Analysis and Design of a monopole whip antenna

[Abstract] This paper analyzes the working principle of a monopole whip antenna, design methods, and gives the model antenna. Measured results of the antenna structure improvement, results show that the antenna operating frequency in the 334 ~ 346MHz center frequency to 340MHz VSWR in the entire frequency band is less than 2, the gain is relatively stable, mean to 2.2dBi. this antenna volume is small, light weight, easy to carry, better repeatability and consistency.

[Keywords] monopole whip antenna the electrical properties VSWR direction Figure 1 INTRODUCTION whip antenna due to the small size, compact structure and in the types of communications equipment widely used in its study design and therefore attracted widespread attention. Various terrestrial radio and car radio is equipped with various forms whip antenna the ships also often see the antenna bristling whip. whip antenna physical size is still small antenna, especially the HF band low-end, resistance small electric Kang Da, matching difficult, so most of the whip antenna applications in narrowband work status, a bandwidth of approximately 5% to 10% [1]. monopole whip antenna belonging to whip class structure is quite simple a suitable for car and portable work, small size, high radiation efficiency, and erected.

2 theoretical analysis of this paper, the design of the whip antenna structure with a length of 1/2 wavelength monopole. Monopole antenna is a half of the dipole antenna, this antenna is almost always higher than the plane of the installation site, such as the structure of its basic principle as shown in Figure 1, the length h of upstanding transducer and infinite floor. impact of the ground available antenna mirror instead, so that the monopole antenna can be equivalent to the arm length in the free space for 2h of symmetric oscillator. Of course, such an equivalent only the equivalent of a half-space on the ground, the reason is that the floor below the radiation field [2].

Unipolar ground of 2.1 radiation field pattern set up in the infinite perfectly conducting plane antenna radiation field can be applied directly to free space symmetric oscillator formula to calculate [3]:
· (1
Wherein Im represents antinode current Im = I0/sinβl, θ = 90 °-δ l = h (I0 of the input current, Δ elevation angle, h is the height of the monopole antenna. Substituting into the above equation to give:
By F (δ shows that the direction of view of the horizontal plane is a circle, i.e., omni-directional in the azimuth plane is the vertical plane in the direction shown in Figure 2. When h is gradually increased, the waveform becomes sharp, when h> 0.5λ, vice identified at h = 0.625λ, sidelobe maximum occurs in the Δ = 60 ° direction, and continue to increase h, inverting on the role of the current in the antenna, along Δ = 0 ° direction. The radiation weakened To this end, h should be limited within 0.625λ. antifading in the medium wave band, in order to require as much as possible to reduce more than 55 °, the radiation in the high elevation angle direction, To this end, h should be as large as possible. Average case, h = the 0.53λ about more appropriate.

As can be seen from Figure 2:

(1 Under normal circumstances, the choice of λ / 4 monopole antenna as a standard antenna. Direction in Figure is a circle in the horizontal plane, the dumbbell-type distribution of the pitch leaving. And its input impedance is close to 50Ω, easy and common characteristics a coaxial line with an impedance of 50Ω match its antenna gain of 5.15dB.

(2 actual engineering omnidirectional antenna using a known 5λ / 8 monopole antenna, its gain of approximately 8.15dB, as shown in Figure 3. Course, the ground plate is generally come with a few metal bars efficiency. Meanwhile, in order to match and 50Ω coaxial, in the bottom of the antenna with loading coil to offset the input impedance of the capacitive portion.

(3 If the height of the monopole antenna is taken of λ / 2, it is equivalent to a full-wave transducer in the free space, theoretically compared to h = λ / 4, the gain to improve 1.67dB. However, such a high input impedance of the antenna , is not easy and the coaxial connection, this must be an impedance switch. reposted in the free papers Download Center
2.2 electrical characteristics of effective height: the effective length of the monopole antenna Introduction is the effective height, it can be said that the strength of the radiation of the antenna, is an important indicator of the upright antenna. assumptions on the antenna current is sinusoidal distribution, and β is the propagation constant is defined in accordance with the effective height of:

When h << λ, that is? H → 0, equation (3 can be simplified as follows:

When the transducer is very short, the current is approximately triangular distribution, therefore the effective height of half of the actual height h = λ / 4, he = 0.5λ / π.

The direction coefficients: first discuss the radiation resistance, and then by the direction of the calculation of the radiation resistance coefficient. Monopole antenna radiation resistance and free-space symmetry in infinite perfectly conducting ground exactly the same method of calculation of the radiation resistance of the oscillator, only because of a monopole antenna The mirror portion is not radiated power, its radiation resistance for a free space of the same arm length (l = h half of the radiation resistance of the symmetric oscillator When h = λ / 4, for the radiation resistance of the thin wire antennas 36.50Ω when h the = λ / 8
Wherein, RRM Rr0 are attributed to the current and input current of the antinodes of the radiation resistance. Figure 4 shows the radiation resistance, with the antenna height variation curve, and the abscissa, i.e. (h / λ × 360 ° in electrical angle when antenna height h decreases, the radiation resistance is decreased quickly.

When Δ = 0 °, by the formula (1 can be obtained:

f (φ = 1-cos? h (6 [paper mesh]

When h << λ, the formula (5 (6 generations into the direction of the coefficient formula:

Because COS? H ≈ 1 - (? H/22 direction coefficient of the electrical low height of a monopole antenna is approximately equal to 3.
3 antenna design according to requirements, the design center frequency of the whip antenna in the 340MHz bandwidth of 12MHz, a coaxial feeder. Antenna coaxially fine line structure, as shown in Figure 5.

The of coaxial thin line structure VSWR curve shows that, VSWR at center frequency 340MHz 1.58,334 MHz 1.98,346 MHz to 1.96, but the bandwidth antenna gain is not ideal, because the size of the chassis ground smaller, the monopole antenna of limited access to the metal ground plate, and the direction FIG somewhat upturned. therefore added in the antenna at the bottom a coaxial metallic sleeve, and the coaxial outer conductor to ground, as shown in 6 shown, the radiator portion of the antenna for the coaxial inner conductor, the length is set to 1/4 wavelength [4]. sleeve monopole antenna can improve the bandwidth and gain [5-6].

Found sleeve diameter of 6mm, a length of 1/4 wavelength better VSWR, the center frequency of 340MHz to 1.28, the gain is 3.18,334 MHz VSWR of 1.78, the gain is 2.47,346 MHz stationed The wavelet coefficients of 1.99, the gain is 2.32, and the curve shown in Figure 7, the direction shown in Figure 8.

Analysis Conclusion This article on the electrical characteristics of a simple structure and a monopole whip antenna, and adding a metal sleeve in the design improvements to effectively improve the bandwidth and gain of the antenna, this antenna sizes small, easy to carry, in the can play an important role in modern communication.


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