US3427624A - Low profile antenna having horizontal tunable top loading member - Google Patents

Description

Feb. 11, 1969 R D. WANSELOW ETAL LOW PROFILE ANTENNA HAVING HORIZONTAL TUNABLE TOP LOADING MEMBER Filed July l3, 1966 Sheet l g-i Y/N M Feb. N, 1989 R. D. WANSELOW ETAL 3,427,624 LOW PROFILE ANTENNA HAVINGHORIZONTAL TUNABLE TOP LOADING MEMBER Sheet Filed July 15, 1966 United States Patent Oflice 3,427,624 Patented Feb. 11, 1969 4 Claims ABSTRACT OF THE DISCLOSURE A vertically polarized low profile antenna having a vertical radiator and a horizontal top loading member connected to the top of the radiator is provided with novel wide-band tuning means comprising a plurality of variable capacitors connected in series with the top loading member of optimum spacings. A preferred embodiment using a multiple-turn spiral top load with three ganged series tuning capacitors is described.

The present invention relates to antennas, and more particularly, to a low-profile, transmission line antenna tunable over a wide frequency range.

The antenna of this invention is of a class called a,

Directly Driven Resonant Radiator, or DDRR. Examples of antennas incorporating DDRR principles are found in US. Patents 3,151,328 and 3,247,515 by Boyer, 3,384,896 by Kriz, and copending application Ser. No. 527,874, filed Feb. 16, 1966- by Milligan and Wanselow.

It is an object of the present invention to extend the DDRR concept to provide excellent selectivity and radiation characteristics while tuning over a wider frequency range than previously attainable.

A further object of this invention is to provide a DDRR antenna and tuning means therefor which is admirably suited for airborne or mobile vehicle use, by virtue of its simultaneous low height and eflicient radiation, and its good performance under the associated environmental conditions.

Our invention comprises a short vertical post radiator top-loaded by a horizontal, unbalanced transmission line member having a plurality of tuning capacitors connected thereto, said capacitors being preferably connected directly in series with said horizontal top-load member and spaced apart a predetermined distance for greatest continuous tunable frequency range.

This invention will be more fully understood by reference to the detailed description of specific apparatus to follow, and to the accompanying illustrative drawings, wherein:

FIGURE 1 is a schematic elevational diagram of one preferred form of antenna, showing a tri-series capacitor tuned arrangement.

FIGURE 2 is a schematic elevational diagram of an alternate embodiment, showing a combination series-shunt tuned model.

FIGURE 3 is a top perspective diagrammatic view of a complete antenna assembly, showing a generally spiral form of the top loading member.

FIGURE 4 is an elevational view of the antenna in FIGURE 3, viewed approximately as indicated by the line 4-4 in FIGURE 3.

Referring first to FIGURE 1 for the general form of the present antenna, a conductive vertical post 1 is connected directly to a conductive ground plane member 2. A conductive top load member 4 is mounted parallel to the ground plane 2 and has one end directly connected to the top of the post 1. Variable capacitors 0,, C and C are attached in series with the top load member 4.

As one or more of the three capacitors are varied, the resonant frequency of the antenna changes.

A feed wire 5 is connected to the post 1 or to the top member 4 near their junction, the exact point affecting the input impedance of the antenna. Feed wire 5 is con nected to the inner conductor of a coaxial transmission line 6, for example, with the outer conductor of line 6 being connected to the ground plane 2.

In FIGURE 2 is shown another plural-tuning capacitor means for a similar antenna. Here, one series tuning capacitor C is directly in series with the top member 4a, while in shunt tuning capacitor C is connected between the far end 7 of top member 4a and the ground plane 2.

As is known, space may be saved by forming the top load member 4 into a loop, ring, spiral, or other shape. Such a design is shown in FIGURE 3 for a tri-series capacitor tuned antenna, including driving means for the capacitors. In FIGURES 3 and 4, a ground plane member 2a is made of aluminum honeycomb material, for example, and constitutes a base for the entire assembly. The ground plane member 2a is preferably solid and continuous, but may be lattice-like or consist of a connected wire grid, for example. Fiber glass brackets 9 erected from the base carry a fiber glass support panel 10 at their upper ends. A generally rectangular spiral top load member 4b of metal tubing is mounted on the upper side of the support panel 10, with the inner end of the spiral bent downwardly through the panel 10 to form the radiating post 1:: of the antenna. The lower end of post In is conductively connected directly to the ground plane member 2a.

At three selected positions, the top member 4b is broken to accommodate metal hangers 11, between each pair of which is mounted a variable capacitor C1, C2, and C3, respectively. (Only one capacitor, C2, is shown in FIG- URE 4 for the sake of clarity.) The variable capacitors are thus just below the panel 10, and each carrier a rotatable toothed pulley 12 by which the capacity is varied. It should be noted that the capacitors are located as nearly as possible in the same plane as the top-load member 4b so that vertical current paths are avoided.

A horizontal drive shaft 14 is supported by end fittings 15 from the panel 10, and positive-drive belts 16 are operatively installed between the drive shaft 14 and the pulleys 12 of the variable capacitors C1, C2, and C3. A servo drive motor unit 17 is mounted on the ground plane 20., with a motor belt 19 providing rotating power to the drive shaft 14. Thus all capacitors are adjusted simultaneously.

Approximately at the upper end of post 1a, a terminal lug 20 is fastened to a bolt 21 installed through the tubing. A feed wire 5a is connected to this lug 20 and extends down through the ground plane 2a a short distance from post In. The feed wire 5a is insulated from the ground plane 2a by insulator 22 and connects to a coaxial transmission line connector 23. As mentioned previously, the point of connection of feed wire 5a to the antenna is predetermined from an impedance standpoint to match the transmission line.

