Technical Article

Is Every Conductor an Antenna?

December 01, 2023 by Robert Keim

Learn how conductive circuit components can function as antennas, even when they aren't intended to.

Hobbyist radio kits sometimes include an “antenna” that is nothing more than a wire of appropriate length. This is a good reminder that the most basic antenna is simply a conductor—and that a simple conductor may become an antenna whether we want it to or not.

 

Current, Voltage, and Electromagnetic Radiation

The Lessons in Electric Circuits online textbook defines an antenna as “a passive component that is used to convert an RF electrical signal into electromagnetic radiation (EMR), or vice versa.” But the components that we call antennas aren’t the only sources of EMR in a circuit.

EMR is produced when a voltage drives time-varying current through a conductor. Energy that is initially present in the conductor as the movement of electric charge is converted into energy that propagates outward into space as electric and magnetic fields. Similarly, electromagnetic energy is converted into voltage and current signals inside a conductor when the conductor is exposed to electromagnetic waves.

Electronic devices of all kinds—including those that don’t involve any form of wireless communication—use time-varying signals that generate EMR, and are also exposed to EMR that induces time-varying signals. In this sense, antennas are everywhere: PCB traces, connector wires, and component leads are all capable of emitting and receiving electromagnetic radiation.

In another sense, though, antennas are not everywhere. Components and circuits, especially those involving high-frequency signals, are designed to reduce unwanted emissions and resist the influence of potentially disruptive EMR. Despite the fact that the PCB for a typical embedded device is filled with conductors that theoretically function as RF transmitters and receivers, these unintentional antennas usually don’t have deleterious effects on real-world functionality.

 

Some Antennas Are Better Than Others

The dimensions and configurations of an antenna determine how efficiently it radiates or receives, which is why the first step with any basic antenna is to adjust its physical length according to the wavelength of the signal. You can read more about this in Mark Hughes’s excellent article, “An Introduction to Antenna Basics.”

In many cases, ordinary conductors can barely be described as antennas—they’re so short relative to the signal wavelength that the effects of transmission or reception are negligible. They’re really not comparable to sophisticated, optimized antennas that achieve high-efficiency conversion and may even apply large amounts of gain, as in Figure 1.

 

The Voyager spacecraft is equipped with a parabolic antenna.

Figure 1. The Voyager spacecraft is equipped with a parabolic antenna that provides 48 dB of gain. Image used courtesy of JPL

 

Longer conductors can also be bad antennas. However, when the conductor isn’t intended for transmitting or receiving RF signals, that’s beneficial. For example, twisted-pair cabling combined with differential signaling makes long conductors both less likely to emit problematic EMR and more resistant to incoming EMR.

 

Digital vs. Analog and RF

We’ve established that every conductor is at least theoretically an antenna, but the extent to which this is relevant to electronic design depends on the nature of the signals involved.

 

Digital Signals

Digital signals, especially high-frequency digital signals, will gladly use your PCB traces (Figure 2), component leads, and interconnects as antennas. The wide range of sinusoidal frequencies present in rapid logic-level transitions ensure that there are plenty of opportunities for higher-efficiency conversion between the electrical and electromagnetic domains.

 

Close-up view of a PCB.

Figure 2. PCB traces can function as antennas. Image used courtesy of Adobe Stock

 

However, digital signaling is inherently robust against received EMR. This is because interference will impair performance only if it’s strong enough to push a voltage to the other side of the logic threshold.

 

Analog and RF Signals

“Analog” and “RF” both refer to non-digital signals. Here, “analog” implies lower frequencies. Analog and RF signals are both more likely than digital signals to suffer noticeable degradation from unintentional antennas in a circuit, but RF presents unique challenges.

The high-frequency signals in RF circuits radiate more effectively from short conductors. Also, short conductors more effectively receive signals that are difficult to ignore or filter out because their frequencies are comparable to the system’s frequencies of interest. For this reason, RF designers need to be attuned to the idea that (almost) every conductor is an antenna.

I learned this lesson the hard way as a novice engineer working on an L-band (1 GHz to 2 GHz) data link. At least one of the PCBs in this system had minor design issues, and these minor issues resulted in functionality that was disappointing and sometimes baffling.

The unit’s performance was surprisingly sensitive to small changes in the operational environment. I remember one field test in particular: the bit error rate increased dramatically right before the cellphone in my pocket started ringing. I mentioned this later in a meeting, and a seasoned engineer assured me that the system couldn’t be affected by cellphone EMR. Fair enough, but I wasn’t hallucinating.

 

Wrapping Up

Though most of my designs haven’t required special attention to unintentional transmission and reception, one of my most notable achievements as an engineer involved finding ways to deal with optical-detector cables that were behaving too much like antennas. I hope that this article has helped you to understand a concept that is sometimes a crucial aspect of electronic design.

 

Featured image used courtesy of Adobe Stock

1 Comment
  • J
    JDenenberg December 08, 2023

    Coaxial cable can unexpectedly act as an antenna. There are actually two transmission lines when you use a coaxial cable; the one inside the cable and the one formed between the shield an ground. This second transmission line is often a resonant structure due to being terminated in a short circuit (sometimes open circuit) at each end. T a resonant frequency the input impedance of the external transmission line at the source end will be extremely low so some of the internal cable signal energy can couple into the external cable and the shied becomes ineffective. Solving this problem requires a carefully designed transformer at the source end to leave the coaxial shield floating and then terminate the external transmission line withe an RC series circuit (a resistance of about 180 ohms will be a good termination). This prevents the energy from building up in the external transmission line and restores the shielding to effectiveness.

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