KPC FIELD TOOL • ANTENNAS & CLEAN SIGNALS

HARMONICS PLANNER

Harmonics work for you and against you. Your antenna can use them to cover many bands with one wire; your transmitter leaks them where they don't belong. This page handles both.

📡 Multiband wire〰️ Random-wire lengths🔇 Transmitter harmonics

PLAN A MULTIBAND WIRE ↓CHECK MY TRANSMITTER

ANTENNA PLANNER

One Wire, Many Bands

A half-wave wire is also resonant at 2×, 3×, 4×… its frequency. Cut an end-fed for 3.550 MHz and the resonances fall at 7.100, 14.200, 21.300 and 28.400 — inside 40, 20, 15 and 10 metres. Pick the right starting frequency and you rarely need the tuner. Drag the slider to see each band move, or let the planner find the best spot.

Antenna

Lowest band

I mostly operate

Wire cut for (MHz)

Bands I want

TRANSMITTER HARMONICS

Where Do My Harmonics Land?

Type a frequency, or tap a band to check the whole band at once. Each harmonic is checked against the US amateur bands and the services you least want to interfere with.

Frequency (MHz)

e.g. 14.285 or 7032 kHz → 7.032

To (optional, MHz)

Fill in to check a range

Show up to

Typed in

Ham band another amateur bandSafety aviation, marine, weather, GPSBroadcast FM, TV, AM, CBOther land mobile, time signals

THE BASICS

What Is A Harmonic?

No transmitter is perfectly pure. Alongside the signal you meant to send (the fundamental), it leaks small copies at whole-number multiples of that frequency. Transmit on 7.100 MHz and there is a faint signal at 14.200, 21.300, 28.400 MHz and up.

Why They Happen

Amplifiers are not perfectly linear. Any distortion of the wave shape creates energy at 2×, 3×, 4× the frequency. Pushing a final amplifier too hard makes it worse.

Many amplifiers cancel even harmonics, so the 3rd is often the strongest one left.

Why Portable Ops Should Care

A multiband end-fed or a random wire is resonant on many bands because those bands are harmonically related. That means your antenna radiates your 2nd and 3rd harmonics very efficiently.

At a campout, the next tent's receiver is a few feet away.

What The Rules Say

Under FCC Part 97.307, spurious emissions below 30 MHz must be at least 43 dB under your signal (for equipment in service after 2003); above 30 MHz, at least 60 dB. And if a harmonic causes harmful interference, you must fix it — meeting the number is not a free pass.

CLASSIC TROUBLE SPOTS

Combinations Worth Knowing

The amateur bands were laid out in harmonic steps, so some pairs bite more often than others.

You transmit on	Harmonic	Lands on	Why it matters
40m (7.0–7.3)	2nd	14.0–14.6	Right in 20m — the classic two-tent problem.
40m (7.0–7.3)	3rd	21.0–21.9	Lands in 15m. A 40m station can wipe out a 15m station next door.
80m (3.5–4.0)	2nd	7.0–8.0	The bottom half of 80m doubles straight into 40m.
20m (14.0–14.35)	2nd	28.0–28.7	Lands in 10m.
10m (28.0–29.7)	3rd	84–89	TV channel 6 and the bottom of the FM broadcast band.
6m (50–54)	2nd	100–108	FM broadcast — the most common 6m neighbour complaint.
2m (144–148)	3rd	432–444	Lands in 70cm. Your own dual-band radio may hear it.
2m (144–148)	2nd	288–296	Military aviation UHF — keep handheld amps clean.

KEEPING IT CLEAN

Fixes That Work In The Field

A multiband wire radiates your harmonics too.

The same trick that makes one wire work on 40, 20, 15 and 10 means it happily radiates your transmitter's 2nd, 4th and 6th harmonics. That's the strongest argument for a low-pass filter on a portable end-fed.

Put a low-pass filter on the rig.

A low-pass filter goes between the radio (or amplifier) and the antenna. Its cutoff must be above your operating frequency but below the 2nd harmonic. The calculator tells you that window for any band. Multiband QRP filters exist that switch with the band.

Don't overdrive.

Too much mic gain, too much ALC, or a small amp pushed to full output all distort the signal. Back off until the ALC barely moves.

A tuner is not a filter.

Some tuner designs knock harmonics down a little; many do almost nothing. Don't count on one to clean up a dirty signal.

Use band-pass filters at multi-station events.

When two stations share a site, a band-pass filter on each receiver as well as clean transmitters is what makes them coexist. Field Day groups have done this for decades.

Listen for yourself.

Have a friend (or a WebSDR) tune to 2× and 3× your frequency while you transmit. If you can hear yourself loud on the harmonic nearby, so can the next tent.

Check it on the bench.

An inexpensive spectrum analyzer shows harmonics directly. Always go through a dummy load and enough attenuation — never connect a transmitter straight into the analyzer.

GOING DEEPER

Official Rules & Allocations

This calculator uses US amateur band edges and a selection of other services. For the full picture, see the FCC rules and the national frequency allocation chart.

FCC §97.307 ↗US ALLOCATION CHART ↗