THE STATIONS THAT GOT AWAY

This article first appeared in the August 2026 issue of my World of Shortwave Listening column for The Spectrum Monitor magazine. Further details on this excellent publication are available at www.thespectrummonitor.com

One of the great conveniences of modern portable shortwave receivers is the ability to automatically scan the bands and store active frequencies in memory. Manufacturers call these features by various names: ATS (Auto Tuning Storage), ETM (Easy Tuning Mode), Auto Scan, and similar terms. The appeal is obvious: press a button, let the radio do the work, and return later to a list of stations currently on the air.

Yet many shortwave listeners have noticed that stations clearly audible during manual tuning sometimes fail to appear in the radio’s automatically generated memory list. This raises an interesting question: can shortwave fading cause ATS or ETM to miss stations? The evidence suggests that the answer is yes.

How ATS Works

Manufacturers usually do not disclose the full details of their scanning algorithms, but the basic principle is straightforward. The radio steps through the selected frequency range using increments appropriate to the band or mode being scanned. On many shortwave broadcast scans, that may be 5 kHz, although some radios support finer tuning steps in other modes. At each frequency, the receiver typically tunes to the channel, allows the circuitry or DSP chip to stabilise, assesses whether a signal is present, and decides whether to store the frequency in memory.

Unlike a human listener, the radio does not remain on a channel waiting for conditions to improve. It makes a quick decision based on what it detects at that moment and then moves on. If the signal appears strong enough during that brief evaluation, the frequency may be stored; if not, it may be ignored.

RSSI and Signal Evaluation

One measurement commonly used in radio systems is RSSI, or Received Signal Strength Indicator. Analog Devices defines RSSI as “a signal or circuit that indicates the strength of the incoming (received) signal in a receiver.” RSSI is often useful, but it is not always a calibrated, laboratory-style measurement in consumer portable receivers. In practice, a reading on one radio may not correspond directly to the same reading on another.

Modern DSP-based portables may combine RSSI with other indicators, including signal-to-noise ratio (SNR), carrier detection, adjacent-channel conditions, and AGC behaviour. Whatever the exact method used, the principle remains the same: the radio attempts to determine whether a usable signal is present during the brief period it examines a frequency.

Enter QSB

This is where shortwave propagation becomes important. Shortwave listeners are familiar with QSB, the fading of signals caused by changing ionospheric propagation and multipath effects. Signal strength can fluctuate dramatically from S9 to S5, from S7 to barely audible, or from perfectly readable to nearly lost in the noise. These changes may occur within fractions of a second, over several seconds, or longer.

Human listeners compensate by waiting and continuing to monitor the channel, recognising that a signal weak at one moment may improve shortly afterwards. ATS systems, however, are far less patient.

In an automatic scan, the signal may be picked up when the signal peaks.

But if the signal happens to dip at the wrong time, the auto tuning misses it!

Suppose a radio samples a frequency for only a short time before deciding whether to store it. If a station happens to be in a deep fade during that exact interval, the received level may fall below the radio’s internal decision point, and the radio may conclude that no station is present. A listener tuning manually thirty seconds later, however, may find the same station perfectly intelligible.

The radio has simply made a decision based on a momentary snapshot of conditions. The difficulty is that shortwave signals are inherently variable, whereas automatic scanning works best when the conditions observed during a brief evaluation period are representative of the channel as a whole. On HF, that assumption is not always valid.

What Manufacturers Say

Some receiver manuals acknowledge this limitation directly. The Tecsun PL-330 user manual states that ATS results for LW, MW, and SW depend strongly on radio-wave propagation conditions, the receiving environment, and other factors. It also advises users that if results are poor, they should repeat ATS at a different location or time.

This is significant because it confirms what many listeners observe in practice: propagation conditions can affect what the scan detects, and stations may sometimes be missed.

A natural question follows: how long does the radio actually spend evaluating each frequency? Manufacturers rarely publish that detail. What is clear is that the evaluation window is very short compared with how a human listener might monitor a frequency. As a result, a scan can easily miss a signal during an unfavourable part of a fading cycle.

If the receiver remains on a channel for only a brief period, that interval may still be too short to capture signals that are intermittently above the detection threshold. While the exact dwell time is interesting, it is not the central issue. The more important point is that shortwave signals can change rapidly enough to defeat a quick automated assessment.

Practical Experience

Many DXers report exactly this behaviour. Stations that are easy to hear during manual tuning may not appear in ATS or ETM memories, while others reappear during a later scan conducted only minutes later. Although anecdotal, these observations are consistent with both the physics of shortwave propagation and the practical limitations of automatic scanning.

In my own listening, I have occasionally encountered stations during manual tuning that were absent from an ETM scan completed only a short time earlier. For this reason, I regard ATS primarily as a convenience feature rather than a comprehensive survey of everything audible on the bands.

Implications for the DXer

For casual listening, ATS and ETM can be extremely useful. They provide a quick overview of stronger signals and can save considerable tuning time. However, for DXers seeking weaker, more elusive catches, their limitations become significant.

Automatic scanning may overlook:

• stations affected by QSB,

• marginal transoceanic signals,

• broadcasters operating close to the receiver’s sensitivity threshold, and

• transmissions that rise above the noise only intermittently.

In other words, the signals that interest serious DXers are often the very ones most likely to evade detection by automated scanning. Manual tuning, therefore, remains an essential skill. An experienced listener can recognise fading patterns, persist through weak conditions, and interpret signals that an automated algorithm may simply classify as absent.

None of this renders ATS or ETM ineffective. These features remain valuable tools, particularly for quickly identifying stronger signals currently present on the bands. However, they should not be regarded as infallible.

The next time an automatic scan seems surprisingly sparse, it may be worth remembering that the bands themselves are dynamic. A station omitted from the memory list may not have been absent at all; it may simply have faded at the wrong moment. For the DXer armed with patience and a tuning knob, that missed signal may still be waiting for you just a few seconds later!

73 and wishing you good dial twidling!

Rob Wagner VK3BVW