Low Pass Filter for Ham Radio: Harmonics and TVI Cure

Every transmitter makes some energy that you did not intend to send. Part of it appears at exactly twice, three times and four times the frequency you are using, and if that energy reaches the antenna it goes out into the world, where it can land in someone else’s receiver. A low pass filter for ham radio is the simple, passive device that stops most of it. It passes your wanted signal and blocks the higher frequencies where the harmonics live, and it is one of the cheapest and most responsible additions you can make to a station, especially if you use a linear amplifier. This guide explains where harmonics come from, how a low pass filter works, how to choose one, where it belongs in the station and how to check that it is doing its job.

Where Harmonics Come From

A perfect amplifier would take a sine wave in and give a larger sine wave out. Real amplifiers are not perfect. When a transistor or a valve is driven hard, or operates in a mode that is efficient but not perfectly linear, the output waveform is distorted, and any distorted periodic waveform contains energy at multiples of its fundamental frequency. Those multiples are harmonics. The second harmonic is at twice the frequency, the third at three times and so on.

The numbers show why this matters on the amateur bands. The second harmonic of a signal on 7.1 MHz is at 14.2 MHz, which is inside the 20 metre amateur band. The third harmonic of a signal on 14.1 MHz is about 42.3 MHz, and the third harmonic of a signal at about 29 MHz is close to 87 MHz, right next to the lower edge of the FM broadcast band. Harmonics of a signal on the lower HF bands can also land in the VHF part of the spectrum where a neighbour’s television or radio is likely to be working.

The general term for emissions on frequencies outside the intended bandwidth is spurious emissions, and the Wikipedia page on spurious emissions gives a good overview of the definition. Regulators set limits for them, and the limits differ from country to country, so check your own rules. The practical point is that a clean signal is part of the licence conditions in most places, and a modern transmitter with its own built-in filtering usually meets the limit, while an overdriven amplifier or an older piece of equipment may not.

How a Low Pass Filter Works

A low pass filter does exactly what its name says. It passes frequencies below a chosen cutoff with very little loss and attenuates frequencies above it. The general theory is described in the article on the low-pass filter, and in the amateur version it is built from capacitors and inductors arranged in a ladder. Each additional section makes the transition steeper.

• Cutoff frequency. A filter for the 40 metre band has its cutoff just above the top of that band, so that the wanted signal passes and the second harmonic, at roughly twice the frequency, is far into the stopband.
• Insertion loss. In the passband, a well-made filter has a loss of a few tenths of a decibel, which means almost all of your power goes through to the antenna.
• Attenuation. In the stopband, the better filters give tens of decibels of suppression at the second and third harmonics and more at higher harmonics.
• Impedance. The filter is designed for a 50 ohm system. If the load is not close to 50 ohms, the filter does not behave as the datasheet promises.

The shape of the response depends on the design. Some designs have the flattest possible passband, others trade a little ripple for a steeper cutoff, and some use extra resonant sections to put deep notches at the second and third harmonics. For a simple station, any well-made commercial or kit filter for your band is good enough. What matters most is that the filter is designed for the band, rated for your power and used with a reasonable load.

Choosing a Filter

The table below lists the practical choices.

Three specifications matter most.

1. Power rating. The filter must handle your peak and average power, with a margin. A filter rated for 200 watts will be destroyed by a 1 kW amplifier. Remember that at a high standing wave ratio the voltages and currents in the filter rise sharply.
2. Bands. Confirm the cutoff for the bands you use, particularly the lowest one, where the second harmonic is the biggest worry.
3. Connector and mounting. Choose connectors that suit your coax and a case that can be grounded. Our guide to buying an HF linear amplifier explains why filtering matters even more once you add one.

Where the Filter Belongs

A low pass filter belongs at the output of the transmitter or amplifier, in the line between the transmitter and the antenna system. If you use a linear amplifier, put the filter after the amplifier, because the amplifier is the main source of the harmonics. Many amplifiers include a filter bank, and an extra external filter is then a second layer of protection. If you use an antenna tuner, place the filter between the transmitter and the tuner, or between the amplifier and the tuner, and keep the lead lengths short.

• Keep the cables short and good. Use short coax jumpers of good quality and proper connectors.
• Ground the case. Connect the case of the filter to the station ground, in line with the advice in our guide to station grounding.
• Do not put a filter after the antenna tuner. At the high voltages on a mismatched line, the filter can be damaged and it will not behave as intended.
• Use it with a good load. A filter designed for 50 ohms needs a load near 50 ohms, so check the standing wave ratio before you rely on the filter.

