Omnidirectional 4G Fiberglass Antenna Design

　　Omnidirectional 4G fiberglass antenna design: principle and technical implementation

　　The core value of omnidirectional 4G fiberglass antenna is to achieve 360° horizontal uniform coverage. Its radiation pattern presents a "donut" distribution, which is indispensable in scenarios that require all-round signal coverage, such as smart street lights, vehicle-mounted communications, and small base stations. Compared with directional antennas, its design difficulty lies in balancing horizontal omnidirectionality and vertical gain distribution, while maintaining stable impedance matching in a wide frequency band. The dielectric properties of fiberglass material provide a unique solution for this balance.

　　Implementation mechanism of omnidirectional radiation characteristics

　　Radiation unit topology design

　　Using a vertical dipole array structure, 3-5 half-wave oscillators (spacing 0.6λ) are arranged axially in the fiberglass tube, and superimposed radiation is formed through a co-phase feeding network (phase difference ≤5°). The oscillator adopts a copper tube silver plating process (thickness 8μm), and the skin depth in the 2.6GHz band is only 1.3μm, which can reduce high-frequency loss (≤0.3dB). The actual measurement shows that the gain fluctuation of the structure on the horizontal plane is ≤2dB (within 360°), which meets the core indicator requirements of omnidirectionality.

　　To expand the vertical coverage range, the length of the vibrator is gradually changed according to the frequency band ratio (the vibrator in the low frequency band is longer than that in the high frequency band), so that the vertical beam width reaches 35° at 700MHz and remains at 20° at 2600MHz, ensuring that the signal attenuation is ≤3dB in multi-story buildings or vehicle pitch scenarios (such as uphill and downhill). By optimizing the vibrator spacing through electromagnetic simulation, the first sidelobe level can be suppressed to below -18dB to reduce energy waste.

　　Wideband solution for impedance matching

　　The omnidirectional antenna needs to cover the ultra-wide frequency band of 698-2690MHz, and a gradient impedance transformer (composed of 5 microstrip lines with different characteristic impedances) is used to achieve 50Ω matching. In the fiberglass antenna cover (εr=3.2), the impedance shift caused by dielectric loading is compensated by adjusting the distance between the radiating unit and the metal reflector (1/4λ is optimal). Test data shows that the design has a standing wave ratio of ≤1.5 in the entire frequency band, and can be as low as 1.2 in the 1800MHz band, which is much better than the traditional single-section matching network (standing wave ratio 1.8).

　　For the impedance sensitive characteristics of the 700MHz low frequency band, an additional LC series resonant circuit (Q value = 25) is added to increase the return loss in this frequency band from 15dB to 20dB, and the signal transmission efficiency is increased by 15%. The feed point adopts an N-type connector flip-chip design to reduce the impedance mismatch caused by the lead length (≤5cm), and the insertion loss is controlled within 0.1dB.

　　Engineering design of glass fiber structure

　　Materials and processes of antenna cover

　　The alkali-free glass fiber cloth (E-glass) and vinyl ester resin are selected for composite molding. The dielectric constant is stable at 3.2±0.1 (25℃), and the change rate is ≤2% in the temperature range of -40℃ to 80℃. Through the winding molding process (winding angle ±45° alternating), the axial tensile strength of the antenna cover is ≥300MPa, the impact strength is ≥25kJ/m², and it can withstand the wind pressure load (≤2000Pa) of a level 12 typhoon (wind speed 32.7m/s).

　　The thickness of the antenna cover is optimized according to the frequency band: 700MHz corresponds to 6mm (1/20λ), 2600MHz corresponds to 3mm (1/30λ), and the penetration loss of different frequency bands is balanced through the stepped wall thickness design (≤0.5dB). The surface is coated with fluorocarbon (thickness 40μm), the UV absorption rate is ≥95%, and the dielectric performance change rate is ≤1% after 1000 hours of QUV aging test, ensuring long-term stability.

　　Lightweight design of mechanical structure

　　The overall weight of the omnidirectional antenna is controlled within 2.5kg (1.2 meters in length), which is 40% lighter than the metal antenna with the same performance. The bottom flange is made of glass fiber reinforced nylon (containing 30% glass fiber) and is integrally formed with the antenna cover to reduce the use of metal parts (only the connector interface is retained). The flange is designed with 4 M8 mounting holes with a distribution circle diameter of 100mm, which is suitable for standard brackets. The installation torque of 25N・m will not cause structural deformation (axial displacement ≤0.1mm).

　　To resist vibration shock (10-2000Hz/10g), a silicone rubber buffer layer (hardness 40 Shore A) is filled between the feed network and the antenna cover, which can absorb 90% of the low-frequency vibration energy and make the standing wave ratio change ≤0.2 after the vibration test. The test shows that the electrical performance of the structure has no obvious attenuation after passing 1000 impact cycles of the MIL-STD-883H standard.

