JS8Call on QO-100 SAT Narrowband – A complete frequency guide for S51SG

Recently, while working HF, S51SG asked for the operating frequency for JS8Call on QO-100. That sounds simple, but once you factor in LNB local oscillators, transverters, SDR dial frequencies, and JS8Call audio offsets, the correct answer depends entirely on your station.

The QO-100 narrowband transponder is linear and non-inverting, meaning audio and sideband orientation are preserved. The uplink to downlink relationship is fixed and straightforward.

Rather than giving a partial answer on HF, this post shows exactly how to calculate everything yourself, step by step.

1. The space segment math

QO-100 narrowband uses a fixed, non-inverting frequency conversion.

For the same transponder spot:

Downlink MHz = Uplink MHz + 8089.500 Uplink MHz = Downlink MHz − 8089.500

The frequency difference is exactly 8089.500 MHz.

A commonly used JS8Call trial spot is:

• Downlink: 10489.548 MHz USB
• Uplink: 2400.048 MHz USB

Verification:

10489.548 − 8089.500 = 2400.048

The current narrowband transponder bandplan is 500 kHz wide:

• Uplink: 2400.005 to 2400.490 MHz
• Downlink: 10489.505 to 10489.990 MHz

The JS8Call spot at 10489.548 MHz sits within the narrow bandwidth data modes segment of the transponder. Always check the current band plan published by AMSAT-DL before transmitting, as segments and operating guidelines can change over time.

2. Calculate your receive frequency

Your receiver does not tune 10489 MHz directly. The LNB converts the 10 GHz downlink to an intermediate frequency.

Formula:

IF MHz = Downlink MHz − LNB LO MHz

Step 1

Determine your LNB local oscillator frequency. Typical examples:

• 9750.000 MHz for a standard Universal LNB
• 10000.000 MHz for many modified or external reference setups

Step 2

Insert it into the formula.

Example with 9750.000 MHz LO:

IF = 10489.548 − 9750.000 IF = 739.548 MHz

Example with 10000.000 MHz LO:

IF = 10489.548 − 10000.000 IF = 489.548 MHz

That IF value is what you tune on your SDR or receiver.

If your LNB is locked to a GPSDO or other external reference, use the exact LO value. Even small LO errors will shift your downlink frequency.

3. Calculate your transmit frequency

This depends on how you generate 2.4 GHz.

Case A: Direct 2.4 GHz transmission

If you transmit directly at 2.4 GHz using a Pluto SDR or similar device:

Set your transmit frequency to:

2400.048 MHz USB

No IF math is required.

Case B: 144 MHz IF transverter

Step 1

Determine your transverter LO frequency. A common value is:

2256.000 MHz

Step 2

Use the formula:

IF dial MHz = Uplink MHz − Transverter LO MHz

Example:

2400.048 − 2256.000 = 144.048 MHz

Set your IF rig to 144.048 MHz USB.

Case C: 432 MHz IF transverter

A common LO value is:

1968.000 MHz

Calculation:

2400.048 − 1968.000 = 432.048 MHz

Set your IF rig to 432.048 MHz USB.

Always verify your actual transverter LO. A small LO error directly translates to the same frequency error on the satellite uplink.

4. Account for JS8Call audio offset

JS8Call operates in audio. Your signal is transmitted at an audio tone offset from the dial frequency.

Formula:

Actual RF = Dial frequency + Audio offset

If your JS8Call audio offset is 1500 Hz and you want the actual RF to be exactly:

10489.548000 MHz

Then:

Dial frequency = 10489.548000 − 0.001500 Dial frequency = 10489.546500 MHz

The same logic applies on transmit.

If your desired uplink RF is:

2400.048000 MHz

With a 1500 Hz offset:

Dial uplink = 2400.048000 − 0.001500 Dial uplink = 2400.046500 MHz

If your offset is 1000 Hz, 1200 Hz, or any other value, subtract that value instead.

Always subtract the audio offset to determine the correct dial frequency.

5. Complete calculation procedure

To determine your exact settings:

Step 1

Choose the desired satellite RF frequency.

Example: 10489.548 downlink and 2400.048 uplink.

Step 2

Calculate your receive IF:

IF = Downlink − LNB LO

Step 3

If using a transverter, calculate your transmit IF:

IF dial = Uplink − Transverter LO

If transmitting direct at 2.4 GHz, skip this step.

Step 4

Subtract your JS8Call audio offset from the dial frequency to determine the correct displayed frequency on your radio or SDR.

6. Example full station calculation

Assume:

• LNB LO: 9750.000 MHz
• Transverter LO: 2256.000 MHz
• JS8Call offset: 1500 Hz

Receive:

IF = 10489.548 − 9750.000 = 739.548 MHz Dial RX = 739.548000 − 0.001500 Dial RX = 739.546500 MHz

Transmit:

IF dial = 2400.048 − 2256.000 = 144.048 MHz Dial TX = 144.048000 − 0.001500 Dial TX = 144.046500 MHz

7. Verifying your signal on the satellite

Once configured, it is strongly recommended to verify your signal using a public QO-100 narrowband WebSDR or spectrum viewer. These receivers allow you to see your downlink signal exactly as it appears on the transponder.

Look for your JS8Call tones at the expected IF location based on your LNB LO and audio offset. Confirm that your signal is correctly placed, stable, and not drifting. Monitoring your own downlink is the fastest way to confirm that your math, LOs, and offsets are all correct.

8. Power levels, full-duplex, and operating etiquette

QO-100 is a shared, linear transponder. Start with the lowest uplink power that produces a readable signal on the downlink.

Operate full-duplex whenever possible, transmitting while monitoring your own downlink. This allows you to immediately detect frequency errors, distortion, or excessive drive.

Avoid overdriving the transponder. Excessive uplink power raises the noise floor and degrades the experience for everyone. Your downlink signal should be clean, narrow, and free of distortion.

Always monitor the transponder while transmitting and reduce power if you see compression, widening, or excessive noise contribution.

Final note for S51SG

The short HF answer is:

• Downlink 10489.548 MHz USB
• Uplink 2400.048 MHz USB

The correct dial frequency for your station depends on:

• Your LNB LO
• Your transverter LO or direct 2.4 GHz method
• Your JS8Call audio offset

Use the formulas above and you can compute everything precisely yourself, without guesswork, and without chasing signals around the transponder.

73

Julian OH8STN

And if you would like to support my work, check out one of my books:

https://www.amazon.com/author/julian-oh8stn