RF Attenuator Pads for Hams: 3, 6, 10 and 20 dB Explained

An RF attenuator is the humblest and most useful accessory on the bench: a small metal cylinder with a connector at each end that reduces the power of a signal by a fixed amount. It protects a spectrum analyzer from a transmitter, lets you test a receiver with a signal that is just strong enough, makes a poor load look better to a transmitter and allows you to measure the output of a QRP rig with a meter that was meant for a lower level. You can buy ready-made attenuators for a modest price, and you can build one from three resistors in an evening. This guide explains what a pad does, how the decibel values translate into power, which resistor values make a 50 ohm pad and where the limits of the design are.

What an Attenuator Does

The Wikipedia article on attenuators describes the device as a passive broadband electronic device that reduces the power of a signal without appreciably distorting its waveform. It adds loss, in decibels, and it keeps a constant impedance at both ports, which is 50 ohm in amateur radio. The power that disappears is turned into heat in the resistors, which is the reason why the power rating matters. According to the same article, a 3 dB pad reduces the power to one half, 6 dB to one fourth, 10 dB to one tenth, 20 dB to one hundredth and 30 dB to one thousandth. The general definition of the unit is in the Wikipedia article on the decibel, and if you want to refresh the decibel arithmetic, the article on antenna gain in decibels is a good start.

• 3 dB: power ratio 1/2, voltage ratio 0.708. A 100 W input becomes 50 W, and 5 W becomes 2.5 W.
• 6 dB: power ratio 1/4, voltage ratio 0.501. A 100 W input becomes 25 W, and 5 W becomes 1.25 W.
• 10 dB: power ratio 1/10, voltage ratio 0.316. A 100 W input becomes 10 W, and 5 W becomes 0.5 W.
• 20 dB: power ratio 1/100, voltage ratio 0.100. A 100 W input becomes 1 W, and 5 W becomes 0.05 W.
• 30 dB: power ratio 1/1000, voltage ratio 0.032. A 100 W input becomes 0.1 W, and 5 W becomes 5 mW.

Attenuators in series add their decibel values, so a 10 dB pad and a 20 dB pad together make 30 dB. This is the property you use when you build up a total from a small set of values: a kit of 3, 6, 10 and 20 dB pads gives you any value in steps of 1 dB up to 39 dB, with the right combination.

The Two Basic Circuits

The same article names the two basic circuits used in attenuators: the pi pad and the T pad. A T pad has two series resistors with a shunt resistor to ground in the middle. A pi pad has a shunt resistor at the input, a series resistor in the middle and a shunt resistor at the output. For a pad with the same impedance at both ends, the circuits are symmetrical, and the resistor values follow from the loss and the impedance. In a 50 ohm system the values are as follows, rounded to one decimal, and the closest standard values work well in practice.

The values come from the standard formulas. With K as the voltage ratio of the loss, which is 10 raised to the power of the loss in dB divided by 20, the pi pad’s shunt resistance is Z times (K + 1) divided by (K – 1), and its series resistance is Z over 2 times (K minus 1 over K). The T pad’s series resistance is Z times (K – 1) over (K + 1), and its shunt resistance is 2 Z K divided by (K squared minus 1). You can type these into a calculator or a spreadsheet to find the values for another loss or another impedance. A reasonable approximation with standard resistors usually gives a small error in the attenuation and a return loss that is still good at HF.

Power Rating: Where the Heat Goes

A pad is a resistor network, and everything that it does not pass on, it turns into heat. For a 20 dB pad that is driven with 5 watts, 4.95 watts is dissipated inside it. The heat is not shared equally. In a pi pad, the shunt resistor at the input sits across the full voltage of the line and takes most of the power: for 5 W into 50 ohm, the voltage is about 15.8 V, and across 61.1 ohm that gives about 4 W in the first resistor. You need a resistor rated for that power with a margin, or several resistors in parallel. For this reason, the ratings on commercial attenuators are shown as the maximum input power, for example 2 W, 5 W or 50 W, and they should be respected. A cheap 2 W pad on the output of a 100 watt transmitter will not survive a single transmission.

• Check the rating of any pad before you use it with a transmitter, and consider the duty cycle. A pad that is rated for 5 W continuous may take more in short pulses, but heavy duty modes such as FT8 give it a long time to heat.
• Put the high-power stage first. When two pads are in series, place the one with the higher power rating nearest to the source, since the second pad sees a much lower power.
• Allow for cooling. A large pad with fins works with free air around it, and a small one can heat up quickly. If it is too hot to touch, switch off.
• Do not exceed the frequency range. A pad built from leaded resistors is good at HF but has a poor response at VHF, while a commercial design with chip resistors in a metal housing is made for much higher frequencies.

Practical Uses in the Shack

Protecting test equipment

The best known use is in front of a spectrum analyzer or a low-power power meter. A transmitter produces far more power than the input can accept, so you connect it through the attenuator and a load, and calculate the level before you transmit. Our guide to the essential test equipment for hams lists the devices that need a pad, and the dummy load guide explains the safe way to test a transmitter without radiating.

