Estimating threshold receive power level for WSJT JT65B/KV

WSJT is often used for weak signal QSOs in the amateur service. This article explores developing a conservative minimum receive power for reliable copy from system noise temperature or gain and G/T.

Fig 1: Performance of JT65B modulation

Fig 1 is Fig 1 from "Open Source WSJT Status, Capabilities, and Future Evolution" August 2006 by Joe Taylor, K1JT. It shows the relationship between decode accuracy and signal to noise ratio in a notional 2500Hz bandwidth using JT65B for a range of decoding options. Taking the figures for the Koetter-Vardy decoder, 96% copy is achieved in a test environment with -23dB S/N ratio and JT65B modulation.

The 2500Hz bandwidth is a notional bandwidth that is not related to the maximum necessary bandwidth of the JT65B signal, nor to the receiver in use. The necessary bandwidth for JT65B is about 355Hz, or 8.5dB lower than the notional 2500Hz which is used in WSJT S/N figures. Actual receiver bandwidth is not a factor in this analysis, but clearly it must be greater than the necessary bandwidth for JT65B.

For the purposes of path prediction, the 23dB S/N in 2500 Hz is normalised to a S/N ratio in 1Hz bandwidth by adding 10*log(2500)dB to obtain a threshold for reliable copy of 10.98dBHz SignalPower/NoisePowerDensity.

The NoisePowerDensity can be determined from the total system noise temperature. NPD=10*log(K*T_s)+30dBm where K is Boltzman's constant and T_s is the total system noise temperature (including ambient noise).

Where Gain and G/T are known, NPD=10*log(K)+G-GT+30dBm where K is Boltzman's constant, Gain is gain in dB and GT is the G/T ratio in dB.

For example, G=22dB, G/T=-5dB, NPD=-228.6+22-(-5)+30=-171.6dBm.

ThresholdRxPower=NPD+S/NPD where NPD is in dBm and S/NPD is in dBHz.

So, ThresholdRxPower=-171.6dBm/Hz+10.98dBmHz=-160.6dBm

ThresholdRxPower for JT65B at 96% copy=-228.6+G-GT+30+11dBm

Path losses can be used to determine the minimum EIRP.

Fig 2: Model of a WJST JT65B tropospheric scatter path

Fig 2 shows a tropo path prediction using Radio Mobile based on the example ThresholdRxPower of -160.6dB (which Radio Mobile terms variously Receiver Threshold and Rx Sensitivity), indicating a received signal 14.4dB above threshold for the scenario.

Changes

Version	Date	Description
1.01	15/07/2007	Initial.
1.02
1.03

V1.01 21/04/10 15:44:50 -0600 .

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