The IC-7300MK2 manual calls out 24.572 MHz by name as a frequency where the radio’s own circuitry can produce internal spurious signals, and it tells operators to remember that receiver spurious responses at this spot are a normal design artifact rather than a fault. The figure appears in the manual’s specifications section, among the receiver’s published spurious-response numbers, and it refers to a single mixing product generated inside the transceiver. If you have seen a faint carrier-like blip near 24.572 MHz on a dummy load, the manual is telling you that this is expected behavior.
That single number matters more than it looks. It sits at the intersection of how a modern direct-sampling SDR front end works, what Icom promises in its published specifications, and what an operator should reasonably expect when the antenna is disconnected. The rest of this explainer walks through the mechanism, the context behind 24.572 MHz, the edge cases where it appears louder or quieter, common misreadings, and the practical steps an owner can take to confirm what is happening.
What 24.572 MHz actually represents inside the radio
The IC-7300MK2 is a direct-sampling software-defined radio. Its receiver digitizes the RF band directly with an analog-to-digital converter rather than mixing down to a traditional intermediate frequency. That architecture eliminates classic superheterodyne images but introduces its own family of internally generated artifacts, the most common of which are spurious responses produced when the radio’s own clock signals, ADC sampling, and digital processing leak into the receive path.
A direct-sampling front end must generate a stable, low-phase-noise sampling clock, typically in the tens of megahertz. Digital logic, the ADC, and the FPGA that does the decimation and filtering all clock from that source. Small amounts of that clock energy, its harmonics, and mixing products between them fall inside the HF band the receiver is tuned to. The 24.572 MHz figure is one such product that the manual acknowledges as a known location. It is not the only internal spur, but it is the one Icom chose to print, which suggests it is either repeatable across units or prominent enough to be noticed.
The mechanism is easy to picture once you accept that a direct-sampling receiver is, at heart, a very good digital radio with a lot of fast digital circuitry sitting next to a very sensitive analog input. A tiny amount of digital switching energy couples into the RF path. The result is a faint, often frequency-stable signal that does not move when you change antennas. That stability is the giveaway: a real external signal drifts, fades, and changes with propagation, but an internal spur is stubbornly fixed.
Reading the manual’s specification table correctly
Icom publishes receiver specifications in standards format. Alongside sensitivity, selectivity, and dynamic range, the table lists spurious and image rejection figures. Spurious responses at named frequencies, including the one at 24.572 MHz, are part of that disclosure. The manual’s phrasing is deliberate: it labels these as internally generated and states that they are inherent to the design rather than a defect. This is language Icom uses to preempt support calls from operators who see a faint signal on a spectrum display with no antenna attached.
The practical consequence is that a faint response near 24.572 MHz is not covered as a warranty fault and is not something a firmware update is designed to remove. Icom’s published specification establishes a baseline, and the radio is expected to meet it. When the manual names a frequency, the manufacturer is effectively saying: this is our number, this is what we ship, and this is what our bench measurements confirm. An owner who sees the spur can therefore compare their observation to the manual rather than to an imagined perfection.
Quantified context: how strong should a named internal spur be
Published receiver specifications for radios in this class typically list spurious responses at or below a stated level referenced to the noise floor or to the sensitivity threshold, often expressed in microvolts or decibels relative to a reference. The exact figure varies by band and measurement method, and it is worth reading the manual’s own table rather than relying on a general rule. The key point is scale: a named internal spur is a low-level artifact, not a blocking signal. It should not degrade reception of a real station of reasonable strength.
For context, a well-designed HF receiver can hear signals well below one microvolt, and a typical noise floor on a quiet band sits in the same neighborhood. An internal spur that appears near the noise floor is functionally invisible to most operating. An internal spur that appears noticeably above the noise floor with the antenna disconnected is the case the manual is warning about, and it is still described as an expected characteristic. The number Icom prints is the manufacturer’s own measured or designed value, so treat it as the reference, not as a target to beat.
Timeline: from DC power-up to a visible spur on the panadapter
| Stage | What happens | What the operator sees |
|---|---|---|
| Power on | Clocks and ADC initialize | Brief settling, no stable spur yet |
| Antenna disconnected | Front end sees only internal noise | Spectrum shows noise floor and any internal products |
| Tuning sweeps HF | Mixing products move with local oscillator | Spur near 24.572 MHz stays fixed |
| Digital filters engaged | Decimation and filtering reshape the floor | Spur remains, often sharper |
| Antenna connected | External signals dominate | Spur may be masked by band noise |
| Dummy load attached | External pickup minimized | Spur becomes most clearly observable |
| Firmware update | DSP algorithms may change | Spur may shift slightly or change level |
The order matters because it tells you how to test. A spur that survives a dummy load, ignores antenna changes, and does not move with propagation is doing exactly what the manual describes.
