Measuring Spurious Emissions and Harmonics

Every transmitter produces not only the intended signal on the operating frequency but also small amounts of energy at other frequencies — harmonics, sub-harmonics, mixing products, and oscillator leakage. These are called spurious emissions. If strong enough, they cause interference on other services, violate FCC regulations, and can even cause problems within your own station by entering sensitive receiver circuits through the same antenna system.

FCC Part 97.307 requires amateur radio transmitters to suppress these spurious emissions to specific minimum levels. Verifying compliance requires a spectrum analyzer and the correct measurement procedure. This lesson teaches you exactly how to perform that measurement, how to interpret the results, and how to document them — as well as what to do if the measurement reveals a problem.

What Are Spurious Emissions?

A spurious emission is any emission from a transmitter at a frequency that is not the intended operating frequency, and that is not necessary for the transmission. The FCC defines them broadly to include: harmonics of the operating frequency, sub-harmonics, parasitic oscillations, intermodulation products, frequency conversion products, and any other unintended signals.

The term "spurious" is used specifically to distinguish these unwanted outputs from the sidebands that are part of the intended modulation. For example, the upper and lower sidebands of an SSB signal are expected and necessary — they are not spurious. But if the balanced modulator in the SSB transmitter is slightly unbalanced and lets through a small amount of carrier, that suppressed carrier is spurious. If the power amplifier generates a second harmonic at twice the operating frequency, that is spurious.

Harmonics — Cause and Effect

Harmonics are the most common and most important type of spurious emission. They are generated when a transmitter's power amplifier operates in a non-linear mode — which it does, by design, in Class C operation (used in CW and FM transmitters for efficiency), and to a lesser degree in Class AB linear amplifiers (used for SSB).

A non-linear amplifier generates multiples of the input frequency: the second harmonic at 2f, the third at 3f, the fourth at 4f, and so on. The amplitude of these harmonics falls with order — the second harmonic is the strongest, the third weaker, and higher harmonics weaker still — but this is not always guaranteed. A Class C amplifier without adequate output filtering can produce harmonics only 20 dB below the fundamental, which is nowhere near adequate for FCC compliance.

Low-pass filters in the transmitter output circuit suppress harmonics. A well-designed transmitter includes a multi-element low-pass filter (often called a harmonic filter) tuned to the operating band. The filter's cutoff frequency is set just above the highest operating frequency of the band, so it passes the fundamental with minimal insertion loss while providing 40–60 dB or more of attenuation at the harmonic frequencies. The quality of this filter largely determines the transmitter's harmonic suppression performance.

In amateur radio, harmonics from HF transmitters often fall on other amateur bands. A 7 MHz (40m) transmitter's second harmonic falls on 14 MHz (20m) — another popular band. Its third harmonic falls on 21 MHz (15m). This is why harmonic suppression matters even when there are no non-amateur services near by: your own harmonics can interfere with yourself on other bands and with other amateurs.

FCC Part 97 Requirements

FCC 47 CFR §97.307 sets the spurious emission requirements for amateur transmitters. The key requirements are:

Note that for high-power transmitters, there are two conditions that must both be satisfied. A 1500 W transmitter must suppress harmonics to at least 43 dBc (which would place harmonics at ≤37.5 mW) AND to at most 50 mW absolute power. In this case, the 43 dBc relative requirement (37.5 mW) is more stringent than the 50 mW absolute requirement, so 43 dBc is the binding limit. For a 10 W transmitter running at 25 W or below: 43 dBc would place the harmonic at ≤5 µW, which is far below 50 mW, so the relative requirement binds.

The Measurement Procedure

The following is the complete procedure for measuring transmitter harmonic and spurious output using a spectrum analyzer.

Equipment Needed

• Spectrum analyzer (TinySA or equivalent)
• 50 Ω dummy load rated for the transmitter's full output power (e.g. 100 W into a 50 Ω load)
• Calibrated attenuator: 30 dB, rated for at least the transmitter power; or a directional coupler with −20 or −30 dB coupling
• Coaxial cables and adapters
• A CW or continuous carrier (to produce a stable, unmodulated signal for measurement)

Step 1 — Calculate the Attenuator Value Needed

Transmitter power (dBm) − Maximum safe analyzer input (dBm) + safety margin (10 dB) = minimum attenuation needed.

