Category Archives: QRP

QRP By The Bay, Chelsea Beach, Melbourne, 25 Nov 2017

QRP By The Bay for November 2017 brought together many familiar and a few new faces and callsigns on an unseasonably hot 32 degree Celsius day. When Peter VK3YE wrapped himself up with radios and aerial parts including an ankle clasp that looked straight out of a Monty Python costume department a small crowd followed him over the dunes and down into the shallows. It was the best place to be on a hot afternoon, toes in the clear and cooling waters of Port Philip Bay.  Or should I say, VK3YE’s State-sized ground plane. The now sub surface mannacle paid off for its wearer.  Forty and 10m contacts rolled in while a few of us chatted at the edge.

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VK5ZVS 630M Band Pass Filter using transistor radio IFTs

I’ve included 630 meters in my 8 band Progressive Receiver project. Not that I expect to hear any low-band CW, but down the track I hope to hook up a shack computer for FT-8, WSPR or whatever weak signal digi mode they use down there.  630m is a playground for homebrew transmitters and receivers. For example, Dimitris VK1SV has some simple but impressive class E transmitters using the familiar IRF5xx series switching FETs. http://people.physics.anu.edu.au/~dxt103/472/  You can get a lot of bang for your buck out of any transistor or FET operating at 470kHz. So including 630m in my receiver creates opportunities for future play!

Using metal can transistor radio IFTs as the tuned elements of a BPF is just such an obvious and neat idea. A pack of four IFTs is available from Jaycar, three cans for the 455kHz IF and one (red) for the oscillator.  VK1SV built a 630m filter with two, whereas Steph VK5ZSV went as far as five. My filter uses four, mainly because the white ones come 2 in a pack, so my investment was 2 packs (about $10). 

The video shows a band scan with the filter out, then in.  Thanks to VK1SV and VK5ZVS for publishing their designs on their sites.  

An SSB receiver from an Arduino/si5351 VFO/BFO

Having finished my latest Arduino Nano/si5351 VFO/BFO/Controller, I wanted to hook it up to some receiver stages to realise a SSB receiver. My grand plan for these modules is to build a multiband SSB/CW transceiver for SOTA and portable operation (another rig in my established ‘Summit Prowler’ series). The architecture will be a single conversion superhet with an IF of 12 MHz. For a base design I chose Leon VK2DOB’s MST3, a proven design which I am very familiar with, having used the MST (circa 2013) on nearly all of my SOTA activations to date.

I scratch-built two boards. The first is the MST3 product detector (SA612), audio filter (NE5534) and audio amplifier (TDA7052A). This module includes receiver muting, sidetone injection and audio-derived AGC which controls the gain of the TDA7052A. The second board contains the transmit and receive mixers (both SA612s), some SA630D RF switches to switch the bandpass filter between transmit and receive signal paths, as well as two 5-pole crystal filters (one for SSB, one for CW), each switched by another pair of SA630Ds.  There is no IF gain stage, although I put some pads on the end of this board for one or two MOSFET stages, if needed.  

As per other recent scratch building efforts these boards are a curious jumble of thru-hole and SOIC ICs, soldered to the copper side of a Muppet style hand drawn and etched board, largely populated with junk box and surplus sales components. 

Once the boards were debugged, temporarily adding a 40m bandpass filter and connecting the VFO/BFO up brought the little receiver to life. I used no si5351 clock filtering or buffering, just CLK0 and CLK2 square waves straight into the SA612 Gilbert cell active mixers, which have some useful gain.  The sound of the receiver is pleasant, to me at least.  The video includes a range of DX and local signals on 40m from my QTH in the north-eastern suburbs of Melbourne, received over a day. 

Next, I will add a third board with a bank of switched bandpass filters and switching logic, all controlled by the Arduino. More to follow.

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My second homebrew Arduino/si5351 Digital VFO-Controller

Back in February 2017 I scratch-built my first Arduino and si5351 VFO. Here’s my second one.  It is a generic Nano/si5351 module wired up to Farhan’s Raduino circuit. The si5351 is on an Adafruit breakout which includes a 3.3v regulator and crystal.  Everything else is as per Raduino other than the presence of a second 7805 regulator for the LCD (and a 1000uF electrolytic storage capacitor) which will allow testing of Pavel CO7WT’s scheme for saving the VFO frequency to the Arduino’s EEPROM when the power drops.