Reducing power for tests

If your radio cannot reduce its output to the level you need, a pad does it. A 100 watt radio through a 20 dB pad gives 1 watt, which is a common level for testing a QRP accessory or an amplifier’s drive.

Improving the match

An attenuator reduces the reflected power twice, once on the way forward and once on the way back. As a result, the return loss of a mismatched load, measured at the input of the pad, improves by twice the attenuation. A load with a return loss of 0 dB, which is an open or a short circuit, appears as 12 dB at the input of a 6 dB pad, which is an SWR of about 1.7. This trick is often used to make a poor test load look acceptable to a signal generator or a receiver, at the cost of signal level.

Setting receiver levels and checking for overload

You can use a pad on the antenna input of a receiver to check whether a strong signal is overloading it. If you add 10 dB and the interference falls by more than 10 dB, or the false signals disappear, the cause was overload. This is the same idea as the attenuator built into the radio, described in our guide to when to use a preamp or attenuator.

Isolating two devices

A pad of 3 to 6 dB between two active stages, for example between a signal generator and a mixer, reduces the interaction between them and improves the match seen by each.

Building Your Own Pad

1. Choose the circuit and the loss. For a first project, a 10 dB or 20 dB pi pad at 50 ohm is a good choice.
2. Calculate the values and pick the nearest standard resistors, preferably 1 per cent metal film types. Parallel resistors can give a closer value and share the power.
3. Build it compactly. Mount the resistors in a small metal box with a connector at each end and keep the leads very short. The box acts as the ground for the shunt resistors, and a layout with a wire loop between the connectors will upset the response at higher frequencies.
4. Measure it. A NanoVNA, with the calibration described in our guide, measures the loss and the return loss of the finished pad over the HF range and shows whether the values are right. A reading of 20 dB loss with a return loss better than 30 dB is a good result.
5. Label it with the loss and the maximum power, so that nobody uses it for more than it can take.

Fixed Pads or a Step Attenuator?

A set of fixed pads is cheap, robust and easy to calculate, and it is enough for most of the jobs above. A step attenuator is a switched box with several sections, for example 1, 2, 4, 8 and 16 dB, and it lets you change the loss in small steps without disconnecting cables. It is useful for receiver tests, such as finding the point at which a signal just disappears into the noise, and for finding the compression point of a preamplifier. The sections are the same kind of pad, and the same cautions about power and frequency range apply, but a small step attenuator is often designed for milliwatts, so it must never be connected to a transmitter. Keep a clear label on every device that says whether it is a power attenuator or a test attenuator, and keep them in separate boxes. This simple habit prevents the most expensive mistake on the bench.

Common Mistakes

• Forgetting the power rating. The most frequent error is to put the pad on the output of a transmitter and burn the resistors.
• Counting the cable. The loss of the coaxial cables and connectors between the pad and the instrument is not zero, so a measurement made at the end of a long lead includes it.
• Using mismatched values. A pad is designed for a particular impedance, and used between loads of other impedance it does not give the nominal loss.
• Assuming that a cheap pad is flat. At higher frequencies the loss and the SWR change with the construction, and a pad for 50 MHz may not be right at 432 MHz.

The Bottom Line

An RF attenuator reduces a signal by a fixed number of decibels, keeps the impedance at 50 ohm and turns the lost power into heat. A 3 dB pad halves the power, 6 dB cuts it to a quarter, 10 dB to a tenth and 20 dB to a hundredth. A pi or T pad built from three resistors does the job at HF, and a table of values or a short calculation gives the resistors for any loss. Always respect the power rating, keep the leads short and add the decibels when you combine pads. With a small kit of 3, 6, 10 and 20 dB pads, you can protect your instruments, test receivers and transmitters safely and make poor loads behave.

Frequently Asked Questions

What does a 10 dB attenuator do?

It reduces the power to one tenth, so a 10 watt signal becomes 1 watt. In voltage terms the signal is reduced to about 0.316 of the input.

Can I connect a transmitter to an attenuator?

Yes, if the attenuator is rated for the power and for the mode and the load after it can also take the power. Check the rating, and remember that a pad turns most of the power into heat.

What is the difference between a pi pad and a T pad?

A pi pad has two shunt resistors and one series resistor. A T pad has two series resistors and one shunt resistor. Both give the same loss and impedance when the values are right.

How do I add attenuation values?

Decibel values add in series, so a 10 dB pad and a 6 dB pad make 16 dB, as long as the pads have the same impedance.

Does an attenuator improve SWR?

It improves the SWR seen at the input of the pad, because the reflected signal passes through the loss twice. A 6 dB pad makes a total mismatch look like an SWR of about 1.7.

Can I build an attenuator from ordinary resistors?

Yes, for HF use. Choose metal film resistors with a suitable power rating, keep the leads short and mount them in a metal box. Check the result with a NanoVNA.