US3604008A - Tunable moderate-q transmitting antenna - Google Patents

Abstract

A tunable moderate-Q transmitting antenna. The antenna can be operated in proximity with a plurality of like antennas in a limited space environment with minimal mutual interference manifested as absorbed power or distorted radiation patterns. The antenna essentially comprises a low-profile, top-loaded short monopole which can be operated above its intrinsic resonant frequency and which can be tuned remotely over a predetermined frequency band by means of a ''''single-element'''' tuner.

Description

United States Patent [72] Inv k A-Tllovlefi 3,253,279 5/1966 Tanner 343/749 X 2] A i N 231; OTHER REFERENCES [RE Performance of Short Antennas by Smith & John- 1 Filed Jan. l4, I970 son, 1947, copy in Op. 255, 343- 752 pages 1029- I038 relied [45] Patented Sept. 7,197] v I 35 N 100 be 1947 {73] Assignee The Unlted States of American upon 0 o represented by the Secretary of the Navy Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. 7 V i Attorney.r-Richard S. Sciascia, G. .l. Rubens and J. W.

"AM McLaren [54] 'IUNABLE MODERATE-Q TRANSMITTING ANTENNA 2Claims,4l)rawlngFigs. y v H y [52] U.S.Cl 343/745, ABSTRACT: A m: modcratcq transmitting Manna 343/749 343/752, 343/843 The antenna can be operated in proximity with a plurality of Illt. n antennas in a space environment minimal [50] Field olSearch 343/745, mutual i t f if t d as b d power or 746, 747,749,750, 752, 848,849, 846, 791 distorted radiation patterns. The antenna essentially comprises a low-profile, top-loaded short monopole which can be [56] References Cited operated above its intrinsic resonant frequency and which can UNXTED STATES PATENTS be tuned remotely over a predetermined frequency band by 2,800 ,658 7/195] 7 Long means ofasingle-element tuner. IO)

PATENTED SEP 7 ml SHEET 1 BF 2 4 3 .Y u i. C

Ln-"q g FIG.

INVENTOR.

E C A. THOWLESS ATTORNEYS PATENTEDSEI 1m 3.604.008

sum 2 or 2 0.000 0.850 0.990 L00 L05 I. I0 L15 I. 20

FIG. 4

INVENTOR- ERIC A. THOWLESS ATTORNEYS TUNABLE MODERATE-Q TRANSMITTING ANTENNA STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION A current requirement exists for a tunable, high frequency transmitting antenna which can be utilized effectively in a limited space environment. Such an antenna must have sufficient selectivity to pennit installation and operation of several of these antennas in proximity with minimal interference manifested as absorbed power or distorted radiation patterns. Existing antennas are not capable of providing this required performance since they cannot suppress reradiation of in cident radio waves.

Broadband monopole-type antennas having more than one transmitter connected thereto by use of multicouplers have been thus utilized. However, design problems and costs involved in the use of high-power tunable multicouplers have limited such installations to the use of fixed-tuned, highpaaIlow-pass filters having power levels of 40 kw. PEP. Furthermore, high-pass/low-pass filters preclude use of a frequency band of about percent at the crossover frequency. Also, the number of high-power transmitters which can be used is limited by the average power rating of a coaxial transmission line.

SUMMARY OF THE INVENTION A tunable, moderate-Q transmitting antenna which can be installed and operated in proximity with a plurality of similar antennas with minimal mutual interference manifested as absorbed power or distorted radiation patterns. The antenna essentially comprises a low-profile, top-loaded, short monopole antenna. The antenna can be operated above its intrinsic selfresonant frequency; thus the intrinsic impedance of the antenna is always inductive. Consequently, the antenna can be remotely or manually tuned over a predetermined frequency range by means of a single-element" tune. The moderate-Q (selectivity) and the low-profile of the antenna enable it to be operated in proximity with a plurality of like antennas.