A plastic cover 24 may be installed over the entire antenna and attached to the edges of the ground member 2a. This assembly is especially well suited for use on aircraft or the like, the ground plane member 2a being preferably electrically bonded to the vehicles metal structure.

In the antenna configuration with the series and shunt tuning capacitors as in FIGURE 2, approximately a 2.5 :1 frequency tuning range is obtained. Resonance is accomplished by tuning the series capacitor C in conjunction with the end shunt capacitor C However, as the resonant frequency is lowered, the radiation efliciency is reduced, due to a higher current through shunt capacitor than shown herein in FIGURES 3 and 4. With the post C which is in opposition to the current in the radiating post 1, and hence tends to cancel the radiation-producing current.

We have found that the above disadvantage can be eliminated by replacing the shunt capacitor C with a series capacitor (or capacitors) and a short additional length of top member. Therefore, no current cancellation occurs and efficiency is good throughout the tuning range. The upper limit on the tuning range with series tuning is determined by the maximum amount of variable capacitance that can be obtained with available capacitors.

In the case of the preferred tri-series tuned model as illustrated in FIGURES 1, 3 and 4, a tuning range of more than 3:1 is provided, e.g., from 30 mc. to 104 me. in one actual embodiment. This tuning coverage exists in two modes; the lower mode frequency band is from 30 to 76 me. and the upper mode is from 74 to 104 mc. Other higher modes exist, but not in a continuous coverage of the frequency spectrum. In other words, the antenna is resonant at two or more frequencies for each combination of settings of the three tuning capacitors C1, C2, and C3. The lower mode is where the total elec trical length of the antenna (post plus top-load) is onequarter wavelength.

In the same example as used in the preceding paragraph, capacitors Cl and C2 may be 5.3 to 102 micromicro farad components, and capacitor C3 may be a 6.0 to 145 micro-micro farad component. The post 1a is 6 to 8 inches in height for example, and the total physical length of the antenna (post la plus top-load member 4b) is approximately 150 inches. Regarding the spacing of capacitors and referring to FIGURE 1, length L is about 23 inches, L about 42 inches, L about 29 inches, and L about 56 inches, for example. For high efficiency, the post and top-load member should be of relatively large diameter to have a minimum RF electrical resistance and thus reduce the antenna circuit loss. Using inch diameter tubing for the post 1a and top-load member 4b, for example, and low loss capacitors, the total antenna circuit loss resistance is less than M. ohm. A 50% or greater radiation efficiency is thereby achieved.

The selection of capacitor spacing and value is made with the object of obtaining the largest tunable frequency range in the quarter wave mode. In the case of the triseries tuned model, the calculations become quite cumbersome, but consist of satisfying the basic relation L,,=90L, at quarter-wave resonance, where L is the equivalent replaced electrical length encompassed by the capacitor values chosen for C C and C and the physical line lengths L L and L Other resonances are formulated such that the sum of L and L is equal to odd multiples of quarter wave resonance, i.e., 270, 450, etc.

It will be appreciated that an antenna as illustrated herein having a six-inch post 1a is less than 0.05 wavelength in height, at the highest operating frequency of the second mode. For increased efficiency, an 8 inch post may be employed.

Resonance over the required band can be accomplished either by tuning each of the series capacitors separately or by tuning all of them simultaneously. The input voltage standing wave ratio of this antenna is well below 2:1, and is especially low if a larger ground plane is used 1 or 1a vertical, a vertically polarized wave is radiated. Of course, the present antenna also functions as a receiving antenna, and can be used in various rotated positions, i.e., with the post 1 or 2a horizontal for horizontally polarized waves.

This antenna is most suitable for applications in the HF, VHF, and UHF frequency bands since its advantages includes optimization of the space or volume required, high efficiency, and good inherent circuit selectivity. The latter is reflected by the relatively high circuit Q, which is or more.

Mechanically, the present antenna is rugged and capable of practical, protected installations on aircraft or land craft of all types. By employing a reflectorneter or error detection phase discriminator circuit and a servo system, the ganged capacitors can be rapidly and automatically tuned to the desired frequency.

While in order to comply with the statute, the invention has been described in language more or less specific as to strucural features, it is to be understood that the invention is not limited to the specific features shown, but that the means and construction herein disclosed comprise the preferred form of several variations of putting the invention into effect, and the invention is therefore claimed in any of its forms or modifications within the legitimate and valid scope of the appended claims.

What is claimed is:

1. An antenna comprising a radiating post element, a top-load member having one end connected at a right angle to one end of said post element, the other end of said top-load member being free, and a plurality of variable capacitors connected in series within said top-load member, the spacing between said capacitors and the values thereof being chosen on the basis of the widest possible continuous tunable frequency range.

2. Apparatus in accordance with claim 1 wherein said top-load member is formed in the general shape of a spiral having more than one complete turn.

3. Apparatus in accordance with claim 1 wherein the total effective electrical length of said post element and said top-load member is one-quarter wavelength at the lowest resonant operating frequency.

4. Apparatus in accordance with claim 1 wherein said capacitors are ganged together, with driving means for simultaneously and continuously adjusting said capacitors between minimum and maximum capacities thereof.

References Cited UNITED STATES PATENTS 2,166,750 1/1939 Carter 343--744 2,283,897 5/ 1942 Alford 343744 2,417,793 3/1947 Wehner 343-830 2,431,124 11/1947 Kees et al 343-845 2,578,154 12/ 1951 Shanklin 343-845 2,850,732 9/1958 Kandoian et a1 343752 3,151,289 9/ 1964 Boyer 343744 3,358,286 12/ 1967 Heins 343750 ELI LIEBERMAN, Primary Examiner.

US. Cl. X.R.

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