HF Preamp and Attenuator: When to Use Them and When Not To
Almost every HF transceiver has a button labelled P.AMP or PRE, and another labelled ATT. Many operators leave the preamp on all the time because it sounds like free sensitivity, and many never touch the attenuator because it sounds like a loss. Both habits are often wrong. On the lower HF bands, an HF preamp adds nothing except noise and overload, and an attenuator frequently makes a crowded band easier to copy. On the higher bands, in a quiet location, the preamp can reveal weak signals that were hidden. The difference comes down to one concept, the noise floor, and a five minute test on your own station shows which setting to choose. This guide explains the idea and gives you rules for each band.
What Limits Your Hearing
Three noises compete at the input of your receiver. The first is thermal noise, the unavoidable background of any electronic circuit, which sets a floor of about minus 174 dBm in each hertz of bandwidth. In a 500 Hz CW filter that is about minus 147 dBm. The second is the noise added by the receiver itself, described by its noise figure, which on a typical HF transceiver pushes the floor up by 10 to 15 dB. The third is external noise, which comes from the antenna: atmospheric noise from lightning around the world, galactic noise, and man-made noise from electronics, power lines and switching supplies.
On HF, the third is the largest by far at the lower frequencies. International studies of radio noise, summarised in ITU-R Recommendation P.372, show that the noise level falls as the frequency rises. At 3.5 MHz it can be 50 dB or more above thermal noise at a quiet rural site, at 14 MHz it is about 35 dB, and at 28 MHz around 25 dB. A residential area with many electronic devices is higher still. When the external noise is far above the receiver’s own noise, the receiver’s noise figure does not matter, because it is buried.
That is the whole principle. A preamplifier improves a weak signal only when the receiver’s own noise is a significant part of the total. If the antenna already delivers noise much larger than the receiver, a preamplifier raises signal and noise together, and the ratio stays the same.
How a Preamp and Attenuator Work
A preamplifier is a low-noise amplifier at the front of the receiver. It adds gain, typically 10 to 20 dB, and lowers the effective noise figure of the chain. Its price is a reduced tolerance of strong signals, because the amplified signals reach the mixer and the later stages at a higher level, where they may produce distortion. The receiver’s dynamic range is therefore reduced, a point explained in our article on receiver dynamic range, blocking and intermodulation.
An attenuator does the opposite. It reduces the level of everything that reaches the first stage, usually by 6, 12 or 18 dB, and it removes the strongest signals before they can overload the receiver. Because it reduces the signal and the antenna noise by the same amount, it does not change the ratio of signal to noise from the antenna, but it can lower the distortion products created inside the receiver by much more than it lowers the wanted signal. On a crowded band, that makes the wanted signal clearer.
A Simple Test for Your Own Station
The rule of thumb is easy to test. Choose a quiet moment on the band you want to check and follow these steps.
- Select a frequency with no signal and set the mode and filter bandwidth that you use for weak signals, for example 500 Hz for CW or 2.4 kHz for SSB.
- Turn off the noise reduction and noise blanker, and set the AGC to a fixed value or switch it off if your radio allows it, so that the meter reading is stable.
- Note the S-meter reading with the antenna connected and the preamp off.
- Turn the preamp on and note the reading again.
- Disconnect the antenna, replace it with a dummy load and read the meter with the preamp on and off.
If the noise with the antenna rises by about 6 dB or more when the preamp is turned on, the external noise dominates and the receiver’s own noise is minor, which means that the preamp does little for weak signals. If the reading with the antenna connected is only a little above the reading with the dummy load, the receiver’s noise is a larger part of the total, and the preamp will help. A dummy load makes a good reference, and our guide to the dummy load explains how to choose one. The test is approximate, since S-meters are not precise instruments, but the difference is usually clear.
Band-By-Band Advice
The table shows typical starting settings for a modern station in a location with moderate noise. Adjust them to your own measurements.
| Band | Typical external noise | Preamp | Attenuator |
|---|---|---|---|
| 160 m and 80 m | Very high | Off | Often useful at night |
| 40 m | High | Off | Useful in the evening |
| 30 m and 20 m | Moderate | Off, test on quiet sites | Use if the band is crowded |
| 17 m and 15 m | Lower | Test, often helps in quiet locations | Rarely needed |
| 12 m and 10 m | Lowest on HF | Often helps | Only near strong local signals |
| 6 m | Very low | Usually helps | Rarely needed |
The pattern follows the noise: the higher the band, the more useful the preamp. Local noise changes everything, though. In a noisy suburb with plasma televisions and chargers, a preamp will not help even on 10 metres. At a quiet rural site on the same band, it can be the difference between hearing and not hearing a weak beacon.
When an Attenuator Is the Better Choice
A strong signal in the passband, or several strong signals near the receiver frequency, generate false signals and a raised noise floor inside the receiver. You can recognise this overload by a hiss or a rough sound that appears when a strong station comes on and disappears when it stops, by signals that seem to appear in several places, or by an S-meter reading that jumps when you turn the preamp on. Switching in 6 or 12 dB of attenuation often cleans this up at once. You lose a little of the weak signal, but the receiver’s distortion falls much faster, so the result is better.