Edge cases: when 24.572 MHz is louder, quieter, or absent
Several variables change how visible the named spur is. The first is antenna efficiency and common-mode pickup. A long wire or an antenna with a poor common-mode choke can couple ambient energy into the front end and mask the internal product, or it can make the spur look worse by adding external noise at the same frequency. A dummy load removes that variable and is the cleanest test.
The second is the radio’s own display and DSP settings. A panadapter with a low reference level and aggressive averaging will make faint, stable signals stand out. Operators who have never noticed the spur often find it after turning down the reference level or switching to a narrow span around 24.572 MHz. That is not a malfunction; it is the display doing its job.
The third is the difference between a real signal and a spur on the same frequency. If you see activity at 24.572 MHz, listen for modulation, check for fading over minutes, and swap antennas. A signal that is perfectly steady, has no modulation, and does not change with antenna swaps is almost certainly the internal product the manual names. A signal that fades, carries sideband or digital hash, or moves with propagation is something else.
A counterintuitive detail worth flagging: an internal spur can behave differently depending on which band is selected and where the local oscillator happens to sit. Because the mixing arithmetic changes with tuning, the exact relationship between the clock and the observed spur frequency can shift slightly across the band. The manual’s single named frequency is a reference point, not a promise that the spur is identical everywhere. That nuance is why published specifications are more reliable than anecdotal reports.
Misconceptions about the 24.572 MHz entry
The most common misreading is that a named spur means the radio is defective. In Icom’s framing, it is the opposite. The manual names the frequency precisely because the behavior is expected and designed for. A second misreading is that the spur proves the receiver is “dirty” or of poor quality. Every direct-sampling radio produces some internal products, and the mark of a well-documented design is that the manufacturer publishes them rather than hiding them.
A third mistake is treating the spur as a reason to distrust every weak-signal observation. A single documented product at 24.572 MHz does not mean other carriers you hear are fake, and it does not excuse ignoring real interference. Operators should use the documented frequency as a known constant, not as a scapegoat for every phantom signal.
Warning: do not attempt “fixes” by opening the radio and adding shielding unless you fully understand the front-end layout and warranty terms. Warning: do not conclude you have a defective unit solely because a steady carrier appears near 24.572 MHz with a dummy load attached. Warning: do not transmit into a fault you have not diagnosed, especially if the spur coincides with an antenna system problem. Warning: do not rely on a single spot check; confirm repeatability across power cycles and bands. Warning: do not ignore real interference at 24.572 MHz just because the manual names that frequency as a spur location.
Practical actions: confirming and living with the named spur
The first step is controlled observation. Disconnect the antenna and terminate the input with a 50-ohm dummy load. Tune to 24.572 MHz, set a narrow span, and lower the reference level until the noise floor is visible. Note whether a steady, unmodulated response appears. Repeat after a power cycle and after a band change to check stability. If it is present, steady, and unmoved by antenna swapping, you have confirmed the manual’s expected behavior.
The second step is documentation. Record the observed level relative to the noise floor and any change after a firmware update. That log gives you a baseline if you ever need to rule the spur out later. The third step is operational awareness: when working weak signals near 24.572 MHz, remember the documented product exists and use the dummy-load test to stay calibrated. If you hear something on that frequency that carries modulation or fades, treat it as a real signal and investigate normally.
The fourth step, and the one most owners skip, is to read the manual’s specification table in full rather than only the sentence that names 24.572 MHz. Icom lists several receiver performance numbers in the same section, and understanding the whole table helps you separate a documented characteristic from an actual fault. The fifth step is to keep expectations proportionate: a faint internal product is not a reason to return a radio that otherwise meets its published specifications, and it is not a reason to dismiss a genuine issue that falls outside them.
Limitations and what the manual cannot promise
The manual gives a named frequency, not a guarantee of identical behavior on every unit, band, and firmware revision. Component tolerances, temperature, and manufacturing variation mean an internal spur may appear a few kilohertz off or at a different level on your specific radio. The manual also does not define the spur’s exact amplitude in every operating condition, so treat its published figure as the manufacturer’s reference rather than a universal constant.
Finally, the manual describes a design characteristic; it does not diagnose your individual unit. If you observe an internal-looking signal that is far stronger than the published specification, or you see multiple unexplained products across the band, that is a reason to contact Icom support or an authorized service center. The named 24.572 MHz location is a known artifact with a documented origin. Anything that departs from that description deserves a proper bench evaluation rather than an assumption about it.