Sunday, April 26, 2020

ARNSW Radio Experimenters Group meeting again via video

Since the COVID-19 isolation came in to effect, we've been having meetings of the ARNSW Radio Experimenters (Home Brew) group via a Skype meeting. Today we held the second "normal" meeting. (We've also been holding a few lunch meetings which are less structured and more social in nature.

Today we had Ali, Gary, John, Stephen, Colin, Kevin and Peter.

Peter, VK2EMU, was mentioned in dispatches on the most recent edition of the Soldersmoke podcast. He had been mentioned in the last episode as the builder of a power amplifier with 6 meters. A misunderstanding! It turns out that it's an amplifier for 40m that has six meters on the front panel. See below.

We had a pleasant chat about what each of us is up to that went for about an hour. I recorded the audio and you can hear it here.

I talked about a few topics of interest to home brewers including:

* QSO today interviewed VK3HN, Paul Taylor where he talks about home brew radios for SOTA.
* A recent podcast from Hackaday drew my attention to the fact that you can buy surface mount components in the form of a book. For example, here's one from Little Bird.

Today, I had the final radio operation from the current QTH. I listened to the Sunday broadcast on 20m with a small dipole in the front yard and participated in the call-back. Next stop is a small unit in Alphington in Victoria. It looks like there's a bit of a yard with a tree so I hope to throw some wire up shortly after moving in.

Wednesday, April 01, 2020

Class E PA for WSPR with home brew variometer in the antenna coupler

Ross, VK1UN and EX0AA, is a frequent traveller and likes to get on air with WSPR under challenging conditions including using stealthy end fed antennas deployed from a window from hotels or flats. Currently he’s transmitting in Bishkek, Kyrgyzstan and getting amazing results even on 160m or lower.

His current configuration is interesting in several ways, he uses an Ultimate3 Beacon from QRP-Labs to drive a very efficient home brew class E PA that is coupled to an end fed wire antenna via a coupler with a home brew variometer.

The Class E PA is designed for 12V and 10.09W which, with an IRF530, gives a MOSFET R of exactly 4 Ohms and the PI output network of exactly 4 Ohms input and output as well. 

Why not 50 Ohms?

These PAs are designed for a single series C from PI out and shunt L (variometer) to match to a long wire. Also, he has a decent 4 Ohm load that can be used to test stage one of these PA's operation at 4 Ohms. The main strategy with these PAs is that, if the antenna changes such that the input |Z| is different, he can change the series C and L on variometer very easily.

L4 is the variometer and the secret to easily tuning different end fed antennas. When Ross had a good match direct from the PA via a series L (FET match) and then series LC match to the wire with a lumped inductor, a change the the wire's |Z| required a lot of work calculating and re-winding a new toroid. A variometer is a low cost and easily adjustable alternative to a roller inductor and can be home built.

The other advantage is, that in a 4 Ohm PI out can use 50V SMD caps without exceeding ratings and can get all values unlike those available as 500V caps.

Ross has built versions for several bands but most recently a 160m version which puts out 3W at 5.75V with good results and has a near perfect D-S waveform and good looking spectrum.

DC power input is very modest and efficiency is about 95%.

Here's the PA.

Ross is well received on WSPR from his remote location. Here's the display in the new WSPR Watch for macOS.

Thanks to Ross for sharing this project with us.