STATEMENT OF THE OBJECTS OF THE INVENTION It is the primary object of the present invention to provide a tunable, moderate-Q transmitting antenna which can be installed and operated in proximity with a plurality of like antennas with minimal mutual interference.

It is another object of the present invention to provide a tunable, high frequency antenna which can be operated above its intrinsic self-resonant frequency.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a simplified side view of a typical antenna structure embodying the inventive concept of the present invention;

FIG. 2 is a schematic representation of the "single-element" tuning circuit of the present invention;

FIG. 3 is a Smith chart representation of the intrinsic impedance and the tuning behavior ofa typical antenna embodying the present inventive concept;

FIG. 4 is a graphical representation of typical response curves ofthe preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 represents a simplified side view of a typical trans mitting antenna structure I0 embodying the inventiye'conce t disclosed herein. The antenna structure 10 comprises a short monopole represented in FIG. I by the vertical radiating structure 12. The structure, or conducting mast, I2 shown symmetrically supported about a rigid coaxial transmission line 14. The transmission line is mounted upon a supporting base portion 16 of a conventional ground plane The ground plane 18 consists of, for example, I20 equi spaced copper wires, 20, which are radially disposed abouit' the base 16. A suitable adapter 22 is attached to the bottoniend of the transmission line 14 to mate it with the transmissioi line run (not shown) from transmitting apparatus (not shown). The structure 12 is shown as comprising four yertically disposed wires (or rigid metal supports) 24, having the {op "ends thereof connected to and extending from each of thefdur bot torn comers of a rectangular capacitive tuner 6 which is symmetrically disposed and supported at the top end of the rigid coaxial cable 14 or other suitable rigid rnetallic support means. The bottom ends of the aforementioned fqui' vertical wires (or rigid metallic supports) are mechanically electrically connected to the supporting base In of the ground plane I8.

Vertically disposed and rigidly connected on top side of the tuner box 26 is the upper portion 28 of the vertical radiating structure 12. The upper portion 28 consists ot'a' (or similar metal) pipe which supportslZ equispaced, cally arranged, tophat radial wires 30. It should that the lower end of the brass pipe 2 8 or condiictor, to the energy feed point 32 of the antenna stru'cjtn 'e 10.

The 12 tophat radial wires are rigidly su k rted abo it the vertical radiating structure I2 by means of ag'ny connected to each radial wire through a conyeritio i I tor 36. n1: opposite end at l gu is rigid aits '9' suitable nonconductive, supporting post B S'sue tha'l 30 extend radially outward from the top of ihe downward to said supporting posts.

In a typical structure constructed according tn concept disclosed herein, the overall of the litieal structure can be approximately l4 feet for an aiiteiina ble over a c range of 4 nc/sec. to 8 mcjs'ecflfli effective length (i.e., the length from the transmission I! to the support post 38 of the tophat radial wires) a gnration would be approximately 32 feet. Hence it can readily seen and appreciated that the antenna structure 10 comprises a low-profile (silhouette) antenna structure which due to its high degree of selectivity can be utilized to advantage where space considerations are critical to minimize deleterious reradiation effects.

As is well known, pattern distortion of the radiating antenna by another antenna or structure in proximity is caused by the effect of reradiated energy from the latter. Two separate currents may be considered to exist on an antenna 'receiving' an incident signal. One current is called the unloaded current. This is the current with a distribution upon the antenna that would exist for the unloaded antenna, that is, for a load impedance of 0 ohms. This current and its distribution are a function of the antenna geometry and of the orientation of the incident plane wave in relation to the receiving antenna, and are unrelated to the transmit current distribution of the na when it is driven at its feed point. The moderate-0 ari'tenna structure has a low profile; hence the short effective height of the antenna functions to minimize the unloaded current.

The other current which exists on an antenna receiving an incident signal is called a loaded current. This is the current permitted by the antenna impedance when driven by a voltage across the load impedance. This current will have the distribution ofa driven antenna. At the incident frequencies, (HF) the structure will not be resonant, that is, the complete antenna structure including the tuner will be detuned. The loaded cur rent will be small because of the antenna's selectivity.