Example: 100 W = +50 dBm. Analyzer max = +20 dBm. Safety margin = 10 dB. Needed: 50 − 20 + 10 = 40 dB. Use a 40 dB, 100 W attenuator.

Step 2 — Verify the Setup

Connect: Transmitter → dummy load (via T-connector or directional coupler sample port) → attenuator → spectrum analyzer. Set the transmitter to CW mode (or FM carrier) at the operating frequency. Set the transmitter power to maximum (or the power level you want to test).

Step 3 — Initial Wide Sweep

Set analyzer: Start = 1 MHz, Stop = 5× the operating frequency (e.g. 0–35 MHz for a 7 MHz transmitter). RBW = 30 kHz, VBW = 3 kHz, Ref Level = transmitter power (dBm) − attenuator (dB) + 10 dBm. This places the fundamental near the top of the display. Identify all visible peaks above the noise floor.

Step 4 — Measure Each Harmonic

For each harmonic: zoom in (span = 2–5 MHz around the harmonic), narrow RBW to 3 kHz, use peak detector. Place Marker 1 on the fundamental. Place Delta Marker on the harmonic. Read the delta in dBc from the marker readout. Record each measurement.

Step 5 — Verify Linearity

Add 10 dB of external attenuation. All displayed signals should drop by exactly 10 dB. If any harmonic drops by more than 10 dB, the original measurement was affected by analyzer overload — the results are invalid. Remove the additional 10 dB and re-measure after adding more external attenuation.

Step 6 — Correct for Attenuation (Absolute Power)

Harmonic power at transmitter output = displayed harmonic level (dBm) + external attenuation (dB). The dBc value is already correct (attenuation affects fundamental and harmonics equally, so the relative measurement is valid regardless of attenuator value — assuming the attenuator has flat frequency response, which any quality attenuator does across HF frequencies).

Spectrum analyzer display of a 7 MHz CW transmitter measured through a 30 dB attenuator. The fundamental reads +13 dBm on the display (actual: +43 dBm = 20 W). The second harmonic at 14 MHz reads −39 dBm (actual: −9 dBm), giving a delta of −52 dBc. The third harmonic at 21 MHz reads −48 dBm (actual: −18 dBm) = −61 dBc. Both pass the FCC ≥43 dBc requirement.

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Harmonic Level Estimator Calculator

Use this calculator to determine the actual harmonic level at your transmitter output (correcting for external attenuation), the suppression in dBc, and whether your transmitter meets the FCC Part 97 requirement.

Interpreting Results and Documenting

Good practice is to document all harmonic measurements for your station log, especially for homebrew or modified transmitters. Record: date, operating frequency, transmitter power, each harmonic frequency, each harmonic level in dBm and dBc, the attenuator used, and the analyzer model. This documentation provides evidence of compliance and a baseline for future comparisons after repairs or modifications.

A useful format for the measurement log:

What to Do When Your Transmitter Fails

If a harmonic measurement shows suppression below the FCC requirement, you should not transmit until the problem is corrected. The most common remedies are:

Add or improve the output low-pass filter. Many commercial HF transceivers include built-in harmonic filters; if these are inadequate or damaged, replace them. For homebrew amplifiers, design and add a multi-element Chebyshev or elliptical low-pass filter (covered in Module 16) with adequate stopband attenuation at the harmonic frequencies.

Check for correct bias and operating point. An amplifier operating outside its design bias point (over-driven or under-biased) produces more harmonic content. For Class AB linear amplifiers, verify the idle current and operating point against the design specifications.

Check for output matching and load impedance. A mismatched load causes reflected power that re-enters the amplifier and can dramatically increase harmonic output due to the non-linear re-amplification. Verify that the antenna system presents a good impedance match at the transmitter's output.

Consider an outboard low-pass filter. Commercial outboard low-pass filters (often called "harmonic filters" or "antenna filters") are available for each amateur HF band. They install between the transceiver and antenna tuner and provide additional harmonic attenuation, typically 40–60 dB, for HF use.