The actual phenomenon is: When operating in SSB/CW mode, reducing the RF-G (attenuator) counterclockwise, which lowers the RF/IF gain, results in an increase in the S reading. Conversely, increasing the RF-G clockwise causes the S reading to decrease.
Initially, I found it very strange: If "S" represents "signal strength received by the antenna," and the input signal hasn't changed, why does adjusting the RF gain cause such a large change in the S value?
After reviewing some Yaesu manuals, my current understanding is:
A traditional S-parameter table is not exactly equivalent to an "RF power meter for antenna port," but rather, it's more akin to mapping the receiver's AGC/Gain Reduction status to S1～S9. When the RF Gain is at its maximum setting, the AGC control primarily depends on the input signal strength, so the S-parameter table can be used to approximate the input signal strength:
The stronger the input signal → the greater the gain reduction by AGC → the higher the S value.
However, if RF-G is intentionally lowered by a human operator, this effectively increases the gain reduction. Even if the input signal to the antenna remains unchanged, the receiver's gain control state changes, which in turn raises the baseline of the S table.
Therefore, we can roughly understand it as:
Traditional S-meter ≈ AGC/Gain Reduction status indicator.
The S value is only relatively close to what we typically understand as "actual input signal strength" when RF-G is at its maximum and there is no intentional reduction in the receiver gain.
I believe that while this traditional design has historical and operational reasons, it can be easily misinterpreted from the perspective of modern SDR users.
If redesigned, I would prefer that the machine display two independent metrics simultaneously:
- The equivalent absolute input power at the antenna port, for example, -103 dBm/S4.
The receiver calculates the actual gain of the entire RX link, including PREAMP, ATT, RF Gain, IF Gain, and AGC, based on the signal power measured by the ADC/IF. Then, it converts this gain back to the antenna interface, taking into account the specific values for each component. This ensures that regardless of how RF-G is adjusted, the input signal remains unchanged, and therefore, the resulting value should remain constant.
- AGC/RX Gain Status, for example, AGC Gain Reduction = 18 dB or RX Gain = 42 dB.
This metric specifically tells the user how much gain they are currently receiving on the link.
This makes the physical meaning very clear:
Input Power: How strong are the signals in the air?
AGC Gain Reduction: What gain reduction is currently being used by the receiver?
The traditional S-curve actually combines these two concepts, so when RF-G is reduced, the "S-curve appears to rise," which seems counterintuitive.
Additionally, it's important to note that there is no official block diagram for the FTX-1 currently available, so the statement "directly display AGC code" is based on an inferred understanding of its operational behavior. Yaesu's official documentation for other HF models clearly states that when RF Gain is reduced, the S-meter will increase and this is related to the increased AGC control.
What do you all think? Have any modern SDR radios already released models that separate the display of "absolute input-referred RSSI" and "AGC/RF Gain status"?
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