Measuring Antennas with a Field-Strength Meter

I still like the old-school alternative to “I got a better WSPR spot, so the antenna must be better.” Put a stable transmitter on the air, place a field-strength instrument at a marked point, change one thing and see whether the reading returns when you restore the baseline. The result arrives locally and immediately, without asking the ionosphere or another operator to remain constant.

That simplicity is valuable, but it has a hard boundary. A local meter measures its response to an electric or magnetic field at one place, orientation, frequency and time. It does not directly report total radiated power, efficiency, elevation pattern or common-mode current. Those require additional measurements or a validated model.

Know What the Instrument Responds To

A short dipole or monopole probe responds mainly to one component of the electric field. A small loop responds mainly to one component of the magnetic field. Rotate either probe and the reading changes with polarization and orientation. A three-axis isotropic probe combines orthogonal sensors, but its frequency response, isotropy, linearity, dynamic range and calibration uncertainty still matter.

An uncalibrated diode meter can be an excellent null finder or comparison indicator if it remains in its monotonic, repeatable region. It is not an absolute V/m instrument. Detector threshold, compression, temperature, harmonic response, nearby cables and the operator’s body can all move the indication.

For a calibrated receiving antenna, the antenna factor relates incident field strength to the voltage delivered to the specified load. NIST’s methodology for standard electromagnetic-field measurements makes the calibration boundary explicit: a transfer E- or H-field probe must first be calibrated in a known standard field, and mismatch to the 50-ohm receiver is part of the antenna-factor result.

Record probe type, serial number, calibration date and uncertainty where available; probe axis or polarization; receiver input impedance; cable loss; detector type; resolution bandwidth; averaging; and overload margin. A spectrum analyser or SDR adds frequency selectivity, not automatic calibration.

Field Regions Depend on Antenna Size and Direction

There is no universal lambda/6, 2 lambda, 5 m or 10 m boundary. Start with wavelength lambda, the antenna’s maximum dimension D, the observation direction and the complete radiating structure. If the feed line or ground system carries unintended current, it belongs in that structure until the current is controlled.

In the reactive near field, stored electric and magnetic energy can dominate and E/H need not resemble the free-space wave impedance. In the radiating near field, propagating energy dominates but the angular distribution can still change with distance. In the far field, the angular pattern is essentially independent of distance and the field components behave locally like a transverse wave.

For a broadside antenna whose maximum dimension is large compared with wavelength, ITU-T K.61 (10/2025) gives 2D²/lambda as a commonly used far-field starting distance in the direction of maximum radiation. It is not a replacement for checking the actual antenna and direction. Small antennas, end-fire arrays, ground interaction and off-axis directions need their appropriate model or range criterion.

Validate the range experimentally. Repeat the same angular cut at more than one radius. After normalization, a far-field pattern should remain stable within the uncertainty, and field amplitude should follow the expected distance trend. If lobes, nulls or polarization response move when the radius changes, the range or site is still part of the result.

The Yard Is a Multipath Range

Ground, fences, buildings, trees, vehicles, utility wiring and even the person holding the meter create direct and reflected paths. At HF, finding a textbook far-field range for a large antenna may be impractical. That does not make local testing useless; it makes the conclusion site-specific.

• Mark the transmitter, antenna, probe and tripod coordinates.
• Keep both antenna heights, probe orientation and polarization fixed.
• Use non-conductive support and route probe cables away consistently.
• Keep people and movable objects out of the range during readings.
• Repeat at several probe positions to reveal standing-wave hot spots and nulls.
• Document weather and ground condition when they can change conductivity or geometry.

A circle around the antenna can provide a useful installed horizontal cut if the radius, height, polarization and range validity remain controlled. It is not automatically a free-space azimuth pattern. Changing probe height at one radius samples a different combination of direct and reflected fields; it does not by itself reconstruct the antenna’s elevation pattern or distinguish NVIS from low-angle radiation.

IEEE 149-2021 treats the antenna, test range, instrumentation, site evaluation and uncertainty as one measurement system. That is the right mental model even when the “range” is two tripods in a field.

Normalize to the Power That Reached the Test Boundary

Field amplitude from a linear antenna system scales approximately with the square root of accepted power. A comparison is therefore meaningless if transmitter output, foldback, tuner state or feed-line loss changed unnoticed.

Choose and declare a power reference plane. Measure forward and reflected quantities with equipment whose directivity and calibration are adequate for the mismatch, or measure accepted power by another validated method. Log transmitter state, amplifier state, duty cycle, tuner setting, feed-line configuration and temperature.

