Comms Connect Show in Melbourne

Today I joined some other amateur radio operators at the Comms Connect trade show at the Melbourne Exhibition Centre. Check in was efficient. Ralph VK3ZZC, Peter VK3YPG and Nigel VK3DZ were there too.

It was mostly pretty dry commercial stuff but Icom was there and even had some amateur equipment on display. Knowledgeable Craig Norris was there with all the Icom news.

Kenwood had a stand but there was no amateur gear, mostly DMR commercial stuff.

Telstra had some sort of life-saving drone idea but "it's going to have to be at least three times bigger than this" to be useful.

After the show we went for a very nice Yum Cha and then headed for one of Nigel's family bars "The Sherlock Holmes Inn".

Very nice. My thanks to Ralph for the bag of semi-conductors and happily I had a bag of NE602s for him.

On the train home I ran in to Ray VK3ACR, the world if full of Amateur Radio operators if you pay attention.

QRP Kits Easy Receiver

As a refreshment after all my recent home brew activity I built a nice little direct conversion receiver kit from QRP Kits aka "Pacific Antenna".

It's a classic NE602 + LM386 direct conversion receiver as described in Experimental Methods for RF Design. I've built these before. This version ads an RF bandpass filter to the front end and has an unbalanced audio chain to the LM386. It drives headphones just fine but is a little soft into my bench receiver.

Even though it only covers a small portion of the band - you move that portion with the trimmer capacitor - tuning is very sensitive and they do suggest that a multi-turn pot would be a good idea. I find it drifts a bit. Some of the supplied capacitors had very tiny and indistinct markings but the kit went together well and it worked on first power up.

Here it is receiving a parks operator:

The documentation is good and available on the web page. My kit arrived here in Melbourne, Australia, three weeks after ordering. Pretty good.

Tech news spot on ABC Radio.

I was a guest on ABC radio's "Nightlife" program talking about the amazing "audio overview" feature of Google's NotebookLM that generates a conversation about a topic you give notes on. 

Also Pig butchering scams are targeting Australians and are social media platforms really biased against conservative users? 

Listen here:

https://www.abc.net.au/listen/programs/nightlife/nightlife-tech-talk-with-peter-marks/104446916  

Dremel cut PCB construction

Prototyping small circuits where it's not worth designing and etching a board can be done in many ways. I've used "ugly" (which I think is rather beautiful) where components float above the copper ground plane; MePads where small copper pads are superglued to the base plane; or strip board with track cuts.

Recently I've converged on a system that's working quite well for me.

The circuit is drawn out in pencil and the outline of the PCB rectangle is traced.

Working through what should not be connected to what, I draw the cut lines under a spaced out version of the circuit. Here's my drawing and below the final working board.

I mark up the board with the planned cuts.

To make the cuts in the copper, I lay the Dremel with a cutting wheel on the bench and lean it so that it just cuts the copper. I draw the board along the wheel, leaving the Dremel stationary. I can get pleasingly straight cuts this way. It is worth checking for non-conduction between areas with a multi-meter as sometimes a strand of copper can be left at a corner.

To hold components against the board for soldering you need some sort of "helping hands" but the one I learned about from an Adam Savage video is called Omnifixo. They are not cheap but are well worth it for all the cleverness.

For years I've been using a little, rather low power, bench soldering iron but I find that quite a lot of power is needed when soldering to large copper areas. I have switched to a 70W iron and it is a revelation.

Naturally, I first had the transistor in the wrong way around, but after that was fixed. The joy of oscillation was experienced.

I'm sure this is not news to any readers but I thought it worth documenting for some who might be getting started with home construction. It's not as compact as other techniques but lends itself to making changes.

One other thing I do these days is add an LED (and resistor) to the power line. It looks nice and I can't tell you how much time I've wasted wondering why an un-powered circuit doesn't work!

Experimenting with an SSM2167 compressor limiter board

The home brew AM transmitter on the bench at the moment has thrown up the issue of either low or over modulation from my mic audio. Some time ago, I purchased a couple of interesting boards described as "SSM2167 Preamp Compressor Limiter Noise Gate Dynamics Processing Module DC 3V-5V Microphone Preamplifier". They are under AU$5 from AliExpress and really tiny.

To aid with experimentation, I've mounted mine on a carrier board.

R1 on the board controls the noise gate, which I'm not too interested in. R2 controls compression and came with a 1k resistor in place. The data sheet has this table:

1k would give almost no compression. A strange choice given how the boards are marketed. Happily there are big through holes available on the board for both resistors.

With the un-modified board I fed a 1kHz tone in and get peak to peak voltages:

50mV -> 440mV

100mV -> 880mV

200mV -> 1.43V

Higher inputs do not give higher output. The output waveform looks pretty good, so not clipping. Gain is rather low for my dynamic mic however.

I replaced R2 with a 100k resistor and it has much more gain.

5mV -> 880mV

10mV -> 960mV

20mV -> 1.06V

Output level stops there and again gain is being reduced.

Stephen, VK2BLQ, brought this idea to my attention as it has been discussed as helping the uBitx radios.

First contact with home brew AM transmitter on 7.125

For too many weeks I've been tinkering with a small transmitter for the active AM frequency of 7.125Mhz and just now I called CQ and was kindly answered by Ross, VK3ARW. He reported good signal strength both where he is (west of Bendigo) and on the Ironstone ridge SDR in South Australia. The transmitter is home brew (although I used a Jaycar mic preamp kit).

I've been testing by listening to myself on the PKLoops SDR.

The transmitter uses an Arduino Nano that simply boots up and tells an Si5351 to put out 7.125MHz. There's an RF buffer using a 2N2222. The magic happens, the generation of AM, using a simple diode ring mixer that is set to be off balance. Mic audio, from the preamp, is fed in and out comes AM. (This technique was mentioned by Dave, VK3ASE, who also uses it). There's another 2N2222 gain stage then a low pass filter. Next I have the three stage RF power amp from Drew Diamond's 80m transmitter. Finally another low pass filter for 40m.

To answer Ross's question.. out of all this I get about 1.5W so, for the sake of getting some contacts I've fed this in to a little RF amp that gives about 25W peak.

I must say I'm astonished that I didn't get RF feedback or other instability given the construction. Despite Ross's kind words on the audio I can see that a peak limiter is needed to keep modulation up without over-modulating.