VK3KHZ modifes the WSPR-TX_LP1 to dual band transmitter.

Updated: May 23, 2019


Paul Karlstrand with radio amateur call sign VK3KHZ in Australia was one of my early customers of the WSPR-TX_LP1 transmitter. He emailed me back the day other with some feedback and the results and a modification he had done on his unit.

The WSPR-TX-LP1 has been created for experimentation in mind so that made me interested, I love to hear what people do with the stuff I sell . Paul took advantage of the expansion port and used it for a novel idea.

Using the WSPR-TX_LP1 Expansion port as a band switch.

I envisioned using the expansion port to drive external relays that can switch between Low Pass filter but Paul instead created a modified Low Pass filter that can serve two bands and used the port as an input to determine what frequency to transmit on.


He has fitted a three way switch that connects to two inputs and Ground. This will set the frequency to either 30m or 40m depending the switch position. In the middle position it goues in to band hopping mode. As the low pass filters serves both bands there is no need to do any switching on the RF side. To make sure the LP filter can be used on two bands he modified it from a straight Low Pass to have resonant elements to help with overtone suppression. Here is Pauls schematic for the 30m/40m filter.


To fit the extra components he soldered them to the lower side of the PCB.


He will use this solution when mobile but did a quick test with his home antenna overnight to check out the performance. I took a screen shoot of the results and I think they are interesting in that clearly show teh difference in contact distances between teh two bands. Short distance contact (Brown lines) on the 40m band and the longer range contacts (Blue lines) on the 30m band. Of course many radio amateurs that have been active on multiple band for a long time knows about these interesting facts how our ionosphere behaves over distances and frequencies. But it sure is neat to see it presented as clear as this.


Paul mentions that for his stationary transmission test he used a fan dipole that covers 7, 10, 14 & 21MHz. When he gets it in to the mobile application he will use a 3.6m tall Terlin Outreach Tapped Whip antenna on his vehicle.

He shared his modified Arduino code so that any fellow Ham that want to try it out can do so.

I know how hard it can be to try to figure out someone software that someone else has written but it seem to have come natural for Paul. Here is the code in PDF format, cut and paste in to the Arduino IDE to use it.

It will interesting to follow Pauls continued experimentation when he goes mobile.

He seems comfortable both with the hardware side and the software side of our hobby and his workshop looks like a great place for further experimentation so I expect some interesting things from him.


Paul runs a SDR on several bands so if you want to check out the reception from his location then head over to http://sdr-amradioantennas.com:8073/?f=7135.00lsbz8

Thanks goes out to Paul VK3KHZ for sharing his work and story with us. Thank you and 73 to you all!

//Harry - SM7PNV


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