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Three Summits in the Yarra Ranges: Federation Range, Mt Strickland and Mt Donna Buang

Activators have been bemoaning the poor state of HF propagation recently. But when planning an August SOTA activation in Melbourne, the propagation forecast runs a distant second to the weather forecast. If the HF propagation is bad you might not get the 4 contacts, but if the weather is bad you might get seriously bogged or snowed in.  If nothing else you’ll come home wet and miserable.  So I studied the weather report carefully for Friday the 4th of August. A series of three low pressure systems of varying strength were on the weather map bringing low temperatures, strong winds, rain and snow down to 900 meters. Friday morning looked to afford a break. I considered Mt Matlock (VK3/VC-001) but when the morning came after an evening of steady rain I opted for Federation Range (VK3/VN-029) near Lake Mountain, figuring that walking 5km each way through slushy snow would be better than driving 50km each way along the pot-holed water-logged C511. As it happened, it was the right choice.

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Homebrew 40m SSB rig gets CW, keyer, break-in, AGC and metering

A couple of ‘solder sessions’ finished all the wiring, debugging and testing required to deploy my Arduino Nano keyer and SWR/power meter, and the LED LDR AGC and S-meter modules into my homebrew 40 meter SSB rig (MST Mk1 from OzQrp).  Before reassembly I sprayed the front and rear panels, and added my preferred DecaDry white lettering and a protective coat of  clear satin sealer. It’s now resplendent with all the expected features of a SSB/CW monoband portable rig.

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VK3YE’s ‘non-invasive’ LED-LDR AGC add-on

When I saw the AGC circuit by Peter Parker VK3YE, I realised it solved a problem common to many simple homebrew receivers.  You cannot automatically control the gain of a receiver with fixed gain stages. Most simple receivers do not have IF stages, let alone variable gain IFs. And the LM386 has no gain control. So it’s easy enough to sample the audio at the volume potentiometer, amplify it, rectify it for a source of AGC. But then you need a voltage controlled gain stage ( a Voltage Controlled Amplifier  or VCA).  If your receiver doesn’t have one, AGC won’t work, and you with have to either add a VCA in the receiver somewhere or replace one of the fixed gain stages with a VCA.  Until now.

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Arduino CW keyer for a BiTx or other homebrew rig

It’s been 3 years since I got my MST400 Mk 1 40 meter SSB transceiver going and started using it for SOTA activations. For those not familiar with it, it’s like a kit version of the BiTx40 from 2013. It has a similarly spec’d monoband superhet receiver, 10MHz IF, and IRF510 5 watt power stage. The circuit on a page is here. It’s been with me for nearly all of my 65 activations (over 400 SOTA activation points) around VK3. Now, with HF propagation approaching cycle lows, it is time to do what I should have done ages ago… add CW.

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Homebrew 160 meter AM/CW transmitter receiver

The VK 160 and AM SOTA event (1st April 2017) provided the deadline pressure needed to complete this build. The rig is a separate transmitter and receiver, so seeing as there are no shared modules, I shall call it a trans-receiver rather than transceiver. The designs for both transmitter and receiver are by Drew Diamond VK3XU. See Drew’s Projects Volume 3  book for full details, available from the WIA or RSGB. I posted details of the 160m AM receiver when I first built it, a year ago. At that time I also built the transmitter and power supply. I got the transmitter going but blew up the PA FET (BUZ90). During the long wait for replacements, I shelved the build and moved onto something else. Now, thanks to SOTA, it is finished.


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Hotham SOTA weekend 2017

Q: How many SOTA activators does it take to change a light globe?

A: Nine. One to alert the impending globe change (subject to weather), one to take the bulb and socket to the top of a summit, one to spot when in sight of the bulb, four to replace the bulb while logging UTC and local conditions, one to take photos as the replacement is done, and one to upload the globe replacement details into an online database and write about it all on a blog.

Nine. That’s how many VK3s spent the weekend at Peninsula ski lodge on Mt Hotham for the 2017 SOTA Hotham weekend. It was my first attendance. I met Brian VK3MCD, Peter 3PF, Alan 3FDIM, Ken 3KIM, Ron 3AFW, Glen 3YY, Allen 3ARH, Tony 3CAT and Ron and Glen’s partners for two days of SOTA activations, map reading and socialising.

 Morning of Sunday 26th Feb 2017, looking out the Peninsula lodge window south.  

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