For calibrated electric-field readings, a useful normalized level is:

If the meter provides only a relative indication, first verify that its detector is stable and linear enough over the comparison range. A 3 dB display change is not meaningful when one reading is near threshold and the other is in compression.

Use A/B/A, Cross-Swaps and Repetition

Repeat the cycle and randomize the order when practical. If comparing two antennas, cross-swap feed lines, matching networks, power sensors and transmitter ports. A result that follows one cable or tuner is not solely an antenna result.

Use a reference radiator or transfer antenna to check the range before and after the session. A stable reference catches transmitter drift, receiver drift, moved cables and environmental changes that a simple A/B sequence can hide.

Report the mean, spread and number of repetitions rather than the most flattering pair. BIPM/JCGM 100 provides the general framework: define the measurand, identify input quantities and state the uncertainty that belongs with the result.

What a Local Field Test Can and Cannot Prove

To estimate radiation efficiency, combine gain and directivity from a qualified full-pattern measurement, use an accepted loss-measurement method, or validate a complete model against current and field data. One local field sample cannot separate directivity from efficiency.

To diagnose common-mode current, put a calibrated current probe around the complete coax or declared conductor set and map the net current at multiple positions. A field meter is useful corroborating evidence, especially when cable routing changes the field, but it does not identify the current path by itself.

Field Strength and Propagation Reports Answer Different Questions

WSPR, FT8 and ordinary signal reports include the remote station, propagation, polarization rotation, interference, receiver state and time variability. They are excellent for end-to-end operating evidence when collected in paired, repeated observations. They are weak evidence for a small hardware change when those other variables are uncontrolled.

A local field test removes most ionospheric variability and gives immediate feedback on one installed geometry. It is strongest for repeatable A/B/A changes, null finding, range debugging and identifying whether a cable or nearby object participates. Use local tests to understand the installation; use propagation reports to learn how the complete radio path behaves over time.

Continuous-Carrier Tests Need an RF-Safety Plan

A continuous carrier creates a high duty cycle. It can heat transmitters, amplifiers, matching components, chokes and feed-line faults much faster than conversational operation. Use the lowest power and shortest transmission that achieves adequate measurement margin, stay within every component’s continuous-duty rating and de-energize before touching or changing anything.

Establish a controlled test area with remote keying and an immediate transmit inhibit. Keep people and animals outside the evaluated boundary. Do not stand beside the probe during a reading, and do not assume that the probe’s displayed field is the field a person would experience at every body position.

At HF and other near-field locations, electric and magnetic fields are not necessarily related by 377 ohms. ICNIRP’s 2020 RF guidelines require both E- and H-field reference levels to be considered from 100 kHz to 30 MHz, regardless of a casual near/far label. IEEE C95.3-2021 covers measurement and computation methods for human exposure in near and far fields.

A hobby field-strength meter is not an exposure-compliance instrument unless its calibration, isotropy, frequency response, spatial averaging, time averaging and uncertainty meet the applicable procedure. Follow binding local rules and use qualified assessment when compliance is in question. The antenna comparison never takes priority over RF safety.

Keep It Old-School—and Make It Auditable

You do not need an anechoic chamber to learn something useful. A modest meter, a non-conductive tripod, marked coordinates, a power record and disciplined A/B/A switching can expose changes that SWR and propagation anecdotes hide.

The honest conclusion may be narrow: “At this frequency, polarization, point and accepted power, configuration B produced 1.4 dB more normalized E-field than A, with 0.5 dB expanded uncertainty.” That is not a lesser result. It is a result another operator can understand, challenge and repeat.

Primary and authoritative technical sources

• IEEE 149-2021—antenna pattern, gain, impedance, test-range, instrumentation and uncertainty practice.
• ITU-T K.61 (10/2025)—current field-region definitions, antenna-size boundary and RF-EMF measurement guidance.
• NIST: Methodology for Standard Electromagnetic Field Measurements—E/H transfer probes, standard fields, antenna factor and receiver-load boundary.
• ITU-R SM.378-7—calibrated field-strength antennas, polarization and measurement-system practice.
• ICNIRP 2020 RF Guidelines—RF-exposure quantities, averaging and near-field E/H requirements.
• IEEE C95.3-2021—best practice for RF-exposure measurements and computations.
• BIPM/JCGM 100—measurement models and expression of uncertainty.

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