[2023-1: Design of A Monopulse Reflector Antenna with Dual-circularly Polarized Sum Channels and Linearly-polarized Azimuth and Elevation Difference Channels

[Back logs

Ominidirectional Circularly-Polarized Antenna Using A Skew Pinwheel Radiator: 오길선

Wideband semi-dual-polarized monopulse feed: 오길선

Design of A Full-Band-Matched Rectangular Waveguide Open-End Radiator: 오길선

Wire-Grid Implementation of A Coaxial Double-Sleeve Balun for Feeding Dipole Antennas: 허지원

Widebeam Circularly-Polarized GPS Antenna Employing Crossed-Dipoles: 서송원, 모네비

Sporer

Fig. 2. Waveguide-to-DR transition. Dimensions: a = 5, b = 11, D = 18, D f = 25, w = 8, Lwg = 35, L1 = 15, L2 = 13, and LDRA = 69 (dimensions in mm). Square waveguide made of copper, conical horn made of brass, and dielectric rod made of PTFE.

Fig. 3. Guidance characteristics of the HE11 mode on a cylindrical rod made of PTFE (εr = 2.1). For electrical thick wires (d λ0), c converges the characteristic velocity of the material c0/ √r thus λg → λ0/ √r.

Regarding the rod diameter at the feed end dmax, we follow the proposal of Mallach [5] who states that for optimal radiation characteristics, λg/λ0 along the rod should be within approximately 0.8 ... 1.0. This range can be mapped to a physical diameter by examining the guidance characteristics of the HE11 mode for a cylindrical rod made of Polytetrafluoroethylene (PTFE) as shown in Fig. 3. The curve was calculated by applying the guidance condition for a dielectric rod formulated by Carson et al. [3] and from [22]. PTFE has been chosen because it turned out that its permittivity εr = 2.1 allows dmax to be equal to the inner side length of the waveguide (dmax = w) while obtaining λg = 0.84λ0 at the feed end. In this way the rod is mechanically supported and a stable mounting of the antenna is simplified (see Fig. 2). Further advantages are good availability and low dielectric losses. A slight exponential taper guarantees good impedance matching to free-space and reduces the antenna sidelobe levels [16]. Therefore, our dielectric rod is tapered exponentially from dmax = w = 8 mm (λg = 0.84λ0) at the feed to dmin = 2.5 mm (λg ≈ λ0) at the end.

The pattern peak approaches end fire (θ = 0) as P → 1. By increasing P beyond 1, the directivity increases and reaches maximum value for a given P, depending on the length [1]

P = 1 + 0.465/L (10-8)

Equation (10-8) is the Hansen and Woodyard criterion for increased directivity of a long end-fire structure commonly approximated by [2]

P = 1 + 1/(2L) (10-9)

The directivity peaks for P given by Eq. (10-8):

directivity = SL

(10-13)

FIGURE 10-2 Directivity of an end-fire traveling-wave antenna.

FIGURE 10-3 Beamwidth of a traveling-wave end-fire antenna.

Figure 10-15 shows a common feeding arrangement for the polyrod antenna. The

rod protrudes from a circular waveguide supporting the TE11 mode, which excites the

hybrid mode HE11 on the rod. At the waveguide exit we use a rod diameter to give

P from 1.2 to 1.3 so that the wave will be closely bound to the rod. The feeding guide (Figure 10-15) has a quarter-wavelength choke to reduce the backfire lobe due

to direct radiation from the transition [28]. The choke region can also be flared in a

short horn [29].

The second region of the rod tapers either to a uniform diameter section to produce

maximum gain or to a tapered section to reduce sidelobes. At the end of the antenna we

taper the rod rapidly in a terminating section to bring the relative propagation constant of the surface wave near 1, to reduce reflection from the end. We calculate P along

the guide and adjust the uniform section diameter or tapered section length to satisfy

the total extra phase shift condition for maximum end-fire radiation.

(Example)

The relative propagation constant for peak gain is independent of the material,

which we compute from Eq. (10-8): P = 1 + 0.465/5 = 1.093. By using Scales 10-2

and 10-3, we read the rod diameters: 0.516λ for Teflon and 0.356λ for Delrin. At the

point where the rod exits from the feeding waveguide, a suitable relative propagation

constant is 1.25. We use the scales to find the rod diameters: 0.822λ for Teflon and

0.456λ for Delrin. These diameters are proportional to the free-space wavelength, not

the wavelength in the rod.