Tuesday, October 8, 2019

A low-cost set-up for 630m WSPR

Contents of G0MRF 5W 630m kit
My latest configuration for 630m operation is now completed. It is my 4th and least expensive set-up so far - the other three are here.

The components total about $70 and consist of three kits: from Hans G0UPL, an Ultimate 3S to use as the driver and a 630m low-pass filter together with a 5-watt amplifier from David G0MRF.

I already had an Ultimate 3S on hand so the cost to add this extra method to my line-up was minimal.

My order of assembly began with the LP filter. It contains larger toroids than the filters offered for other bands but it is still impossible to get all the windings (64 and 70) on it in a single layer. Hans mentions that wrapping them on top of previously wound turns is okay and still results in the theoretical cut-off frequency.

I was only able to get 50 turns onto each toroid before needing to wind the remaining turns on top of
G0UPL LP filter for 630m
the others. I spaced those remaining turns onto the toroid in such a way as to use the entire coil to occupy the turns rather than bunching them up tightly at one end or the other.

A look at the filter with my NanoVNA confirms that the filter does indeed offer the required cut-off with minimal insertion loss.

Despite the larger and taller coil on the filter, it fits easily into the filter relay board in my housed Ultimate 3S, right alongside the other LP filters with no clearance problems in any dimension. I then tested the U3S barefoot on 630m. It provided a 740mW output that was easily dialed down to the amp's 4mW requirement.

NanoVNA filter sweep showing insertion loss and 3dB cut-off

On to the G0MRF amplifier kit:

As with David's other kits, this one is very economically priced and uses a high quality PC board. The wire for winding the two transformers came pre-twisted, instructions were clear with no ambiguity.

Amplifier construction begins

Parts count is minimal and there is plenty of room on the circuit board. The SMA connectors shown in the photo are not part of the kit but the kit's circuit board is drilled for them. I ordered mine here from Amazon and they fit perfectly.

The completed amp

Assembly time was ~3 hours with most of that being spent winding the coils. There is a minor mod required to the board that requires scraping away a bit of the green masking material to expose the board's ground plane underneath. All in all, it was smooth sailing.

The moment of truth:

With the U3S power dialed back to 5mW and the amp's bias set as instructed in the manual, I attached the amp to the U3S and a dummy load.

The OHR wattmeter shows 7 watts out with 5mW of drive and 5 watts out with 4mW. Success! Monitoring locally on another rig, my own WSPR signal was properly copied and decoded on 475 kHz.

7 watts out with 5mW in = 31dB gain

I don't yet have my antenna up - that is this Saturday's job - nor do I have a heat sink on the final transistor of the amp. That will have to wait until I install the amp into a housing, hopefully later this week.

My expectations with 5 watts of WSPR on the 630m band are based on previous results with 25 watts - I was heard all over NA and out to Hawaii on a regular basis. I'll know soon enough what 5 watts will do.

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