LiPo Battery box - a modest project

I have a 12.8V 12Ahr Lithium battery but I wanted to avoid a mess of wires when using it in the field so went looking for a box to house all the bits.

Annoyingly hardware stores don't seem to list the internal dimensions of these boxes on their web sites but I found one at Bunnings for $35 that is perfect.

Not much to this project. There's an in-line automotive fuse holder, I've fitted a 30A fuse for now. A nice big switch and a panel mount Anderson connector.

The fit is tight enough that the top of the case presses a little on the top of the battery and stops it moving around.

Next step.... SOTA!

Raspberry Pi Pico RF transmit

Some great work by Jon Dawson on his 101 Things project "A collection of cool projects to make!". Jon clearly describes how he has built:

• SDR Receiver
• SDR Transmitter, including SSB, AM & FM
• A Class E (and D) power amplifier
• Multi-effect Guitar unit

Source code is shared on his Github repository. 

He has a video about the transmitter:

I set up the native C++ build tools for the Raspberry Pi PICO on an Intel Linux machine. I got the FM transmitter working, well it builds, and creates a carrier on 88MHz but the audio I'm streaming it it isn't modulating it for some reason.

There's no comments about how to run this but I deduce that you use a python program to stream a sample wav file over serial to the PICO which should then generate 88Mhz and FM modulate it.

python3 transmit.py Audio_Sample_-_The_Quick_Brown_Fox_Jumps_Over_The_Lazy_Dog.wav 

Available Ports

0 /dev/ttyS0: ttyS0 [PNP0501]

1 /dev/ttyACM0: Pico - Board CDC [USB VID:PID=2E8A:000A SER=E669BCF8E72E682F LOCATION=1-2:1.0]

Select COM port >

1

I did run into a build issue on Intel Linux so I raised a bug report. Jon looked at it overnight and did some updates.

The pico_setup.sh script runs on my machine right up to the point where it tries to install Visual Studio Code. The different samples all now compile. Jon seems to have removed the 19_fm_transmitter example which I was trying to get going.

I have some FST3253 on the way so I can try the transmitter sample.

This work has really driven home to me what a fantastic chip the RP2040 is.

Update

I wrote to Jon about the issues I found and he has updated the code and things are building and working correctly now. If you got the source code early on, do a git pull now and try again.

Here's my version of the board:

And here's some very rough looking AM:

"The quick brown fox..." Here's some sideband:

Tried a raspberry pi pico WSPR transmitter with no external oscillator

Someone mentioned that it's possible to generate RF with a Raspberry Pi PICO with no external oscillator. There is a library by Roman Piksaykin, called pico hf oscillator and it's used in a simple WSPR transmitter available here.

To build this project you need the native C/C++ toolchain. There are good instructions around for getting this going on a Raspberry Pi, but I did run in to a few issues getting it going on my Apple Silicon Mac. In the end it compiles.

The code dedicates one of the two CPU cores to generating the RF signal and it can go up to 33Mhz.

The waveform out of GPIO pin 6 looks like it has a bit of jitter on it.

It sounds good on the receiver and was easily decoded locally. 

The BNC to the CRO was unplugged and connected to a 40m dipole and stations heard me.

It looks like it drifts slightly during a transmission but not too much. Thanks to Roman Piksaykin for his good work on this. 

The Pico is a remarkable platform and I've started looking at this amazing work to produce other signals including sideband from 101 Things.

World Economic Forum lists AI as the second top risk

This week I discussed the World Economic Forum's risk report which lists AI as second only to climate change.

You can listen here if you wish.

I'll be back again next Tuesday evening for more tech talk.

AI noise reduction on sideband radio

David, VK3KR, drew this to my attention. There is amazing work being done to remove radio noise from sideband signals. Check out the demonstration on the RM-Noise Video page. 

RM-Noise only has a Windows client and does the work in the cloud. I look forward to the day when this is built in to HF radios.

Powering a QRP radio for portable operations with a Powertech battery inverter

I was looking around for a nice way to power a small radio when operating in the field. I have various batteries but nothing in a convenient box. A solution was right under my nose! I have a little (155Whr) Powertech rechargeable battery. It has USB outputs and a 240V "modified" sine wave inverter. This will run a Dometic fridge in the car during a trip to the supermarket to get frozen food.