Even the best low pass filter does not cure every interference problem. If a neighbour’s receiver is overloaded by the strong fundamental signal rather than by a harmonic, a filter at your transmitter does nothing, and the cure is a filter at their receiver. The ARRL has helpful guidance on this on its radio frequency interference page, and the general topic of television interference is summarised in the article on television interference.

How to Test That It Works

Do not assume that a filter is working just because it is in line. A few simple tests give you real confidence.

1. Use a dummy load for the first tests. Our guide to the dummy load explains why, and it keeps your test signals off the air.
2. Measure the passband. A vector network analyser such as a NanoVNA can show the transmission through the filter across a range of frequencies, so you can see the cutoff, the insertion loss and the attenuation at the harmonics. Our article on the NanoVNA shows how to calibrate it and run a sweep.
3. Measure the transmitter output. A spectrum analyser with a suitable attenuator shows the harmonics directly. If you do not have one, a friend at the local club may.
4. Listen at the harmonic frequencies. With the transmitter on a dummy load at low power, a second receiver tuned to the second or third harmonic will show whether a signal is present. With the filter in line, the signal should drop sharply.
5. Check at full power with a safe load. Do this only if you are sure that the dummy load and any attenuators can handle the power.

Be careful with test equipment. A spectrum analyser or a receiver connected directly to a transmitter can be destroyed in a fraction of a second. Always use attenuators and a sampling method that is rated for the power, and follow the instructions of the equipment manufacturer.

Other Causes and Cures

When a neighbour reports interference, check the whole chain before you buy anything.

• Overdriven stages. Reduce the drive and the audio level so that the transmitter is operating linearly. A clean signal at lower power is better than a dirty signal at higher power.
• Common-mode current. RF travelling on the outside of the coax can enter nearby equipment without any harmonics being involved. A choke on the feedline helps; see common mode chokes.
• The neighbour’s receiver. Poor shielding or a lack of filtering at the other end is common. A polite visit and a high-pass filter at their antenna input may solve it quickly.
• Your own antenna. A badly matched antenna produces high standing waves that can raise the harmonic level, so check the match.

A Worked Example

Take a station with a 100 watt transceiver and a 1 kW linear amplifier that works on 40 and 20 metres. The amplifier has its own switched filters, but it is old, and a neighbour has complained about noise on a VHF radio whenever you transmit on 40 metres. The first step is to measure, not to guess. Connect the amplifier to a dummy load, drive it to its normal power with a low-power signal and look at the output with a spectrum analyser and a suitable attenuator, or ask a club member who has one. If the second harmonic of 7.1 MHz at 14.2 MHz is only a few tens of decibels below the fundamental, the built-in filter is not doing enough.

Now add a good external filter for 40 metres, rated well above 1 kW, between the amplifier and the antenna tuner. Repeat the measurement. The harmonic should fall sharply, and the neighbour’s problem may disappear. If it does not, the cause is probably not harmonics at all, and the next steps are a common-mode choke on the feedline and a conversation with the neighbour about their receiver. Write down the readings before and after so that you can show what you have done. The record is useful if you ever need to prove that your station is clean, and it makes a polite and convincing argument for the neighbour to fit a filter at their end if needed.

The Bottom Line

A low pass filter is a small, inexpensive box that protects your neighbours and your licence. Use one at the output of your transmitter or amplifier, choose it for your bands and your power, keep the station load close to 50 ohms and test it with a dummy load and an analyser. Remember that it only cures harmonics, so check for other causes of interference too. Clean transmissions are part of being a good amateur, and a filter makes them easy.

Frequently Asked Questions

Do I need a low pass filter if my transmitter has one built in?

Often not, as most modern transceivers filter their output. If you add a linear amplifier, check the amplifier’s own filtering, and consider an external filter if the amplifier has none or if you have had interference reports.

What is the difference between a low pass filter and a band pass filter?

A low pass filter passes everything below a cutoff and blocks the harmonics above it. A band pass filter passes only one band and blocks signals above and below it, which makes it useful for receiving in multi-transmitter stations.

Where should I place a low pass filter?

At the output of the transmitter or amplifier, before the antenna tuner and the feedline, using short good-quality jumpers and a grounded case.

Will a low pass filter cure interference to my neighbour’s television?

Only if the cause is harmonics from your transmitter. If their receiver is overloaded by your fundamental signal, the cure is a filter at their receiver, such as a high-pass filter at the antenna input.

How do I know the power rating I need?

Use a rating above your peak power, with a margin, and remember that a high standing wave ratio stresses the filter. Never use a low-power filter with an amplifier.

Can a low pass filter cause problems?

A good filter adds a few tenths of a decibel of loss. A filter used with a bad mismatch or beyond its power rating can overheat or fail, so keep the load close to 50 ohms.