700MHz omnidirectional antenna for DTMB digital TV reception

　　700MHz Omnidirectional Antenna for DTMB Digital TV Reception

　　DTMB (Digital Terrestrial Multimedia Broadcasting) is China's independently developed terrestrial digital television standard. The 700MHz frequency band (often referred to as the "digital dividend" spectrum) has become a core carrier for its wide-area coverage due to its strong diffraction characteristics, low propagation loss, and excellent penetration of obstacles such as buildings and foliage. For DTMB reception, omnidirectional antennas are crucial in scenarios requiring 360° signal coverage (such as homes and small communities). They eliminate the need for precise directional alignment while ensuring stable reception of multiple nearby DTMB transmitters. The following is a detailed analysis of their design, performance, applications, and technical optimizations:

　　1. Why 700MHz is an Ideal Choice for DTMB Reception

　　The synergy between the 700MHz band and DTMB stems from the band's physical characteristics and policy-driven spectrum planning, making it a natural partner for digital terrestrial television:

　　1.1 Frequency Band Characteristics that Meet DTMB Requirements

　　Long Wavelength and Strong Diffraction Ability: The 700MHz wavelength (≈42.8 cm) enables the signal to diffract around obstacles such as trees, low buildings, or hills—critical in rural, suburban, and even urban fringe areas where DTMB transmitters are sparsely located or far away. At the same transmit power, the coverage of a 700MHz DTMB signal is 2-3 times that of a 1.5GHz signal.

　　Low Propagation Loss: At a distance of 10 km, the free-space path loss of 700MHz is approximately 8dB lower than that of 1GHz. This means that 700MHz DTMB transmitters can reach remote areas (e.g., villages 15-20 kilometers from a transmission tower) without the need for high-power amplifiers, reducing infrastructure costs for broadcasters.

　　Vertical Polarization Alignment: DTMB terrestrial transmitters primarily use vertical polarization (to match terrestrial reception), and 700MHz omnidirectional antennas can be easily designed with vertical polarization, ensuring efficient signal coupling (avoiding the 10-20dB polarization mismatch loss).

　　1.2 Policy and Spectrum Planning

　　Following China's "analog TV switchoff" (completed in 2020), the 700MHz band was reallocated for the following uses:

　　DTMB Digital TV: The 748-798MHz sub-band (10 channels, each 8MHz) is dedicated to DTMB, covering nationwide terrestrial digital TV services (e.g., CCTV and provincial satellite TV).

　　5G Coexistence: The adjacent 703-748 MHz sub-band is used for 5G n28, but strict frequency separation (enforced by regulatory standards) ensures minimal interference with DTMB reception—this requires antennas with strong out-of-band rejection (see Section 3.2).

　　2. Core Design Requirements for 700 MHz DTMB Omnidirectional Antennas

　　DTMB omnidirectional antennas must balance three priorities: 360° uniform reception, compatibility with DTMB signal characteristics, and adaptability to user scenarios (indoor/outdoor). Key design highlights include:

　　2.1 Radiation Pattern: Strictly Omnidirectional

　　Horizontal Plane (Azimuth): The radiation pattern must be as close to circular as possible, with a gain variation of ≤3dB across 360° (e.g., 4dBi at 0°, 3dBi at 180°). This ensures the absence of "blind spots" in multi-transmitter scenarios (e.g., a single home receiving signals from both north-facing and east-facing DTMB towers).

　　Vertical Plane (Elevation): A narrow vertical beamwidth (typically 30°–60°) focuses energy on ground-based DTMB transmitters (avoiding unnecessary radiation toward the sky or ground). For example, outdoor rooftop antennas typically have a 45° vertical beamwidth to capture signals from towers 10–20 km away.

　　2.2 Gain and Bandwidth: Matching DTMB Technical Specifications

　　Gain Balance: Omnidirectional antennas trade high gain for 360° coverage—indoor models typically have a gain of 3–5 dBi (for compact size and ease of installation), while outdoor models (for long-range reception) achieve 5–8 dBi (through optimized radiator structures such as helical coils or stacked dipoles). Excessive gain (>8 dBi) narrows the horizontal pattern, resulting in a loss of omnidirectionality.

　　Bandwidth Coverage: Full coverage of the DTMB 700MHz band (748-798MHz, 50MHz bandwidth) is required, with a standing wave ratio (VSWR) ≤1.5:1, to ensure no signal loss due to impedance mismatch. Advanced designs (such as loaded dipole antennas) can even extend coverage to adjacent bands (703-748MHz) to accommodate future DTMB expansion while maintaining a low VSWR.

　　2.3 Interference Mitigation: Coexistence with 5G and Other Signals

　　Sharing the 700MHz band (DTMB + 5G n28) requires stringent interference mitigation:

　　Band-edge filtering: A bandpass filter (BPF) is integrated into the antenna to suppress 5G n28 signals (703-748MHz) with a ratio of ≥40dB, preventing strong 5G signals from overloading DTMB receivers (operating at lower power levels).

　　EMI Shielding: For indoor antennas, use a metal shielding can around the feed circuit to reduce interference from home devices operating at 2.4GHz/5GHz (e.g. Wi-Fi routers, microwave ovens). This can reduceross-band interference to below -85dBm, avoiding DTMB picture freezing or pixelation.

