Thursday, June 15, 2017

A brief WSPR transmitter comparison

WSPR is a fantastic mode to use for investigating propagation paths. I'm often surprised at openings that defy conventional wisdom - 10m openings late at night, long-path openings detectable with sub-watt power levels and others.

There are several ways of transmitting a WSPR signal that don't require a computer and I've tinkered with most of them. Each has its own advantages and disadvantages:


This is the only method of those listed here that I haven't used. As supplied, it is a dual bander (20 and 30 meters) with additional filters available for 40, 80 and 160 meters. It is tiny (easily portable), operates from a 5V power supply and puts out 200mW.

If you have no interest in discovering elusive openings on the higher bands, it seems ideal and is often used in conjunction with SOTA-Beams DXplorer website that plots the stations that receive you on a time vs. distance graph. My own experience with testing reception reports and this website lead me to question the resolution of points needed in order to be plotted. You can read more about that here.

Raspberry Pi 3 + TAPR shield

I've detailed the steps necessary to converting a $30 R-Pi to a 20m WSPR transmitter here. Set up properly, the RPi generates 100mW on 20 meters. It requires the TAPR shield which is both a LP filter and an RF amplifier that plugs into the GPIO pins on the RPi. Time synchronization is maintained via your router and NTP.

So far there are no TAPR shields available for other bands.

WSPR-AXE (kit)

I'm not sure if this item is still available as parts of the website describing it appear abandoned. I have a 10m version of this transmitter and no longer use it. I don't particularly agree with the idea of transmitting constantly as I think it causes undue QRM to other stations. When 10m is open only sporaticly, this may not be so much of an issue. When the band is open, an always-on transmission could be regarded as excessive. The WSPR-AXE also lacks the ability to synchronize to an external time source and relies on the operator to manually keep the time within WSPR spec. The onboard clock is sufficient to keep accurate time for several days though - more than adequate for most operations.

Ultimate 3S (kit)

This is, by far, the best method of operating a WSPR beacon for the following reasons:

  • Time sync maintained via GPS with optional module
  • Operation on any band from 2m to 2200m
  • Up to 6 bands, one after the other, may be used automatically in sequence
  • Can be configured to transmit from once to 30 times per hour
  • Other modes available as well
  • Optional 5 watt amp available

The U3S produces 200mW out and can be purchased with or w/o a case and with as few or as many LP filters as needed. It operates from 5V.

My own experiences with building and using this transmitter can be found here.

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