Setting WSPR Power

Minimum WSPR Power.
I see remarks on WSPRnet Chat or Forum that users claim their rig power will not go below 5 watts.
With all transmitters when using SSB the RF power out is a reflection of the level of audio that is applied to the input, the microphone or data port.
The power control of a radio when on SSB/Data modes limits the maximum power, not the minimum. All it does is apply ALC if the audio input exceeds a certain level. It is often advisable NOT to drive a transmitter into ALC with digital modes.
Many say they cannot measure power below 5 watts so do not try. There is a method where low powers may be set with fair accuracy.

Reduce Audio Level.
There are several ways to reduce the input to a transmitter.
With Windows select the soundcard playback properties of the card selected in your WSPR setup.


Right click the speaker icon on the taskbar. Click "Playback devices". Select your soundcard, go the the "Levels" tab. Here the "Speaker" slider controls the audio to your transmitter.
 If you see another like this the "Line-in/Mic-In" must be muted. Click the speaker icon on that tab.



Right clicking on the slider on the "Level" tab enables you to choose dB readings. BUT I have found these dB scales may be very inaccurate, actually useless!
While you have this open make sure all special effects are disabled. On this soundcard the "Sound Blaster" tab has a box that needs to be ticked to disable them. Explore these tabs, maybe other buttons lead to other effects that need to be disabled. Enhancements may sound good through speakers but will not help WSPR!


WSPR itself has a digital output control. Shown version 1.6.0 Hover your mouse cursor over the "Pwr" slider and the level is shown in dBs. Use your mouse wheel for easy adjustment. Earlier versions had the control at "Setup - Advanced"
"Transmit digital gain" may be reduced by up to 45dB. The mouse wheel will move the slider one step. Unfortunately it is necessary to move the cursor off, then on again to see the value. It is not possible to get exact dB values with steps of -0.3dB. Ignore the decimal point 1dB will make 1dB difference in SNR so a few tenths of a dB do not matter.

Steps I see:-0dB, -0.3, 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.6 3.9 4.2 4.5 4.8 5.1 5.4 5.7 6.0 6.3 6.6 6.9 7.2 7.5 7.8 8.1 8.4 8.7 9.0 and this seems to continue, -0.3dB steps so each 3rd dB is even.

To set audio levels so these controls will set the power.

If, for example you do not feel your power meter will read accurately below 5 watts then set your power control to about 10 watts. Then adjust either or both of the sliders shown above so that 5 watts is indicated. Now your power is controlled by the audio and power sliders. You may have to reduce levels significantly.  Note the dB readings on these sliders.

You now have the ability to reduce power by a known number of dB by using the WSPR Pwr control.

The box that shows the power value transmitted by your WSPR signal also shows dBm. 37dBm is the same as 5 watts.
So if you make the dB figure 7dB more negative on this or the audio control slider then you will be transmitting 7dBm less, 30dBm, 1 watt.
Then maybe reduce by a further 10dB. 37dBm -17dB is 20dBm, 100mW.
If your first step of setting 5 watts output is accurate then these powers will also be accurate.

Many SWR meters may be used to show lower powers more accurately. A meter that has a scale on it's reflected power display may be carefully used connected in reverse.
The reflected power meter will then read forward power.
Be careful, make sure your power control is at minimum and that you do not allow high power to be transmitted while you are testing.

Microphone Inputs.
Feeding audio from a computer into a microphone input is not ideal.A soundcard output is far too high to feed a microphone input directly. If the radio has a data input then use that.
To use a microphone input an attenuator will be required, just reducing the microphone gain is unlikely to be sufficient, If you manage to reduce the output power enough there may still be distortion and noise on your transmitted signal.

A suggested circuit, maybe one potentiometer may be enough but two give finer control. Almost any may be used, preset trimmers are small and convenient.

Using a 100 watt transceiver like this is not ideal, think about using a low power rig or maybe a home made transmitter, A Softrock or QRP Labs "U3S" is good. Others perhaps a little more complex are available maybe using the Raspberry Pi or Red Pitaya.

Another way to help reduce power is by using an attenuator in the antenna feed.
A 10dB attenuator will not affect reception, just about any commercial radio is sensitive enough. Note this attenuator must be built to be able to comfortably deal with at least 5 watts input.
A 100 watt transmitter will have a relatively high amount of  broadband noise present on the output. An attenuator will have the effect of reducing the amount sent to the antenna.
Commercial attenuators are available. This page has an Excel calculator for building yourself. https://a29.veron.nl/hams/pa1b/pa1b-power-attenuator-calculator/

If you use a SDR at the bottom of this page https://sites.google.com/site/g4zfqradio/hdsdr_wsjt-x I describe a way to control and display your WSJT-X power output in HDSDR.

Comments