Measuring Antennas with a Field Strength Meter: A Practical Guide

When most hams think about antenna performance, they often rely on indirect methods—WSPR spots, FT8 reports, or signal reports from friends. While these tools are undeniably useful for real-world propagation checks, they tell you little about how your antenna radiates energy. Are you radiating efficiently? In the right direction? Is your new balun helping or hurting?

A field strength (FS) meter—also called a field energy meter—is a simple yet powerful tool for understanding your antenna’s actual radiation behavior, both in the near and far field. In this article, we’ll walk through the how and why of field strength measurements, and show you how to extract meaningful performance data from your antenna setup.

Why Use a Field Strength Meter?

Unlike WSPR or FT8 reports, which depend on ionospheric conditions and other stations' setups, a field strength meter gives you a local, direct, and immediate measure of your antenna’s radiated power. Here’s what it can reveal:

  • Radiation pattern shape (qualitatively)
  • Changes in efficiency after modifying the antenna, balun, or ground system
  • Nulls, lobes, and directionality
  • Near-field anomalies (e.g., strong common-mode radiation on the feedline)
  • Differences between vertical and horizontal polarization

Understanding the Measurement Zones

Before measuring, it’s crucial to understand where you're standing relative to the antenna:

Zone Distance from Antenna What It Tells You
Reactive Near Field 0–λ/6 Complex E and H field behavior; not ideal for measurement
Radiating Near Field (Fresnel Zone) ~λ/6 to 2λ Useful for comparative field strength measurements
Far Field (Fraunhofer Zone) >2λ Where true radiation pattern and gain are stable

For HF bands (say, 20 meters = 14 MHz = 21.4 m wavelength), you want to take measurements at least 5 to 10 metersaway for meaningful trends. For VHF/UHF, even 2–3 meters might be enough.

What You Need

  • Field Strength Meter or RF probe (homemade or commercial)
  • Optional: Calibrated reference antenna
  • Non-conductive tripod or handheld boom
  • Transceiver with controlled output power
  • Optional: Step attenuator or RF power meter

If you have access to a spectrum analyzer or SDR with a calibrated antenna, you can create a highly sensitive, directional FS meter with dB resolution.

How to Measure

1. Establish a Baseline

  • Set your rig to transmit a continuous carrier (e.g., AM, FM, or CW).
  • Use a known-good antenna as a baseline, or take measurements before/after any antenna changes.
  • Choose a fixed distance—ideally in the radiating near field (1–2 wavelengths).

2. Walk the Circle

  • Place your FS meter on a boom or tripod and move it around the antenna in a circle, maintaining a fixed radius.
  • Record FS readings every 30° or so.
  • Plot the values in polar or linear format to visualize lobes and nulls.

3. Change Height

  • Take readings at different heights (e.g., 0.5m, 1m, 2m) to examine elevation behavior—especially relevant for NVIS vs DX patterns on 40m or 80m.

4. Compare Setups

  • Change one variable at a time: radial count, balun placement, feedline length, antenna height.
  • Record FS meter readings for each scenario to evaluate relative efficiency.

Real-World Use Cases

Balun or Choke Effectiveness
If moving the feedline causes large changes in FS readings, you may be dealing with unwanted common-mode radiation. A good current balun should minimize this.

Efficiency Check
Two antennas that load up equally well on the same band may radiate very differently. FS meter readings at 1–2 wavelengths can reveal which one actually puts more power into the air.

Radiation Pattern Mapping
You’ll see how directional (or not) your antenna is. A 5/8-wave vertical might radiate much stronger at low angles than a 1/4-wave whip—even if SWR is perfect in both.

Caveats and Tips

  • Don’t chase absolute dB values unless you’re using calibrated gear. Focus on relative comparisons.
  • Avoid reflections—try to do measurements away from buildings, fences, and large metallic objects.
  • Normalize measurements to account for power output—especially when testing antennas across different bands.
  • Be patient. FS readings can vary with tiny changes in placement or cable routing.

Field Strength vs. WSPR: Complementary Tools

Feature Field Strength Meter WSPR/FT8 Reports
Local Radiation? ✅ Yes ❌ Not directly
Directionality? ✅ Yes ⚠️ Only via multiple reports
Propagation Info? ❌ No ✅ Yes
Quantitative Gain Comparison? ⚠️ With effort ❌ Not reliably
Common Mode Detection? ✅ Yes ❌ No

Use both: WSPR for real-world propagation through the ionosphere, and FS meters for antenna efficiency and pattern behavior in your own yard.

Final Thoughts

Too many hams rely exclusively on digital mode reports to assess antennas. But these are often more about propagation than performance. If you really want to understand what your antenna is doing—how well it radiates, and in which directions—a field strength meter is a surprisingly powerful, old-school tool. You don’t need a lab-grade setup; even basic tools can reveal shocking truths about your antenna’s behavior.

And once you’ve seen the difference for yourself, you’ll never blindly trust SWR again.

Interested in more technical content like this? Subscribe to our notification list — we only send updates when new articles or blogs are published: https://listmonk.rf.guru/subscription/form

Questions or experiences to share? Feel free to contact RF.Guru or join our feedback group!

Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.