How Fiberglass Antennas Work: Complete Guide to High-Gain FRP Antenna Principles

Many engineers have used fiberglass antennas, but only partly understand how they work internally. Why can a simple-looking fiberglass rod achieve high gain above 10 dBi? Does the fiberglass enclosure block signals? Is it really necessary to make the antenna so long? Why must it be installed vertically?

Understanding these principles helps you not only choose the right antenna, but also get the best performance from it. With the same antenna, correct versus incorrect installation can more than double the communication distance.

1. Antenna Radiation Basics: From Electrical Signal to Electromagnetic Wave

To understand fiberglass antennas, you first need to understand the basic principle of antenna radiation. An antenna is essentially an energy converter: it converts the high-frequency AC signal output by the wireless module’s power amplifier into an electromagnetic wave that propagates through free space. When receiving, it does the opposite: it converts electromagnetic waves in space into electrical signals for the receiver.

1.1 Dipole and Monopole: The Foundation of Omnidirectional Antennas

The most basic omnidirectional antenna is the half-wave dipole. It consists of two metal elements, each 1/4 wavelength long, for a total length of about 1/2 wavelength. A half-wave dipole has 2.15 dBi gain and radiates uniformly 360° in the horizontal plane. It is the reference for all omnidirectional antennas.

A monopole antenna is a variant of the dipole: the lower half of the dipole is replaced by a reflecting ground plane, while the upper half retains a 1/4-wavelength element. The Ebyte XPL suction-cup antenna is a typical monopole structure, where the metal surface to which the suction cup attaches acts as the ground plane. The BLG fiberglass antenna uses a co-linear dipole array structure, which can be understood as multiple dipoles connected in series along the same axis. This is the key to achieving high gain.

1.2 Relationship Between Wavelength and Antenna Size

The electromagnetic wavelength formula is:

λ = c / f

Where c is the speed of light (about 3 × 10⁸ m/s) and f is the operating frequency. The lower the frequency, the longer the wavelength, and the larger the physical size of the antenna:

Operating Frequency	Free-Space Wavelength	1/4 Wavelength (Monopole Length)	1/2 Wavelength (Dipole Length)
170 MHz	1.76 m	44 cm	88 cm
433 MHz	69 cm	17.3 cm	34.5 cm
868 MHz	34.6 cm	8.6 cm	17.3 cm
915 MHz	32.8 cm	8.2 cm	16.4 cm
2.4 GHz	12.5 cm	3.1 cm	6.25 cm
5.8 GHz	5.2 cm	1.3 cm	2.6 cm

This explains why a high-gain 433 MHz antenna can be 1.75 meters long, while a 5.8 GHz antenna can reach 11 dBi with a much shorter physical length: the higher the frequency, the shorter the wavelength, and the shorter the physical length required for the same number of elements.

2. The Core Secret: Multi-Element Co-linear Array

The key technology that enables fiberglass antennas to achieve high gain—up to 12 dBi—is the internal multi-element co-linear array structure. This is also the fundamental difference between a fiberglass antenna and an ordinary single-section whip antenna.

2.1 What Is a Co-linear Array?

A co-linear array, often called a CoCo antenna, arranges multiple half-wave dipole elements along the same axis, usually vertically, and uses a precisely designed feed network to excite all elements in phase, meaning the current phase is the same.

When all elements radiate with equal amplitude and in phase, their electromagnetic waves interfere in space:

In the horizontal direction—perpendicular to the antenna axis—the path lengths from all elements to a distant point are almost equal. The electromagnetic waves add in phase, field strength multiplies, and gain increases significantly.

In the vertical direction—along the antenna axis—the path differences between elements cause signals to cancel each other, suppressing radiation.

The result is that the donut-shaped pattern of a single half-wave dipole is flattened: the horizontal plane remains 360° omnidirectional, but vertical-plane energy is concentrated toward the horizontal direction, forming a pancake-shaped pattern. It is like replacing a light bulb with the reflector of a flashlight, concentrating light that would otherwise spread up and down into the horizontal direction so it reaches farther.

2.2 Relationship Between Gain and Number of Elements

The gain of a co-linear array is directly related to the number of elements, which is also the array length:

Number of Elements	Theoretical Gain	Vertical Beamwidth	Example BLG Model
1 half-wave dipole	2.15 dBi	78°	TX433-BLG-120 (2.5 dBi / 1200 mm)
2	About 5 dBi	About 40°	TX433-BLG-40 (4.5 dBi / 400 mm)
3	About 6 dBi	About 28°	TX433-BLG-55 (6 dBi / 550 mm)
4	About 8 dBi	About 20°	TX433-BLG-120 (8 dBi / 1200 mm)
6	About 10 dBi	About 13°	TX433-BLG-175 (10 dBi / 1750 mm)
8+	11–12 dBi	About 10°	TX2400-BLG-120 (12 dBi / 1200 mm)

Note: Actual gain is also affected by feed network loss, element matching, and enclosure loss. It is normal for actual gain to be 0.5–1.5 dBi lower than the theoretical value. The BLG series optimizes these parameters during design so actual gain stays as close to theoretical as possible.

