The small antenna elements at microwaves facilitate the construction of highly directive, high gain antennas with high front-to-back ratios. At frequencies below about 2 GHz, 12- to 24-element Yagi arrays, enclosed in plastic shrouds for weather protection, may be used. At higher frequencies, antennas with dish reflectors are the norm.
The aperture ratio (diameter/wavelength) of a dish governs both its power gain and beamwidth. The power gain of a parabolic dish is given to a close approximation by:
Gain= 10 log10 6(D/λ)2×N,dBiwhere D = dish diameter and N = efficiency. Dimensions are in metres. The half-power beam width (HPBW) in degrees is approximately equal to 70λ/D.
A microwave antenna with its dish reflector, or parasitic elements in the case of a Yagi type, is a large structure. Because of the very narrow beamwidths – typically 5æ for a 1.8 m dish at 2 GHz – both the antenna mounting and its supporting structure must be rigid and able to withstand high twisting forces to avoid deflection of the beam in high winds. Smooth covers, radomes, fitted to dishes and the fibreglass shrouds which are normally integral with Yagis designed for these applications considerably reduce the wind loading and, for some antenna types, increase the survival wind speed.
The electrical performance of a selection of microwave antennas is given in Table 4.1 and the wind survival and deflection characteristics in Table 4.2 (Andrew Antennas, 1991).
Table 4.1 2.1–2.2 GHz antennas – electrical characteristics Type Dia. Gain (dBi) Beam Cross F/B VSWR number (m) width pol. ratio max. Bottom MidTop (deg.) disc (dB) band (dB)Ultra High Performance Antenna, F-Series Unpressurized – Radome Inc. Single polarized
UHP8F-21 2.4 31.9 32.1 32.3 4.2 32 61 1.10 UHP10F-21 3.0 33.7 33.9 34.0 3.6 33 64 1.10 UHP12F-21 3.7 35.4 35.6 35.8 2.9 32 65 1.10 Dual polarized
UHX8F-21 2.4 31.9 32.1 32.3 4.2 30 58 1.20 UHX10F-21A 3.0 33.8 34.0 34.2 3.6 32 62 1.20 UHX12F-21A 3.7 35.4 35.6 35.8 2.8 32 67 1.20
High Performance Antenna, F-Series Unpressurized – Radome Inc. Single polarized
HP6F-21B 1.8 29.4 29.6 29.8 5.5 30 46 1.12 HP8F-21A 2.4 31.9 32.1 32.3 4.1 30 53 1.12 HP10F-21A 3.0 33.8 34.0 34.2 3.4 32 55 1.12 HP12F-21A 3.7 35.4 35.6 35.8 2.9 32 56 1.12
Standard Antenna, F-Series Unpressurized
Single polarized
P4F-21C 1.2 26.4 26.6 26.8 7.6 30 36 1.15 P6F-21C 1.8 29.8 30.0 30.2 4.9 30 39 1.12 P8F-21C 2.4 32.3 32.5 32.7 3.8 30 40 1.12 P10F-21C 3.0 34.0 34.2 34.4 3.4 30 44 1.12
Grid Antenna, F-Series Unpressurized
Single polarized
GP6F-21A 1.8 29.6 29.8 30.0 5.4 31 36 1.15 GP8F-21A 2.4 32.0 32.2 32.4 4.0 35 39 1.15 GP10F-21A 3.0 34.0 34.2 34.4 3.3 40 42 1.15 GP12F-21 3.7 35.5 35.7 35.9 2.8 40 44 1.15
Table 4.2 Wind survival and deflection characteristics Antenna types Survival ratings Max. deflection in 110 km wind (degrees)Wind Radial
P4F Series
Without Radome
With Radome
Standard Antennas (except P4F Series) Without Radome
With Standard Radome UHX, UMX, UGX
Antennas
160 12 0.1 185 12 0.1
be required. At higher frequencies, air-spaced or pressurized nitrogenfilled cables are frequently used with waveguides as an alternative.
The normal mode helix antenna shown in Figure 4.16 produces an omnidirectional pattern when the antenna is mounted vertically. The diameter (D) of the helical coil should be one-tenth wavelength (λ/10), while the pitch (i.e. S, the distance between helix loops) is onetwentieth wavelength (λ/20). An example of the normal mode helix is the ‘rubber ducky’ antenna used on VHF/UHF two-way radios and scanners.
An axial mode helical antenna is shown in Figure 4.17. This antenna fires ‘off-the-end’ in the direction shown by the arrow. The helix is mounted in the centre of a ground plane that is at least 0.8λ across. For some UHF frequencies some manufacturers have used aluminum pie pans for this purpose. The helix itself is made from either heavy copper wire (solid, not stranded) or copper or brass tubing. The copper tubing is a bit easier to work. The dimensions are:
D ≈ λ/3
S ≈ λ/4
Length ≈ 1.44λ
A ‘rule of thumb’ for the circumference is that maximum gain is obtained when circumference C is:
C = 1.066+ [(N − 5)× 0.003]
Small loop antennas are used mostly for receiving, although some designs are also used for transmitting. One application for the small loop antenna is radio direction finding (RDF). Another use is for providing a small footprint antenna for people who cannot erect a full sized receiving antenna. Perhaps the greatest use of the small loop antenna is for receiving stations on crowded radio bands. The small loop antenna has very deep nulls that make it easy to null out interfering co-channel and adjacent channel signals.
Figure 4.17 Axial mode helical antenna