　　Key technical challenges and solutions

　　Balance between wide bandwidth and omnidirectionality

　　The long wavelength of the low frequency band (700MHz) is prone to cause horizontal radiation pattern distortion. By adding a metal reflection ring (diameter 0.8λ) at the bottom of the antenna, the horizontal gain fluctuation can be compressed from 3dB to 1.5dB. The reflection ring adopts a hollow design (opening rate 30%) to reduce the shielding of the high frequency band (2600MHz) and ensure the horizontal consistency of the full frequency band.

　　The gain distribution in the vertical direction is optimized by loading the top load. A disc-shaped metal sheet (diameter 0.3λ) is added to the top of the antenna to increase the vertical beam width of the 700MHz band by 10°, while having little effect on the 2600MHz band (change ≤ 2°). This design is particularly suitable for outdoor omnidirectional coverage, and can take into account the communication needs of both the ground and low altitude (such as drones).

　　Temperature stability control

　　Changes in ambient temperature will cause the size and dielectric constant of the vibrator to drift. A temperature compensation feeder (composite of iron-nickel alloy and copper) is used, and its thermal expansion coefficient (1.2×10⁻⁶/℃) is only 1/15 of pure copper. A negative temperature coefficient (NTC) capacitor is connected in series in the impedance matching network. When the temperature rises by 1℃, the capacitance value changes by -0.2%, compensating for the positive shift of the vibrator resonant frequency (about +50kHz per ℃).

　　The test shows that within the range of -40℃ to 80℃, the compensated antenna resonant frequency offset is controlled within ±3MHz, and the standing wave ratio change is ≤0.3, which is much better than the uncompensated design (±10MHz, standing wave ratio change 0.8). This temperature stability ensures communication reliability in extreme climate areas (such as northern winter and desert summer).

　　Design adaptation of typical application scenarios

　　Smart street lamp integrated antenna

　　It adopts a low-profile omnidirectional design (height 60cm, diameter 8cm), integrated on the top of the street lamp pole, with a horizontal gain of 8dBi and a vertical beam width of 30°. Through the camouflage coating (RAL 7035 light gray) integrated with the street lamp pole, the light transmittance of the glass fiber material is < 5% (to avoid visibility of internal components). Within a radius of 100 meters, the 4G signal coverage rate reaches 98%, meeting the omnidirectional access requirements of IoT sensors (such as traffic monitoring and environmental monitoring).

　　Vehicle-mounted omnidirectional communication antenna

　　Designed with a magnetic adsorption base (suction force ≥ 50N), antenna height ≤ 15cm (to avoid height restrictions), horizontal gain 6dBi, and signal attenuation ≤ 2dB when the vehicle turns. The fiberglass shell adopts a streamlined design (drag coefficient Cd = 0.5), and the wind load at a speed of 120km/h is ≤ 50N, reducing the impact on the roof. Through dual-polarization (vertical + horizontal) switching technology, the signal can still be kept stable when the vehicle tilts ±30° (interruption time < 50ms).

　　Portable emergency communication antenna

　　Adopts a retractable structure (expanded length 1.2m, retracted to 40cm), fiberglass tube wall thickness 3mm (weight 1.2kg), equipped with a quick locking mechanism (3 seconds to complete the expansion). The horizontal gain is 10dBi, achieving 360° coverage within 500 meters around the emergency command tent, and supporting 20 4G terminals to access simultaneously (throughput ≥50Mbps). The bottom is integrated with a waterproof connector (IP67), which can maintain smooth communication in heavy rain.

　　Performance test and optimization indicators

　　Key electrical performance indicators

　　Horizontal omnidirectionality: gain fluctuation within 360° range ≤2dB (typical value 1.5dB)

　　Frequency band coverage: 698-2690MHz, standing wave ratio of each band ≤1.5

　　Gain: 8dBi in 700MHz band, 10dBi in 2600MHz band

　　Polarization: vertical polarization (customizable ±45° dual polarization)

　　Impedance: 50Ω±1Ω (full frequency band)

　　Environmental adaptability indicators

　　Working temperature: -40℃ to 80℃ (standing wave ratio change ≤0.3)

　　Humidity: 95% RH (40℃, no condensation), insulation resistance ≥100MΩ

　　Vibration: 10-2000Hz/10g, electrical performance change after test ≤0.5dB

　　Waterproof grade: IP67 (1 meter underwater / 30 minutes without damage)

　　The core of the design of the omnidirectional 4G fiberglass antenna is to achieve horizontal uniform coverage in a wide frequency band through the coordinated optimization of material properties and electromagnetic structure, while taking into account environmental adaptability and mechanical reliability. Its design process needs to balance the relationship between gain, bandwidth, and size, and finally meet the omnidirectional communication needs of diverse scenarios through the iteration of electromagnetic simulation and prototype testing.