How to Get the Most from Attenuators and Preamps
By Gary Breed, K9AY
Most amateur transceivers include a preamp and/or attenuator, accessed either by switches on the front panel or via the radio's software menu. Unfortunately, many hams do not make the best use of these features, partly because most radios do not provide a sufficient range of preamp gain or attenuation to cover all situations. This article will explain why these gain-control functions are important and how to use them (or augment them) to get the maximum performance out of your radio.
Factor #1-Dynamic Range
Receiver dynamic range is an important fundamental performance parameter. It is one of the most thoroughly tested features in technical product reviews like those found in QST or RadCom. I won't try to explain the intricacies of dynamic range. If you are unclear in your understanding of the concept, I recommend that you read through the material covering this subject in the ARRL Handbook.
Basically, you receiver's dynamic range includes signal levels ranging from the noise floor of its own circuitry to the level that causes audible intermodulation distortion (IMD or intermod). For the purposes of illustration, let's assume that your receiver has an optimum dynamic range of 90 dB, starting at an input signal level of -130 dBm. This is diagrammed using the simple bar graph of Figure 1. This is a typical intermod-free dynamic range specification for low-cost to mid-range HF transceivers, using ARRL methods. Top-of-the-line receivers typically have a dynamic range around 100 dB.
Figure 1. How signal levels relate to receiver performance and real-world conditions.
Our objective is to control the incoming signal levels with gain or attenuation, keeping the receiver operating within its optimum range whenever possible. To so this, we need to know the range of levels we can expect for arriving signals. Figure 1 also shows some of the common situations you will encounter, and how they relate to the receiver's optimum performance range. I have limited this illustration to HF, because VHF and UHF have much lower noise floors that require special attention to preamplification alone. As you can see from the graph, HF communications includes a range of signals that far exceeds the 90 dB dynamic range of our receiver.
Let's start our analysis by looking at the situation on the higher bands of 20 meters and above. On the high bands, atmospheric noise may be 30 dB lower than it is on 160 meters. Especially on 15 and 10 meters, the background noise level can be lower than the bottom of the receiver's dynamic range. (Jansky made the first observation of galactic noise at 21 MHz, so we know it can be very quiet!) At these frequencies, we may need to add preamplification to hear signals that are above the noise level, but below the threshold of our receiver's sensitivity. Most radios offer 10 dB of preamp gain, with a few providing more. This may not be enough, and an external preamp may be needed to raise the level of incoming signals into the receiver's dynamic range.
However, when the high bands are wide open, signal levels can be very strong! If we still have a lot of preamplifier gain turned on when those big signals are present, we will exceed the upper limit of our desired dynamic range and generate signal-covering intermods. The challenge is to be aware of the overall range of signals. We want to turn on the preamp when we need to hear weak signals, but turn it off (or even use some attenuation) to avoid filling our receiver with intermod "crud" from strong signals. In other words, the high bands are not "set and forget" when it comes to adding or removing gain from our receiver.
The low bands require special attention to attenuation. the 40-meter band is a unique case, since it can be either quiet or noisy. The band has the added factor of megawatt international broadcast stations. Like the other low bands, you will never need a preamp on 40 unless you are using an inefficient receiving antenna. On all the low bands, the question is, "How much attenuation do I need?" As noted regarding the high bands, we must consider the range of signal levels. However, those 40-meter broadcasters might force you to add so much attenuation that weak signals are lost-but they might be lost under the intermods anyway without attenuation! Sometimes you must accept some annoying intermods in order to hear weaker signals.
Factor #2-AGC Action
Eighty and 160 meters have the added dimension of high atmospheric noise levels. The broadband, random nature of noise makes it an insidious foe! When our problem is just strong signals, we can live with some intermods, since they are discrete "phantom signals." Intermod products resulting from the mixing of noise and strong signals are-you guessed it-more noise. To combat the effects of noise, we must not only deal with signal levels; we must also consider our receiver's AGC behavior.
AGC offers some protection against strong signals, but only in a portion of our receiver's circuitry. AGC is designed for listening comfort, not the preservation of intermod-free dynamic range. When AGC is operating, it reduces the overall gain of our receiver, usually in the IF stages, but sometimes also in front-end RF stages. The methods used for AGC gain reduction almost always reduce the usable dynamic range. If a high noise level hangs your S-meter at S-9, half of your receiver's dynamic range is no longer available. If we attenuate the input so that the noise barely activates the AGC, we have nearly all the dynamic range available to handle signals.
There is another problem with the effect of noise on AGC. AGC reduces the average signal level with its relatively long recovery time, but AGC detectors have a fast attack which responds to peaks. Noise has a disproportionate effect on AGC, since noise has very high peak levels with low average power. As a result, it takes less noise power to activate the AGC than signal power. When noise is reduced ahead of the AGC detector (using your attenuator), the signal-to-noise improvement is larger than the amount of the attenuation.
A more down-to-earth way to put this is-if you can add enough attenuation to limit the noise level to S-1 or S-2, you will hear many more signals that were previously inaudible because the noise was keeping the AGC unnecessarily high. Try it! Tune to 80 or 160 and listen on your transmit antenna. Now add the 30 dB or more of attenuation it takes to get the S-meter down to the low end of its range. You will almost certainly discover that listening with your transmit antenna works a lot better than you expected.
Obtaining Enough Gain or Attenuation
There are two significant problems with implementing the fairly simple principles noted above. First, your receiver may not have enough gain or attenuation available and, second, connecting external devices may not be easy.
Some mid-priced radios have only two gain options: normal gain and a 10 dB preamp. Others may add a 10 dB attenuator. Higher-level radios usually add more attenuation options and may have extra preamp gain. When you use inefficient receiving antennas like short Beverages, EWEs, K9AY loops, flags and pennants, you will almost certainly need to add an external preamp to obtain more gain. If you want to use your transmit antenna on receive, you may need more attenuation than is available in the rig. For additional interference protection, highpass or bandpass filters may also be desirable. How can we accommodate these external devices with the greatest flexibility?
The diagram of Figure 2 shows the ideal manner for connection of external signal control equipment. A few transceivers are wired this way (my ICOM 765 for example), but not all models are this flexible. most have a single jack for an external antenna and do not allow you to put anything between the transmit antenna and the receiver circuits. Hams using a directional transmitting array such a four-square often use it as their primary receiving antenna. They want to have filters, preamps and attenuators available all the time, not just for an external receive-only antenna. If I owned a radio without a "loop-through" connection from the transmit/receive relay to the receiver, I would not hesitate to install it-access to the receiver's antenna connection is essential for flexible implementation of external enhancements.
Figure 2. Flexible installation of external preamps, attenuators and filters requires a "loop through" connection between the transmit/recevier relay and the receiver input.
Successful contesting on the HF bands requires us to get the most out of our stations. Nowhere is this more pronounced than in our receivers. No radio system in the world requires greater ability to handle a wide range of signal levels than HF ham radio (except maybe the intelligence community). The best, most expensive radios on the market do not have enough dynamic range to deal with the entire range of noise and signal levels we encounter between 1.8 and 30 MHz.
We need to be smart about the way we control signal levels with preamplification
and attenuation in order to allow our receivers to give us their maximum available
performance. Hopefully, these notes on how and why we use preamps and attenuators
will result in a few more QSOs in that next contest!