Loop-on-Ground: Passive Transformer or Active Interface?
Loop-on-Ground: Passive Transformer or Active Interface?
A quiet S-meter is not an efficiency measurement. A Loop-on-Ground should be judged by wanted-signal SNR, stable pattern, common-mode control and the ability of its first interface to preserve the field-derived signal.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
The Loop-on-Ground is a low, lossy, ground-coupled receive sensor. That can be useful: it is easy to deploy and may have a favourable installed noise response. But “quiet” can mean good interference rejection, low output, transformer loss or simple attenuation. Only an SNR comparison separates them.
The Loop, Ground and Feed Form One Circuit
A wire loop placed on or close to soil is coupled capacitively and inductively to a lossy, frequency-dependent medium. Its terminal impedance and pattern depend on perimeter, shape, height, soil conductivity and permittivity, insulation, feed location, nearby conductors and frequency.
It is therefore misleading to assign the loop one fixed impedance and choose a transformer ratio from that number alone. Measure complex impedance over the operating range with the installed geometry, then decide what the first interface must accomplish.
What a Passive Transformer Can Do
A passive transformer can provide impedance transformation, galvanic isolation and a balanced-to-unbalanced transition, depending on topology and construction. It needs no power at the antenna and can tolerate strong signals well when its core and winding remain within their limits.
A nominal 9:1 impedance ratio corresponds to a 3:1 turns ratio in the ideal transformer model. The installed LoG load is rarely a pure resistance equal to the transformed cable impedance across a wide span. Winding capacitance, leakage inductance, magnetizing impedance, core loss and source imbalance all change the transfer.
Passive loss ahead of the receiver reduces wanted signal and antenna noise together. It becomes harmful when the following receiver noise is no longer sufficiently below the delivered external noise, or when the transformer and feed arrangement disturb the desired balance and pattern.
What an Active Interface Can Do
An active interface can present a deliberately chosen impedance to the loop, provide gain before feedline loss and drive a defined cable impedance. A balanced input can preserve the loop’s differential signal while rejecting some common-mode voltage.
The trade-offs are power, noise, linearity, filtering, protection and recovery. Gain that overcomes cable loss can also reduce blocker headroom. A low input-referred noise result can still be irrelevant if external noise dominates, while poor linearity can create false signals in a strong RF environment.
CMRR and Installed Common Mode Are Different
Circuit common-mode rejection ratio is the differential gain divided by common-mode gain under stated source impedances, frequency, amplitude, bias and termination. In decibels:
CMRR: 20 log10|Ad/Acm| dB
A high bench CMRR does not prove that an installed feedline exterior carries no current. Unequal source impedances at the balanced input can convert common-mode voltage to differential output. Coax, power wiring, bias-T, ground bonds and the receiver can form another receiving path.
Measure exterior current and perform controlled cable-routing, bonding and choke changes. If the received signal changes when the feedline moves while the loop stays fixed, the installation is not listening through the loop alone.
Shielded and Unshielded Receiving Elements
RF.Guru LoG systems use both shielded and unshielded receiving elements. The choice is an implementation variable, not a universal ranking. A shield can control electric-field coupling and the physical current path when its gap and bonding are correctly arranged; it can also add capacitance, loss and construction constraints. An unshielded wire is simpler and may be entirely suitable when balance and common mode are controlled elsewhere.
Future-proof comparison should therefore identify the actual element and interface rather than infer performance from a product generation or shield label.
Compare the Two Architectures Fairly
- Use the same loop geometry and site unless geometry itself is the variable under test.
- Declare reference planes at the loop terminals, interface output and receiver input.
- Measure transfer and impedance across frequency, not at one convenient spot.
- Fix receiver settings and compare wanted-signal SNR with rapid A/B/A switching.
- Check external-noise margin with known attenuation so passive loss is interpreted correctly.
- Stress the strong-signal case with representative blockers and verify compression and intermodulation.
- Audit common mode by mapping exterior current and changing one cable treatment at a time.
- Repeat with weather and soil changes because the ground-coupled load can move.
The Better LoG Is the Better Receive System
A passive transformer-fed LoG can be excellent when its loss still leaves adequate external-noise margin and its balance, impedance and feedline behaviour are controlled. An active LoG can be better when it preserves a weak sensor voltage, drives a long cable or establishes a more useful balance and transfer—but only if it remains quiet enough and linear enough.
Do not choose from a transformer ratio or amplifier headline. Choose from complex impedance, antenna factor or transfer, CMRR, exterior-current evidence, blocker margin and wanted-signal SNR in the installed environment.
Primary and authoritative references
- IEEE 145-2025 — Standard for Definitions of Terms for Antennas
- IEEE 149-2021 — Recommended Practice for Antenna Measurements
- ITU-R P.372-17 — Radio noise
- Analog Devices MT-042 — Common-mode rejection ratio
- Texas Instruments — CMRR and balanced-interface limitations
Mini-FAQ
- Does a quiet LoG prove high efficiency? No. Low noise level can result from useful rejection or from attenuation and loss. Compare wanted-signal SNR.
- Is a 9:1 transformer always correct for a LoG? No. The installed complex loop impedance changes with frequency, geometry, soil, insulation and surroundings.
- When is a passive interface attractive? When its transfer, balance and loss preserve enough external-noise margin and its simplicity and strong-signal tolerance suit the station.
- When is an active interface attractive? When a defined sensor load, gain before cable loss or balanced drive improves the complete chain without sacrificing noise or blocker headroom.
- Does high circuit CMRR eliminate feedline pickup? No. Source imbalance and exterior current on coax, power and ground paths remain installed-system measurements.
- Is a shielded receiving element always better? No. Shielding changes electric-field coupling, capacitance, loss and construction. The installed SNR and common-mode evidence decide.