The attenuator is particularly useful in a contest or a pileup on 40 and 20 metres, at night on the low bands and in a multi-radio station with another transmitter nearby. It is also useful for judging a signal, because turning it on tells you how strong the signal really is. If a station is still readable with 18 dB of attenuation, it is a strong signal, and there is no reason to listen with maximum gain.
The same idea applies to the other receiver controls. A narrow filter, described in the article on receive filter bandwidth and roofing filters, removes strong signals that are not on your frequency. A well-set AGC prevents the audio from pumping. Using all of these together gives a much cleaner receive path than relying on gain alone.
Receiving Antennas and Low Gain
There is one situation on the low bands in which a preamp is valuable. Receiving antennas such as a Beverage, a small loop or a short active antenna may have a very low output level, much lower than a full-size transmitting antenna. A Beverage is directional and rejects noise, but its signal level can be 10 to 20 dB or more below that of a dipole, so the signal and noise from the antenna may fall below the receiver’s own noise. A preamplifier then restores the level, and the improvement in signal-to-noise ratio of the antenna system is real. This is described in our article on low band receive antennas. In this case the test above shows a small rise in noise when the antenna is connected, which tells you that the preamp is needed.
Use a low-noise design with good strong-signal behaviour for such a job, and consider placing it at the antenna, so that cable loss does not reduce its benefit. Many receiving antennas include an amplifier of their own, and adding another one in the radio can overload the receiver. Check the combined gain and use the attenuator if signals sound distorted.
Reading the Radio Menu
Radios label these controls in different ways, and the labels can mislead. Some models offer two preamp stages, often called P.AMP 1 and P.AMP 2, with different gain, and an attenuator with several steps. Yaesu radios use the term IPO, which stands for intercept point optimisation, and switching it on bypasses the preamp for better strong-signal handling. Other radios offer a separate RF gain control that reduces the gain of the front end continuously. Direct sampling receivers sometimes include a digital gain setting or a dither option, which has a similar purpose. Read the manual for each setting, because the same name can mean different things on different models.
A practical approach is to make a small table of your preferred settings for each band, with the preamp, the attenuator and the RF gain, and keep it near the radio. Many transceivers can store these settings in band memories, so that the radio returns to the right choice when you change band. Update the table once or twice a year. You will notice that the best setting changes with the season and with the noise level in your neighbourhood, and that the table reminds you to check it.
Overload Symptoms and Fixes
- Rough or harsh sound when a strong signal appears. Switch on the attenuator, turn off the preamp, or reduce the RF gain.
- Signals that appear in several places. These are intermodulation products. Reduce the gain and narrow the filter.
- A noise floor that rises on a busy band. This is usually receiver overload, not propagation. Test with the attenuator.
- Clicks and pops from a nearby transmitter. Use a band-pass filter or a stub filter on the second radio.
- Preamp helps on one antenna and not on another. The antenna with the lower noise benefits, which is normal.
Common Mistakes
- Leaving the preamp on for every band. On the low bands it adds overload and no sensitivity.
- Judging by loudness. A louder signal is not a better one. Compare the signal to the noise, not the volume.
- Ignoring the attenuator. It is one of the most effective tools in a crowded band.
- Adding gain to solve a noise problem. If the noise comes from your own house, find and fix it, as our guide to finding RF noise in a modern home describes.
- Testing only once. The result changes with the band, the hour, the weather and the season.
The Bottom Line
An HF preamp helps only when the receiver’s own noise is a significant part of what you hear, which means on the higher HF bands and at quiet locations, or when a very low-level receiving antenna is used. On 160, 80 and 40 metres it usually does nothing useful, and an attenuator is often the better switch when the band is busy. Run the five minute test on each band with your own antenna, write down the result, set the radio accordingly and revisit it when the noise at your location changes. The right setting is the one that gives the best readability, not the loudest sound.
Frequently Asked Questions
Should I leave the preamp on all the time?
No. Use it on the higher bands and in quiet locations when the test shows that it helps, and leave it off on the lower bands where external noise is much higher than the receiver’s noise.
Why does the preamp make no difference on 80 metres?
Atmospheric and man-made noise on 80 metres is far above the receiver’s own noise, so a preamp raises signal and noise together and the ratio does not improve.
What does the attenuator do?
It reduces all signals entering the receiver by a fixed amount, which lowers overload and intermodulation caused by strong stations and can make weak signals easier to copy.
How do I know if my receiver is overloaded?
Signs include harsh distortion when a strong station is present, false signals that move when you change the gain, and a noise floor that rises when you turn the preamp on.
Is a preamp useful with a Beverage antenna?
Often yes, because the antenna has low output and the receiver’s own noise may then matter. Test it and use the attenuator if strong signals cause problems.
Can the noise blanker replace the attenuator?
No. The noise blanker removes short impulse noise, while the attenuator reduces overload. They solve different problems and can be used together.