Technical Articles

Receivers and Busy Bands

Beginning in early fall, weekends start to fill up with “the majors”—contests that attract thousands of stations which fill the bands with strong signals. November and December are the most crowded with both modes of ARRL Sweepstakes, the CW weekend of CQ World Wide, and both the ARRL 160 and 10 Meter contests. (CQ World Wide SSB is the last full weekend of October, which is almost November!) During these months, non-contest stations start to feel a little like hikers during deer season. Don’t give up—put your receiver adjustments to work and it’s amazing how well you’ll get along with all the other band’s inhabitants.

There are two basic issues that get harder to deal with on contest weekends—overloading from strong signals and interference from stations closer together. These can be problematic at any time, of course, but they tend to get worse when the bands are full of stations. (Transmitting a clean signal is another way to avoid causing interference, but that’s for a different article.)

Additional Resources: This article touches on topics explored more fully in the ARRL Handbook chapters on receiving and filter design. An excellent presentation by Rob Sherwood, NCØB, on how to get the most from today’s modern receivers was given at the 2023 Contest University. Rob’s talk begins at the 7:21 mark.

What Is Overload and What Causes It?

Several types of interference are referred to as “overload.” What we’re concerned with is fundamental overload by a strong signal that travels through the normal signal path from the antenna system into the receiver circuitry. It is so strong that some circuit in a receiver is unable to process the signal as intended. This circuit is usually a mixer, an RF amplifier, or a filter. In a direct-sampling SDR without front-end analog circuits, fundamental overload occurs when the analog-to-digital converter’s input range is exceeded.

Overload anywhere in the receiver signal path, analog or digital, can create spurious signals in the receiver that sound like splatter or “buckshot” from a strong signal. Once they are generated, there is no way to get rid of them. The correct approach is to reduce the signal level before it can cause overload.

Using the Attenuator

The first step is limiting the input signal level. Receivers generally have two such tools: a preselector and an attenuator. The preselector is a bandpass filter at the receiver input. Some preselectors are automatically tuned by the receiver and others are manually tuned by the operator (i.e., you). These pass signals in the band on which you’re operating but remove out-of-band signals, such as from broadcast stations. Multi-op stations often add a bandpass filter in the antenna system feed line to prevent interfering with stations on the other bands and that also helps reduce out-of-band signals coming in from the antenna.

The attenuator seldom gets a workout, but it can be your biggest friend when dealing with strong signals. 10 to 20 dB of attenuation cures a surprising number of problems at the cost of just a couple of S-units of signal strength. Remember that the goal is to maximize signal-to-noise ratio for readability, not necessarily absolute signal strength. Even with the attenuator on, you should be able to hear the other station.

A clue that the interference is due to fundamental overload is the threshold effect. As signal strength is reduced or gain is reduced, below some threshold the interference quickly disappears. For example, if you add 10 dB of attenuation and the interference or noise drops by more than 10 dB, it is a sign that your receiver was being overloaded. Add more attenuation and the problem may disappear entirely!

Managing a Preamplifier

After a preselector or filter, the next stage is often a preamplifier. (SDR receivers may have an RF amplifier that increases the signal to an appropriate level for the analog-to-digital converter input.) Preamplifiers are easily overloaded by strong signals. These strong signals can also drive the preamplifiers hard enough to overload following stages in the receiver. 

Preamplification is only needed when it is necessary to make an input signal stronger than the receiver’s internal noise. You can tell if you need to turn on a preamplifier by using antenna noise gain. With the preamp turned off, switch your receiver between the external antenna and a dummy load. The increase in background noise with the external antenna is the key. If the noise increases by 6 dB or more (one S unit), you generally don’t need a preamp because it will only increase the noise along with the signal, resulting in no improvement in the signal-to-noise ratio.

Modern receivers have plenty of sensitivity and very low internal noise. (See Rob Sherwood’s presentation referenced above.) Except on 10 meters, preamplification is rarely needed. The default setting for your preamplifier should be OFF.

Adjusting RF Gain and the AGC

Late-Breaking News—RF Gain controls are not welded in the full-on position! This makes your receiver very sensitive, but also leaves your IF (and sometimes the RF) amplifiers susceptible to overloading. Experiment with reducing RF Gain to see if it doesn’t improve your receiver’s performance in a strong-signal environment.

Even during casual operating, turning down RF Gain can dramatically reduce background noise. Try tuning to a quiet frequency and turning down RF Gain until the S meter just begins to rise above the noise floor—this is the AGC threshold. With the RF Gain set this way, input noise has a minimal effect on audio output volume.