For a specific 868 MHz high-gain LoRa option, see the TX868-BLG Series 868MHz High-Gain LoRa Fiberglass Antenna.

2.3 Feed Network: The Key to Keeping Every Element in Step

Making multiple elements work in phase is not as simple as connecting wires in series. A precisely designed phase-shifting network is required to compensate for the phase delay as the signal travels along the feedline, ensuring that the signal reaches each element with the same phase. The BLG series uses two mainstream feeding methods:

Coaxial co-linear structure (CoCo): Uses the inner and outer conductors of a coaxial cable alternately as radiating elements. It is simple and low-cost, and is widely used in low-to-medium gain models below 6 dBi.

Series-fed dipole array: Each dipole is fed through an independent matching network. It offers more uniform gain, wider bandwidth, and better consistency. It is used in high-gain models above 8 dBi and wideband models such as 4G/5G wideband antennas.

3. Fiberglass FRP Enclosure: Why Choose This Material?

Many people wonder when they first see a fiberglass antenna: the enclosure is fiberglass—glass fiber plus resin. Will this material block the signal? The answer is no. Fiberglass has become the standard material for outdoor antenna enclosures precisely because it satisfies three demanding requirements at the same time: mechanical strength, weather resistance, and RF transparency.

3.1 What Is FRP Fiberglass?

FRP stands for Fiber-Reinforced Polymer, commonly known as fiberglass. It is a composite material with glass fiber as the reinforcing skeleton and epoxy or polyester resin as the matrix. Simply put: glass fiber provides steel-like strength, while resin provides cement-like bonding and shaping. Together, the composite outperforms either material alone.

3.2 Five Key Properties of Fiberglass

Property	Parameter / Performance	Significance for Antennas
Mechanical strength	Tensile strength 300–500 MPa, close to ordinary steel	Resists wind, hail, and snow loads; long antennas do not bend
RF transparency	Dielectric constant εr ≈ 4–5, loss tangent tanδ < 0.01	Almost transparent to 170 MHz–6 GHz electromagnetic waves; signal loss < 0.1 dB
Corrosion resistance	Resistant to acid, alkali, and salt spray; does not rust	Long-term use in coastal, chemical plant, and wastewater plant environments
UV resistance	Outdoor life over 10 years with UV stabilizers	Does not age, become brittle, or discolor after long-term sun exposure
Temperature resistance	Stable operation from -60°C to +120°C	Covers the BLG series -40°C to +85°C industrial temperature range

3.3 How Is IP67 Sealing Achieved?

The waterproof sealing process of BLG series antennas mainly includes:

Overall enclosure sealing: The fiberglass outer tube is one-piece pultruded, with no longitudinal seams, avoiding the risk of water ingress through seams.

Top cap sealing: The top uses an engineering plastic cap plus epoxy potting, providing complete sealing.

Bottom connector sealing: The N-type connector and enclosure use a silicone O-ring plus threaded fastening. The bottom cable outlet is potted with waterproof adhesive.

Factory immersion test: Each batch is sampled and tested by immersion at 1 meter for 30 minutes to ensure no water enters.

Note: Although the antenna body is IP67, after field installation the N-type connector between the antenna and feedline should be wrapped with self-amalgamating silicone rubber waterproof tape, such as 3M 2228, to prevent long-term rain exposure and oxidation at the connector.

4. Vertical Polarization: Why Must It Be Mounted Vertically?

Almost all fiberglass omnidirectional antennas use a vertical polarization design, meaning the electric field of the electromagnetic wave is perpendicular to the ground. This is not accidental; it is determined by the physical characteristics of ground-based communication.

4.1 Physical Meaning of Polarization

Electromagnetic waves are transverse waves. The electric field direction and magnetic field direction are perpendicular to each other and both are perpendicular to the propagation direction. The polarization direction is the direction of the electric field:

Vertical polarization (V-Pol): The electric field is perpendicular to the ground—up and down. The antenna must be installed vertically.

Horizontal polarization (H-Pol): The electric field is parallel to the ground—horizontal. The antenna must be installed horizontally.

4.2 Why Is Vertical Polarization Preferred for Ground Communication?

Ground-wave propagation advantage: In the VHF/UHF bands, ground waves propagating along the ground are vertically polarized. Horizontally polarized ground waves attenuate very quickly and disappear within a few kilometers.

Mobile terminal antennas are naturally vertical: Whip antennas on mobile phones, walkie-talkies, and vehicle radios are all vertical. When polarization matches, received signal strength is highest. Polarization mismatch—when transmit and receive antennas are perpendicular—can cause more than 20 dB attenuation, about a 100× power loss.