　　2.4 Environmental Adaptability: Indoor vs. Outdoor Scenarios

　　Antenna designs vary by usage environment to balance performance, size, and durability:

　　Indoor Models: Prioritize compactness and aesthetics (e.g., desktop, wall-mounted shapes) with sizes typically 10–20cm in height (e.g., Shenzhen Antenna’s DTMB-700I: 15×5×3cm, weight 80g). Materials like ABS plastic ensure safety and match home decor, while adhesive or magnetic mounts simplify installation.

　　Outdoor Models: Focus on ruggedness for harsh conditions:

　　Waterproof/Dustproof: IP65/IP67 ratings (e.g., Huawei’s ODU-700: IP67, submersible in 1m water for 30 minutes) to resist rain, snow, or dust.

　　Weather Resistance: Use FRP (fiberglass-reinforced plastic) or UV-resistant ABS for the radome—withstanding -40℃~+65℃ temperatures, 120km/h winds, and 10+ years of UV exposure (critical for rural rooftop installations).

　　3. Key Performance Indicators for DTMB Reception

　　The effectiveness of a 700MHz omnidirectional antenna is measured by how well it supports DTMB’s core requirements (stable reception, high picture quality, weak-signal capture):

　　3.1 Radiation Efficiency

　　Requirement: ≥60% for indoor antennas, ≥70% for outdoor models (free-space conditions). This ensures minimal signal loss between the antenna and DTMB set-top box. For example, an outdoor antenna with 75% efficiency can receive a -85dBm DTMB signal (weak rural signal) and boost it to -70dBm—enough for the set-top box to decode 1080p video with a bit error rate (BER) ≤1e-6.

　　Impact on User Experience: Low efficiency (<50%) leads to frequent signal dropouts, especially in areas with marginal DTMB coverage (e.g., urban high-rises with shadowing from taller buildings).

　　3.2 Signal-to-Noise Ratio (SNR) Enhancement

　　DTMB requires an SNR of ≥15dB for stable 1080p reception. A high-performance 700MHz omnidirectional antenna should:

　　Capture weak DTMB signals while suppressing background noise (e.g., thermal noise, adjacent-channel interference).

　　Deliver an SNR improvement of 5–10dB compared to a basic "rabbit ears" antenna. For example, in a suburban home 12km from a DTMB tower, an outdoor 700MHz omnidirectional antenna can raise SNR from 12dB (unwatchable) to 18dB (smooth 1080p).

　　3.3 Multi-Transmitter Compatibility

　　Many areas receive DTMB signals from 2–3 transmitters (e.g., local municipal TV + provincial satellite TV). The antenna’s omnidirectional pattern uniformity (gain variation ≤3dB) ensures all transmitters are received equally well—avoiding situations where one channel is clear (strong transmitter) and another is pixelated (weak transmitter due to antenna directionality).

　　4. Typical Application Scenarios & Product Examples

　　4.1 Indoor: Urban/Suburban Family Homes

　　Use Case: Apartments or houses within 8–15km of a DTMB tower, where signals are relatively strong but may be blocked by nearby buildings.

　　Product Example: Taoglas DTMB-700IN Indoor Omnidirectional Antenna

　　Specifications: 4dBi gain, 748–798MHz bandwidth, VSWR ≤1.5:1, magnetic base (for easy placement on TVs or fixtures), size 12×4×2cm.

　　Performance: Receives 8–12 DTMB channels (e.g., CCTV 1–5, local news) with SNR ≥18dB, no pixelation during peak viewing hours.

　　4.2 Outdoor: Rural/Remote Areas

　　Use Case: Villages or farms 15–25km from a DTMB tower, where signals are weak and require long-range capture.

　　Product Example: ZTE DTMB-700OUT Outdoor Omnidirectional Antenna

　　Specifications: 7dBi gain, IP67 rating, 748–798MHz bandwidth, FRP radome, wind resistance ≤120km/h, mounting bracket for rooftops/poles.

　　Performance: Covers 20–25km, receives 6–8 DTMB channels (including provincial TV), maintains stable reception even in heavy rain.

　　4.3 Mobile: RVs/Camping Vehicles

　　Use Case: Temporary DTMB reception for mobile scenarios (e.g., RV travel, outdoor events), requiring portability and quick installation.

　　Product Example: Haier DTMB-700M Mobile Omnidirectional Antenna

　　Specifications: 5dBi gain, foldable design (size 20×8×3cm when unfolded, 10×8×3cm when folded), IP65 rating, USB-powered signal booster (integrated).

　　Performance: Sets up in 5 minutes, receives 4–6 DTMB channels within 10km, compatible with portable DTMB TVs.

　　5. Design Challenges & Solutions

　　5.1 Balancing Omnidirectionality & Gain

　　Challenge: Higher gain often narrows the horizontal radiation pattern (losing omnidirectionality), while pure omnidirectionality limits long-range reception.

　　Solution: Use stacked dipole arrays or helical omnidirectional structures:

　　Stacked dipoles: 2–3 vertical dipoles stacked along the antenna axis, increasing gain by 2–3dBi without significantly narrowing the horizontal pattern (gain variation remains ≤3dB).

　　Helical coils: A vertical helical radiator (1–2 turns) with a shorted bottom—achieves 6–7dBi gain while maintaining 360° coverage (used in outdoor rural antennas).