 3 db increase in the antenna's 
output signal above this noise is reduced. Hence, an improved sensitivity. 
It should be clear that the values of the various circuit components 
depicted in the drawing and described above will vary in dependence upon 
the intended use. In a presently preferred embodiment of a Q spoiled rod 
antenna used in a receiver to detect the radio signals in the Omega 
Navigational System, the table below sets out exemplary values which have 
been found satisfactory. 
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TABLE OF EXEMPLARY VALUES 
Reference Designation 
Value 
______________________________________ 
M ferro-magnetic high perme- 
ability material 
Q1 2N 6550 
A1 4136 by Raytheon 
L1 1200 turns of #30 wire 
L2 2 turns 
C1 1800 pf (C1 + C2 
C2 select for center 
nominal = 
frequency 2100 pf) 
C3 2.2 micro-farads 
C4 2.2 micro-farads 
C5 10 micro-farads 
R1 1.21 K 
R2 510 
R3 1.21 K 
R4 18.7 K 
R5 42.2 K 
R6 1.21 K 
R7 20 ohms 
Rf 1.5 K (selected for desired 
Q) 
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In describing the invention, reference has been made to its preferred 
embodiment. However, those skilled in the art and familiar with the 
disclosure of the invention may recognize additions, deletions, 
substitutions or other modifications which would fall within the purview 
of the invention as defined in the appended claims.