It also has three 2.1mm coaxial sockets marked "DC Output 12V" and I wondered if these were fed by a buck converter and might be noisy. It turns out there is no noise on these DC outputs.

The box has these specifications:

I thought I'd have a peek inside partly to see if the markings on the battery match the claimed capacity. It's fairly straightforward to open except I think the bottom screw might have been glued in. Obviously a unit that generates 240V is potentially dangerous.

The unit is a quality construction from what I can see. The battery pack is made up of 18650 cells.

Markings on the battery pack match the claimed capacity. The voltage is marked as 11.1V but when fully charged I see 12.4V so it's likely that a radio powered by this would need to be able to work well below 12V.

This unit is an old model and the new version has a USB-C output, although I wonder if it's USB-C PD?

Full wave loop for 80m

Fortunate to have space for this, I have put up a horizontal full wave loop for 80m. I cut 82m of wire, put up the loop and trimmed it down until it's resonant in the band. The resonance is quite sharp though.

I initially used a 1:1 balun to feed the loop and it matches quite well.

Now I can switch between the full size dipole for 80 and the full size loop for 80m.

Noise, and signals are lower on the loop. The real test is measuring signal to noise ratio and I've been switching back and forth while receiving WSPR to get an idea on this.

So far I can't see that SNR is better on one or the other. HF conditions vary all the time so perhaps I need to set up two receivers to really compare.

The loop is quite low to the ground, perhaps 2m high for much of it which might explain why it's not superior to the dipole which is quite a bit higher up.

Thanks to Frank, K4FMH, for bringing his excellent presentation on optimising horizontal HF loops to my attention:

Clearly my height above ground is too low for 80m use. The shape of my loop is far from square, more like 5 sides attached to trees.

Here's an SWR plot using nanovna-saver with the 1:1 balun I used at first:

After watching Frank's excellent talk, I changed to a 4:1 balun and things are improved on the higher bands:

The loop works on several bands very well. (Today I learned that NanoVNASaver-saver has an option to show the amateur bands on the SWR plot).

I joined the weekly MRARC 80m net tonight and switched back and forth between my high dipole and the loop. Several stations reported that the signal strength from the loop was 10dB down on the dipole. I think the loop is too low.

Antenna performance makes more difference than anything else

Last night I joined the weekly Macedon Ranges Amateur Radio Club 80m net on 3685 for a chat with the group. I called in as usual and was acknowledged. Net controller Joe commented that my signal was a bit down but I was fully readable. 

After my first over I had a look and noticed that my transmitter was transmitting just 2% of the normal 100W as I'd been using it for WSPR earlier in the day! Despite this, everyone could hear me, including Peter, VK3RV, who was operating from South Australia. 

Note also, that there was a very high level of noise from remote lightning happening in NSW.

I'm fortunate to have the nice dead tree you see here as the central support for my dipoles. The 80m dipole was raised with the generous assistance of Dallas, VK3EB, shortly after I moved to this location. The ends of the dipole are quite slack and have survived quite high winds over two years. (One end did come down on one occasion). 

The 80m dipole slowly moves lower in resonance due to wire stretching so I pull it down periodically and fold back the ends a bit to move it up again.

I've used trap dipoles, G5RVs, off centre fed dipoles, and loops in the past with good results but I find plain old single band half wave dipoles work the best for me. I now have separate dipoles up for 80, 40, 20, and 10 meters.

When not in the shack I leave WSPR running so I can take a look at band conditions. At the moment I'm running a QDX from QRP-Labs. This runs under 4W but is heard around the globe. I've noted in the past that transmissions from the QDX are especially pure as they are not mixed audio and seem to be very well decoded.

Here's a transmission on 20m just now with 25 decodes in one over. Amazing for a transceiver that fits in a shirt pocket.

Later I was spotted by 72 stations in one transmission:

A simple but decent home brew dipole makes more difference than any fancy gear or additional power.

Talking Tech on ABC Radio

I joined Lisa Pellegrino last night to wrap up the past year in technology and look at what I expect for the new year.

Topics included:

• Twitter X train wreck
• Wearable health sensors
• Crypto's bad year
• Home automation still a mess
• USB-C won

It's amazing to me how much the radio audience interacts with the Nightlife program.

My audio this week was via a Sennheiser USB mic. We used Zoom and I set the audio to a higher quality by choosing the "Original sound for musicians".