Reradiation from the moderate-Q antenna will be weak at frequencies away from resonance. The amount of power being reradiated (scattered) and the distribution of power in dif ferent parts of the distant universe are unknown and not capable of being calculated.

Antenna patterns were obtained of a radiating moderate-Q antenna located in a cluster of models of arbitrarily self-resonant, moderate-Q antennas with a spacing of about 0.44 wavelength at the lowest frequency. Patterns were also taken of a similar cluster of monocone antennas having a lower frequency spacing of one wavelength. The radiation patterns of a moderate-Q antenna exhibited much less pattern distortion than did the monocone, even though the moderate-Q antennas were located much closer together.

For further comparison, measurements were also made of the radiation patterns of a simple, short, vertical whip antenna when (1) a passive parasitic moderate-Q antenna was placed close to it, and (2) when a passive parasitic conical monopole was placed close to it. The results showed that the pattern distortion caused by reradiation from the moderate-Q antenna is not serious. The greatest effect the moderate-Q antenna has upon the short whip is at the resonant frequency of the moderate-Q antenna, and the effect is negligible for frequencies more than percent away from resonance. Second, the pattern distortion caused by reradiation from the conical monopole is generally severe, particularly for a reactive load. Except at the operating frequency, an antennas load impedance will generally be highly reactive.

HO. 2 is a schematic of the electrical circuit of the singleelement tuner 26 shown in FIG. I. The actual tuning circuit 46 consists of a fixed-value capacitor 48 and a fixed-value shunt coil 5t]v Tuning of the circuit is accomplished by means of the variable capacitor 52. Input terminal 42 is connected to the inner conductor of the rigid coaxial transmission line 14 and terminal 44 is connected to the outer conductor. Capacitor 54 represents the stray capacitance of the circuit and A A represents the intrinsic impedance of the antenna structure without the tuning circuit.

P C. 3 is a typical Smith chart representation of the intnnsic impedance and tuning response of the antenna structure 10 for three distinct exemplary frequencies such that 2,, Z and Z, represent the intrinsic antenna impedance at the frequencies af4 mc./sec. 6 mc.lsec. and 8 mc./sec., respectively.

As can be seen 2, can transformed to Zion the unity conductance circle by the series variable capacitor 52 of FIG. 2.

It would be expected that Z. would be transformed in a similar manner to the point 58 on the unity conductance circle. The shunt stray capacitance, C,, however, causes to be shifted "along" the unity conductance circle as shown in FIG. 3. The antenna admittance at 8 mc./sec. is shown as Y,. Shunting this admittance is the susceptance from the stray capacitance, C,, which transforms a to Y is equivalent to the antenna impedance 2, at 8 mc./sec. transformed to Z by the shunt stray capacitance C,. Z can thus be transformed to g lby the series variable resistor 52 of the tuning circuit 46 shown in FIG. 2.

X4" and )3: represent the admittance equivalents of Z and Z It can be seen that the admittance spread" is 4:1, and that without the stray capacitance it would have been 3:1 instead of the exemplary frequency spread of 2:1 (e.g., 4 mc./sec. to 8 mc./sec.

Thus, it is obvious that a single shunt coil having a 2:l spread of negative susceptance cannot by itself effectively match or tune a load with a 4: I spread of positive susceptance. A single shunt coil will have too much shunt susceptance at 8 mc./sec. and not enough at 4 mcJsec. so that a VSWR of approximately l.8:1 at the ends of the frequency range is the optimum value that could be achieved.

To achieve the exemplary 2:] frequency spread, a shunt susceptance having a shallower slope" of susceptance versus frequency is required. A shallower slope can be achieved, for example, by connecting a capacitor, 48, in parallel with the inductor S0 and by a consequent slight decrease in the inductance value of the coil.