Omnidirectional characteristics are easier to achieve: A vertically installed co-linear array naturally forms a horizontal 360° omnidirectional pattern. If mounted horizontally, the omnidirectional pattern is destroyed and becomes bidirectional.

Installation note: BLG fiberglass antennas must remain vertical, within 90° ± 5° relative to the ground. A 15° tilt can reduce gain by 2–3 dBi, and a 30° tilt may interrupt communication in some directions. During installation, use a mobile phone level app to help confirm the angle.

5. Gain and Beamwidth: Why Is Higher Not Always Better?

Gain and beamwidth are one of the most easily misunderstood points in antenna selection: higher gain does not mean better. It means more concentrated. Gain and beamwidth are two sides of the same coin.

5.1 What Is Beamwidth?

An antenna pattern is not an ideal uniform disk; it is petal-shaped. Beamwidth usually refers to the half-power beamwidth, or HPBW: the angle between the points where radiated power drops 3 dB, or to half, from the maximum direction.

For an omnidirectional fiberglass antenna, there are two key beamwidths:

Horizontal beamwidth (H-Plane): Always 360°, omnidirectional. It does not change with gain. This is the definition of an omnidirectional antenna.

Vertical beamwidth (E-Plane): Narrows as gain increases. This is the key parameter that determines the coverage shape.

5.2 Gain vs. Vertical Beamwidth Comparison

Gain	Typical Vertical Beamwidth	Coverage Shape	Typical Applications
2.5–4 dBi	About 80°–125°	Near hemispherical coverage	Short-range omnidirectional coverage, indoor/outdoor fill-in, 5G wideband
5–6 dBi	About 30°–50°	Mainly horizontal, with some vertical coverage	Medium-range base stations, light pole coverage, Wi-Fi hotspots
8–10 dBi	About 13°–25°	Flat pancake shape, concentrated in the horizontal plane	Long-range base stations, plain coverage, 4G high gain
11–12 dBi	About 8°–12°	Extremely flat, very narrow in the vertical direction	Ultra-long-distance backbone links, marine/plain long-distance communication

5.3 The “Dead Zone Beneath the Antenna” Problem with High-Gain Antennas

When you install a 12 dBi antenna with a vertical beamwidth of only 10° on a 10-meter-high light pole, the most concentrated energy radiates horizontally to hundreds of meters or even several kilometers away. However, directly beneath the pole—at short range and large depression angles—the signal may be very weak because it is in a pattern null. This is the classic dead zone beneath the antenna.

Solutions:

Choose appropriate gain based on the farthest coverage distance, rather than blindly choosing the highest gain.

In urban, campus, or factory environments with many obstructions, 4–6 dBi may provide more even coverage than 10–12 dBi.

For high-gain antennas, a downtilt of 3°–5° can improve short-range coverage, but it sacrifices far-end distance.

When coverage of tall buildings or hillsides is needed, a lower-gain, wider-beam model is more suitable.

6. VSWR: The Indicator of Antenna Matching Quality

VSWR, or Voltage Standing Wave Ratio, measures the impedance matching between the antenna and the feedline/module. It is one of the most important parameters for evaluating antenna quality.

6.1 Physical Meaning of VSWR

The wireless module output impedance, feedline characteristic impedance, and antenna input impedance are all designed to be 50Ω. When all three are perfectly matched, all high-frequency energy output by the module is transmitted through the feedline to the antenna and radiated, with no reflection. If the antenna input impedance deviates from 50Ω—due to incorrect frequency, antenna damage, a loose connector, proximity to metal, and so on—part of the energy is reflected back to the module, forming a standing wave on the feedline. Forward-propagating energy and reflected energy superimpose, creating a standing waveform with high and low voltage points along the feedline.

6.2 Reflected Power Corresponding to VSWR Values

VSWR	Reflected Power Ratio	Transmission Efficiency	Engineering Evaluation
1.0	0%	100%	Ideal match, not achievable in practice
≤1.5	≤4%	≥96%	Excellent match, standard for BLG main models
≤2.0	≤11%	≥89%	Good match, standard for BLG wideband models
≤3.0	≤25%	≥75%	Workable but not recommended; part of energy is reflected
>3.0	>25%	<75%	Poor match; long-term use may burn out the module

BLG series fiberglass antenna main models have VSWR ≤ 1.5, with reflected power ≤ 4%. Wideband models, such as 4G/5G wideband antennas, relax VSWR to ≤ 2.0 because they must cover an extremely wide frequency range. This is still a good level.

Final Thoughts

Fiberglass antennas achieve high gain through a carefully designed co-linear array, use an FRP radome for weather resistance and RF transparency, and rely on vertical polarization for practical ground communication. Understanding gain, beamwidth, and VSWR helps you choose the right antenna and install it correctly.

For 868 MHz LoRa deployments, the TX868-BLG Series 868MHz High-Gain LoRa Fiberglass Antenna is a practical high-gain fiberglass antenna option designed for long-range and outdoor industrial wireless applications.