Experiment with changing the AGC time-constant settings. A fast time constant will make the AGC system respond aggressively to interference and noise. On a very crowded band, this can cause the signal output volume to continually change, which is harder to listen to. Another option is to turn the AGC system OFF and use the RF Gain and Volume or AF Gain controls instead. If you are listening to a strong signal, this can reduce background noise and interference considerably.

Noise Blankers and Noise Reduction

Most analog noise blankers (NB) operate by sensing wide-bandwidth noise pulses in the receiver’s initial stages. Because narrow, sharp noise pulses have such a wide bandwidth, noise blankers must look at an entire band, not just what gets through the narrow filters further along the signal path. When a pulse is detected, the noise blanker mutes or blanks the receiver for the duration of the pulse. SDR noise blankers work similarly but on the digitized signal.

A strong signal can confuse the noise blanker, causing what sounds like severe distortion products over many kHz. If the band is full of strong signals, noise blankers are useless or worse. Unless you have really strong local power line noise, turn your noise blanker OFF. In any case, adjust the noise blanker level to the minimum you need to reduce the noise at your station. If it sounds like a station is severely over-modulated or distorted, turn the noise blanker OFF to be sure your receiver isn’t the culprit.

Noise reduction (NR) is a DSP process that removes audio noise from a received signal. It generally works by sensing signals of the desired type and removing whatever else is present. The amount of noise reduction is also typically adjustable. Your receiver may have multiple types of noise reduction for different signal types. Using the “wrong” type of noise reduction can distort the received signal, so be sure you select the right type.

Dealing With Crowding

Modern receivers with DSP functions have wonderful signal filtering capabilities! Your analog receiver may have any or all of Passband Tuning, IF Shift, Variable Bandwidth or similar controls as well. There’s no time like the present to find the receiver’s manual and learn what these controls do. If you can adjust the slope of the filters to be more abrupt (i.e., “sharp” instead of “soft”), you can reduce QRM from adjacent channels substantially. It’s a good idea to practice under regular conditions to get used to operating the controls under stress.

Reducing your tuning rate or step size will help you make small adjustments to minimize interference. In general, having each full turn of the main tuning knob correspond to one signal’s bandwidth is a good compromise between speed and adjustability. Don’t expect stations to be exactly on even values of kHz or even tenths of kHz. Everyone will be fine-tuning to deal with conditions and interference.

What Sounds Like Overload but Isn’t?

There are other types of receiver problems that you might think are overload but really aren’t:

  • Common-mode “breakthrough”—Your station’s cables act as antennas, picking up signals as common-mode current. When this current gets into a receiver (or any other piece of equipment) it will be amplified and processed, interfering with other signals that follow the normal path. Common-mode signals can be blocked by good shielding and ferrite RF chokes on the cables picking up the signals.
  • External Intermodulation (IMD)—Strong signals can mix together in any non-linear circuit or device, including rectifiers accidentally created from corroded or rusty metal-to-metal contacts and bad antenna or cable connections outside. IMD products are generated on specific combinations of the strong-signal frequencies, and when both signals aren’t present, the interfering product goes away.
  • Diode harmonic generators—It only takes one strong signal to create harmonics in a rectifying junction. The diode can be outside (like corrosion) or inside your station. Common-mode RF current on rotator cables, relay control wiring, power supply wiring, speaker cables, etc. can get to un-bypassed diodes or transistor junctions where harmonics are generated. The harmonic signals flow back along the same cables and are radiated like any other signal. Like IMD products, the harmonics are on specific frequencies. 

Make a Plan B

I get unhappy when my plans are clobbered by something I don’t expect. Bands full of other signals are a good example. Education and planning will help avoid these unpleasant surprises. There are plenty of contest calendars online such as by WA7BNM and the ARRL. Special events and DXpeditions are announced months in advance. This information shows which mode or modes will be affected and if certain frequencies will be busy. Knowing where the activity is NOT allows you to avoid congestion and have a reasonable backup plan. Be willing to plan ahead and be flexible!

If you will be participating in a net or have a regular schedule, be sure to have a “Plan B” if you expect the bands to be full. You might decide to meet on a pre-planned alternate frequency, which is good to practice anyway. Next, use your receiver’s controls and functions to reduce and avoid interference—don’t expect a clear channel of 3 kHz or more! Be prepared to change frequency a little bit or a lot, taking full advantage of the amateur service’s almost unparalleled flexibility of frequency and mode. If there is a contest on 20 meters, then 17 meters will be contest-free. If a DXpedition is filling the band on CW or the digital modes, try phone instead.

In Summary

By effectively using the capabilities of a modern receiver, you will surely find that the band is quieter and nearby signals less disruptive. In fact, you will find yourself making better use of your receiver’s controls every day!

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