那些PCB天线/倒F型天线，为什么其中一端要接地，这样子不会短路吗？这个接地有什么用？PCB天线设计有什么要注意的？
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When using an antenna, our core objective is to have an external electromagnetic field excite a standing wave on the antenna. Recalling the characteristics of a standing wave: there exist wave nodes where the amplitude is always 0, and wave antinodes where the amplitude is always at its maximum.
Analyzing the distribution patterns of voltage waves and current waves from the perspective of electromagnetic waves: In a circuit, the voltage at the grounding point is clamped at 0V, so the voltage there is always 0; while at the open end (the other end) of the antenna, the voltage reaches its maximum value. The distribution of current is completely opposite: the current is maximum at the grounding point and minimum at the open end.
Thus, the antenna structure of one end grounded, one end open essentially dictates the form of the standing wave — the wave antinode must appear at the open end, and the wave node must appear at the grounding point. Only electromagnetic waves with a wavelength four times the length of the antenna can stably produce a standing wave on this structure (in reality, there are multiple sets of wavelengths that meet the condition; here we only discuss the fundamental wave with the lowest frequency). Therefore, this structure can be used as a narrowband antenna.
Simultaneously, from the grounding point to the open end, the voltage amplitude gradually increases from 0, while the current amplitude gradually decreases from its maximum value to 0; the absolute value of the ratio of voltage to current will cover all values from 0 to infinity. And this ratio can precisely be used to match the characteristic impedance of the subsequent circuit’s input.
The structural advantages of the F-type antenna are quite significant: the antenna length and the feed point position are extremely easy to control. Therefore, in scenarios lacking computer simulation tools and professional testing methods, it is easy to complete the design for frequency and impedance matching — this is also the core reason why it frequently appeared in early circuits.
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Can it be understood as: adjusting the frequency by controlling the length from the ground end to the open end to be 1/4 wavelength, and performing impedance matching by selecting the position of the feed port between these two points?
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Yes 
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The inverted-F antenna’s ground connection is not a DC short circuit (the ground at RF frequencies is an RF ground, which has different characteristics from a DC ground). Its core functions are: 1. To form a resonant structure with the radiating element, determining the antenna’s operating frequency; 2. To provide an RF reference ground plane, ensuring effective electromagnetic wave radiation; 3. To work in conjunction with the length of the radiating arm and shorting stub to adjust impedance matching (typically targeting 50Ω), thereby improving signal transmission and reception efficiency.
Considerations for PCB antenna design:
- Ensure ground plane integrity (avoid large-area slots, which can affect radiation performance);
- Strictly match the dimensions corresponding to the resonant frequency (the lengths of the radiating arm and shorting stub require simulation/actual measurement calibration);
- Control 50Ω impedance matching (adjusted via trace width, dielectric thickness, and dielectric constant);
- Keep away from interference sources such as power supplies and clock circuits (maintain an isolation zone of at least 20mm);
- Select appropriate PCB material (e.g., FR-4 has stable dielectric constant, while PTFE-based materials are suitable for high frequencies).
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Great question! It seems like it should short out, right? But at radio frequencies, it doesn’t act like a simple DC short.
That grounded leg is a key part of the design. It does two main things:
- Impedance Matching: It helps “tune” the antenna to work with the 50-ohm feed line from your chip, so power transfers efficiently instead of reflecting back.
- Shrinks the Antenna: It lets the physical antenna be about a quarter-wavelength long, instead of a half-wavelength, by using the ground plane as the other half.
For PCB antenna design, the big three are:
- Follow the Reference! Never wing it. Use the IC manufacturer’s reference layout exactly—their keep-out zones, ground plane shape, and component placement.
- Mind the Keep-Out Zone: Never put anything, especially metal or noisy circuits, in the antenna’s specified empty area.
- You MUST Tune It: Every PCB is different. You’ll always need to tune the matching network (those tiny capacitors/inductors near the feed) on real boards to get peak performance.
Hope that helps
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The shorted pin is there on purpose – it sets the impedance and lets the antenna resonate at a lower frequency than its physical length. It’s not a “DC short” that kills the RF; for RF signals that stub is part of the antenna structure, not a harmful short circuit. That “ground” connection is mainly for matching and to create a virtual ground reference for the radiator.
About design:
- Keep a solid ground plane under the feed line, but clear copper (no ground) directly under the antenna trace. Leave a keep‑out area around the antenna element.
- The antenna size/shape is tuned to your frequency; even small changes can shift resonance, so follow the reference layout as closely as you can and tune with a matching network (pi network, etc.).
- Keep metal objects, batteries, connectors, and big copper pours away from the antenna area to avoid detuning and efficiency loss.
- Always verify with a VNA or on‑air testing; simulation alone isn’t enough.
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PCB antennas (especially inverted-F) being connected at one end to “ground” is not a “short” — it gives the antenna a loop, an image and matching in the high‑frequency world. Below are three things explained at once:
- Whether grounding actually shorts it;
- What grounding actually does;
- Pitfalls to avoid during layout.
- Why connecting one end to ground won’t “short” it
- DC view: indeed 0 Ω, but RF signals are AC — on the wavelength scale that ground leg behaves like a distributed L/C, showing inductive/capacitive reactance, not 0 Ω.
- High-frequency return: current must return to the source; the ground plane is the lowest-impedance return path. Without it, currents on the radiating arm cannot “close the loop” and no radiation occurs.
- Image principle: the ground plane acts like a “mirror”, letting a 1/4λ monopole produce the same radiated field as a 1/2λ dipole while halving the required height.
- Three main roles of grounding
(1) Impedance matching
The inverted-F adjusts the distance between the feed point and the shorting pin to tune the antenna impedance from the original 30–70 Ω to 50 Ω, allowing direct conjugate match to the feed without an extra π/T matching network, minimizing return loss.
(2) Forming the current loop & image radiation
The shorting pin provides a “0 potential” reference for high-frequency current and excites image currents on the ground plane; their superposition efficiently sends energy into space. Without it, the antenna is just a “dead copper strip”.
(3) Tuning the resonant frequency
The closer the shorting pin is to the feed point, the larger the equivalent parallel inductance and the higher the resonance; vice versa if farther away. Moving 0.5 mm in layout can shift 2.4 GHz to 2.5 GHz — the cheapest “passive tuning knob”.
- Seven hard rules for PCB antenna design
- Continuous ground plane: all layers beneath the antenna must be cleared (keep-out), but leave an unbroken reference ground around and on the back side; if the return path is cut, gain immediately drops 3–6 dB.
- Shorting pin width: typically 0.3–0.5 mm — too wide reduces reactance and shifts resonance high; too narrow gives high Q but narrow bandwidth, causing frequency shift in mass production.
- 50 Ω feedline: microstrip width is calculated from board thickness/dielectric constant; a 2 Ω difference can change return loss from –20 dB to –10 dB; place ground vias beside it,
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