As can be seen from FIG. 3, the transfonned antenna admittances Y, and Y "are on the unity conductance circle. Thus, since only the susceptances are being adjusted, the shunt inductive susceptance (-B and shunt capacitive susceptance (+8 required to obtain the necessary matching susceptance, B can be determined as follows:

B in active susceptance at 4 mc./sec.

B =inductive susceptance at 8 mcJsec.

B =capacitive susceptance at 4 mc./sec.

c =capacitive susceptance at 8 mcJsec.

lhe matching susceptance, B is a function of the desired VSWR and can be determined by noting where the maximum allowable VSWR circle crosses the unity conductance circle. For example, for the antenna structure 10, B. will be negative at 8 mcJsec. and positive at 4 mc./sec.

The capacitor element in the electrical circuit 46 located within the tuning box can be controlled, for example, by a reversible motor. The control apparatus for this motor can be located at the transmitter site. A syncro-transmitter can be coupled to the capacitor tuning shaft to thereby drive a syncro-receiver which receiver can in turn drive a conventional mechanical counter. A tuned matched condition can be indicated by a forward/reverse directional RF wattmeter which samples the forward and reverse power in the coaxial transmission line leading to the antenna If desired, local tuning of the antenna (without RF power being applied to the antenna) can be accomplished manually by means of a conventional handcrank on the side of the tuner box.

The following procedure can be used to tune the antenna structure [0. The antenna is first tuned to what is expected to be a matched condition which is an approximation based upon available information. With a transmitter output of about 5 kw. average feeding the antenna, the antenna is tuned for minimum reflected power. Full power is then applied to the transmitter at which time the antenna is fine-tuned, if necessa- HG. 4 represents in graphical form typical response curves for the antenna structure 10 when the structure is tuned to 4, 6, and 8 mc./sec. ln FIG. 4, F, represents the resonant frequency of the structure.

Arching between components or corona within the tuner box can be eliminated by substantially covering the components with a dielectric oil coolant. The use of bronze wire rope for the tophat radials can be advantageously utilized to prevent corona at the expected potentials and also dirtying of the wires.

The tenninating hardware and insulators at the outer end of the tophat radials should be of sufficient size to keep potential gradients low, thereby making the use of corona rings unnecessary.

1 has it can readily be seen and appreciated that a tunable, moderate-Q transmitting antenna structure capable of being installed and operated in proximity with a plurality of like antenna structures with minimal interference manifested as absorbed power or distorted radiation patterns.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

l. A high frequency moderate-Q transmitting antenna structure which can be installed and operated in proximity with a plurality of like structures with minimal mutual interference comprising:

means;

a ground plane;

said radiating structure means being mounted perpendicular and grounded to said ground plane;

top-loaded monopole radiating structure

Claims (2)

1. A high frequency moderate-Q transmitting antenna structure which can be installed and operated in proximity with a plurality of like structures with minimal mutual interference comprising: low-profile, top-loaded monopole radiating structure means; a ground plane; said radiating structure means being mounted perpendicular and grounded to said ground plane; said radiating structure means including rigid, coaxial transmission line means symmetrically disposed in the center and in the lower portion thereof; frequency tuner apparatus mounted on and connected to the top end of said transmission line means; said tuner apparatus having a first input terminal thereof connected to one of the two conductors of said transmission line and having a second input terminal thereof connected to the other of the two conductors of said transmission line; said tuner apparatus including single element, variable capacitor means for tuning said antenna structure over a frequency range above the self-resonant frequency thereof by means of said variable capacitor means only; a plurality of tophat radial wires; said radial wires being connected to the top of said radiating structure means and extending downwardly to supporting means in a symmetrical manner with respect to said radiating structure means; said tuner apparatus further including feed means for feeding energy to be transmitted to said radial wires of said antenna structure, said antenna structure being operated above its self-resonant frequency such that the intrinsic impedance of said antenna structure is always inductive.

2. The antenna structure of claim 1 wherein said radiating structure means is fed energy to be transmitted through coaxial feedline means at said ground plane.

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