It’s like trying to Tune a Piano with a Feather

“Like trying to tune a Piano with a Feather” that was my description of getting a home brew MEPT tuned in the 100Hz segment of the QRP-QRSS sections of the bands.

When Bill of Soldersmoke fame built his MEPT the only RX he had was a very inaccurate solid state rig that drifted. These are some sugestions I made to help him find his TX frequency. The number of new stations coming onto QRSS and the same problem being encoutered prompts me to write this. In addition my memory does not get better, I use my blogs to record things so that if I go away for a few months and forget something I have a reference.

Like most things a good understanding of the Receive side is essential before venturing a signal onto the air. If you can receive other MEPT’s and know their frequencies then you are 99% there.

First the RX needs to be warmed up for quite some time to make sure it has settled.

Here is my Kenwood TS870, a Transceiver I consider to be very accurate and very stable after 5 minutes from switch on the display should read above 070, it will never quite get there it is a few Hertz low but it can be seen to be heading up that way.

Here is my ‘G’ MEPT signal at this time received by the Italy I2NDT Grabber I have just adjusted my TX frequency to 10.140073MHz plus or minus a bit.

So how do we get Spectran or Argo and our RX to display MEPT’s.
First we need to get the received signals in the audio passband of the RX, for convenience 1KHz is a good frequency to choose, it is near the middle of the passband and a round number. No matter what band we are receiving on always use USB (Upper Side Band) and the standard 2.4KHz bandwidth. To produce a 1Khz audible beat note of a particular radio frequency we need to tune 1KHz below its frequency. Hence to receive 10.14000 MHz we need to set our RX dial to 10.13900 MHz.

Next we need to set up Spectran to match.
From the Menu select Mode – Preset QRSS3. The display will now scroll Horizontally.
At the bottom select Show Controls – in the Freq Offset box enter -1000 (minus 1000) as shown.

Now point your mouse to the grey bar between the top and bottom spectrun displays. In the top display is a vertical red line it is reading -150 or more at the red line, Left click the mouse and drag the scale to the left through Zero to place 100(+100) at the red line.
The Right hand vertical grey scale now reads 10 – 20 – 30 up to 100 if it was not hiding at the top. This scale now shows the 100Hz of the 10.140000 to 10.140100 QRSS Band.

When you close Spectran these settings will be saved so you only need to set up once.

If you are new to Spectran do some thinking and tune to the frequency you need to to display WWV on what ever frequency you can hear it on, check the accuracy of your RX dial.

If you are seeing MEPT’s you can put your newly built TX into a Dummy Load and adjust its trimmer to sit your signal where you want in the QRSS band
But beware there will be more than one of you, even with a few mWatts into a screened dummy load you will find it hard to get rid of your signal.

Here I have re-tuned my RX 70Hz higher than 10.13900MHz the signal is some 30dB down but strong, comparison with the Left Hand side of the trace makes it obvious which is the fundamental.

This is where the Feather comment comes in, a tiny adjustment of the trimmer can shift your signal way out of the 100Hz window, you are using a Dummy Load so tweek and twiddle to get used to it, like all things after a time minute adjustments become easy.

When you are fully confident in your skills, settings and accuracy you can put your MEPT on the air. If you are lucky like me, (I2NDT Grabber is almost a certainty during daylight hours) your signal will pop up right on your determined frequency on a Grabber. Alterantively you can get a friend to listen / look for you but beware many Hams do not know within 200 to 300 Hz where they are on the band despite what their dial reads.