Why Traveling-Wave Antennas Are Less Prone to Common-Mode Noise Pickup

(Beverage • T2FD • Rhombic • Terminated loop / “TermiLoop”)

The short version

Common-mode (CM) noise rides on the outside of your coax and other leads, then sneaks into the receiver at the feedpoint as if it were a wanted signal. Traveling-wave antennas—those with a resistive termination that prevents standing waves—tend to excite the feedline far less than resonant, high-Q antennas. Add their mostly balanced geometries and directional patterns that put deep nulls on local noise, and you get the reputation these antennas have earned for “quiet” reception. Proper feedline choking is still mandatory, but the architecture stacks the deck in your favor.

What is common-mode noise—and why do antennas “make” it?

• Differential vs common mode. Inside a coax, the desired RF flows as equal-and-opposite currents on the center conductor and the inside of the shield (differential mode). Unwanted current can also flow on the outside of the shield (common mode). That outside current is effectively another antenna coupled to every switching supply, router, and power line around you. If it isn’t stopped at the antenna end, it enters the receiver as added noise.
• What excites CM current? Imbalance—geometric or electrical—and resonances that place high RF voltages at the feedpoint. Chokes presenting several kilo-ohms of CM impedance at the antenna sharply reduce this current.

Why traveling-wave antennas behave better

They’re non-resonant—by design. A resistor at the far end of the wire absorbs the traveling wave, preventing reflections and the large voltage/current peaks that go hand-in-hand with standing waves. A quieter, flatter current distribution at the feedpoint means less tendency to drive the feedline’s outside surface. (Reflection coefficient Γ = (Zᴸ − Z₀)/(Zᴸ + Z₀) ≈ 0 when Zᴸ ≈ Z₀.) This is the core idea behind terminated Beverages, T2FDs, rhombics, and terminated loops.

They’re usually balanced. T2FDs, rhombics, and terminated loops are symmetric and commonly fed through a transformer or balun. That symmetry reduces the net electric-field imbalance at the feedpoint that would otherwise launch CM current on a coax. You still add a 1:1 current choke at the transition to coax.

Their patterns reject local junk. Receive-directivity factor (RDF) and deep, steerable nulls are the traveling-wave superpower. Beverages reach roughly 6–14 dB RDF as length increases; small terminated loops (K9AY/Flag/Pennant family) are around 6–8 dB RDF and can be aimed to put a null on a noisy power line or grow-light. Nulling the direction of noise reduces what ever reaches the antenna terminals in the first place.

Some designs couple less to troublesome E-fields. Blunt-ended loops have fewer high-voltage hotspots that are prone to precipitation/static crackle and other electric-field nasties compared with open-ended wires—a side benefit that many operators interpret as “quieter.”

How the main traveling-wave families suppress CM pickup

Beverage (terminated long wire, close to ground)

Mechanism. A long, low wire with a ground reference and a far-end termination forms an end-fire traveling-wave receiver. The resistor absorbs reverse-traveling energy, sharpening forward directivity and avoiding strong standing-wave voltages at the feedpoint that would pump the feedline.

Noise behavior. Because its pattern is narrow and low-angle, a Beverage often reduces the amount of local RFI that ever reaches the feedpoint. With proper feedline isolation, RDF improves markedly with length (roughly 7–12 dB from 75–300 m (≈ 250–1000 ft)).

Keys to keeping it quiet. Use a transformer at the feedpoint and serious CM chokes on the coax; route and/or bury the feedline away from the wire. Ferrites and burial consistently deepen nulls and stabilize patterns.

T2FD (Tilted Terminated Folded Dipole)

Mechanism. A folded dipole with a broadband resistor across the far conductor. The termination damps resonance and keeps impedance smooth so the feedpoint doesn’t develop high Q and E-field hotspots. Feed it with a 4:1 transformer into a 1:1 current choke.

Noise behavior. Users prize the T2FD as a “steady,” low-drama receiver across wide HF spectrum. You give up some efficiency (resistor loss) but get less propensity to light up the feedline or shack. As with any balanced antenna transitioning to coax, the choke is what stops CM.

Rhombic (terminated)

Mechanism. Four long legs forming a diamond; the far-end termination (typically 600–800 Ω) matches the antenna’s traveling-wave impedance. Result: unidirectional pattern, wide bandwidth, and minimal standing waves.

Noise behavior. High directivity rejects off-axis noise. Balanced feed (often open-wire) and a proper balun minimize CM injection into any coax transition.

Terminated loop family (K9AY, Flag, Pennant)

Mechanism. These small receive-only loops use a resistive termination to shape a cardioid-like pattern with a deep rear null. The geometry and termination damp resonance and help avoid large feedpoint voltages that would otherwise launch CM currents.

Noise behavior. Steerable nulls make them effective local-noise fighters. Feedline choking and burial are critical to preserve pattern integrity and maintain a deep null.

The TermiLoop (RF.Guru)

Mechanism. The TermiLoop is a compact folded traveling-wave loop intended for both RX and TX. It uses a resistive termination placed at a low-current, high-voltage node and maintains a smooth impedance curve (SWR < 3 : 1) across 160–6 m. Its geometry is broadband by design—more akin to a folded end-fed traveling-wave element than a resonant loop.

Noise behavior. The TermiLoop’s termination flattens voltage peaks, suppressing CM excitation, while its loop symmetry and feed transformer preserve balance. It remains quiet even on mixed-band operation thanks to low standing-wave voltage and proper feedline choking.

Feedline discipline still matters

• Install a high-impedance 1:1 current choke at the antenna. More CM impedance → less CM current. Practical designs by K9YC present several kΩ across HF; higher is better.
• Choke every attached cable—coax, control, and DC—and for deep nulls with small loops, bury runs ≈ 30 cm (≈ 12 in) to suppress re-radiation that spoils the pattern.
• Keep transitions balanced. Use an appropriate transformer or balun, and route the feedline away at right angles for several meters before turning toward the shack.

Comparison at a glance

Reality checks and tradeoffs

• “Quiet” ≠ “high gain.” Many traveling-wave receive antennas have low absolute signal output, but their SNR is better thanks to pattern and lower CM injection. That’s why preamps are standard with small terminated loops and short Beverages.
• Termination costs watts (on transmit). T2FDs, rhombics, and TermiLoops deliberately dissipate power in the resistor—acceptable for broadband stability but an efficiency tradeoff.
• CM can still bite. Even the best traveling-wave antenna gets noisy if the coax becomes part of the antenna. Ferrites and disciplined cable management are non-optional.

Quick setup tips that pay off

• Place a choke at the feedpoint (and again where the coax enters the shack). Target multi-kΩ CM impedance across your bands of interest.
• Aim and height matter. Beverages improve RDF with length; small terminated loops should be near ground and aimed so the null sits on your worst RFI direction.
• Bury or route feedlines away at right angles for several meters before turning toward the shack; burial often deepens nulls.

Bottom line

Traveling-wave antennas earn their “quiet” reputation because terminations kill standing-wave conditions that tend to pump common-mode current onto the feedline, while their balanced geometries and directional patterns keep local noise out of the antenna in the first place. Add proper chokes and disciplined routing, and you’ll get the low-noise performance these designs are famous for.

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