About a year ago I set upon a new adventure, to determine if the dummy load I own, traditionally seen as a device that doesn't emit any RF, to determine if it was in fact imbued with the ability to absorb RF, rather than share it around like an antenna might.
At this point you might be shaking your head, but this wasn't embarked upon lightly. I had spent several years experimenting with lower and lower power levels and came to learn that even 10 mW can make it to the other side of the planet, 13,945 km away.
In other words, extremely low power can, under the right conditions, make it across the globe.
So, with that in mind I connected a dummy load to my WSPR beacon and configured it to transmit across each band between 80m and 10m and then set-up an RTL-SDR dongle to monitor those same bands as well as the remainder of the amateur bands within the range of the dongle.
I did this because I wanted to make sure that if there was any chance of a harmonic appearing unexpectedly, there'd be a good chance I'd see it. Then for a year I did nothing. Well, not exactly, but we'll get to that.
After downloading 16 months of WSPR reports I extracted all the records that had my beacon in it and separated them into two groups. Those that had VK6FLAB as the reporting station and those that didn't.
Both contained a significant number of signal reports.
After doing some analysis, charting the efforts and discovering other phenomena, I can report back a little of what I learned.
The dummy load does radiate, but I only saw reports from the dongle which is in the same room. I have already talked about this and at the moment the working theory is that the coax between the beacon and the dummy load radiates, or the beacon itself does, or something else, because the RTL-SDR dongle is clearly and accurately decoding WSPR signals. I'm not sure how I'd go about verifying where the radiation is coming from exactly or if I'd need equipment that I don't have or know about.
That said, somewhat to my surprise, the signal reports were not static, varying considerably by as much as 40 dB. Notice that we're talking about a signal that is being, at least ostensibly, transmitted into a dummy load and being received on an external antenna.
The signal reports don't appear particularly affected by time of day or band, that said, there's a visible range of reports across the year, but it's hard to observe anything definitive since most of September and October is missing. At this point I cannot tell you why that's the case. More on this in a moment.
I did notice that some odd things happened to reports throughout the year. For example, on 40m, the reports were significantly poorer in the first half to 2025. 17m and 15m on the other hand were worse in the second half of the year.
On the 25th of April here in Australia we're permitted to change our callsign to use AX as a prefix, so my VK6FLAB callsign would be AX6FLAB. On that day I configured my beacon to use 40m and connected it to the antenna for the first time in a year.
I had several reports across the Great Australian Bight over 2,000 km away. At midnight UTC the next day I changed the callsign back to its normal VK prefix and changed the bands to use 80m, 40m, 15m and 10m, and left it connected to the antenna.
While the antenna is a mono-band antenna, I wanted to know if it would still radiate on bands it wasn't intended for.
Turns out that yes, it does, very nicely in fact. Since activating it like this, I've been heard by 79 different stations, as far away as 15,649 km in the United States, and in Antarctica as well as across Europe and several reports in Africa, the bulk in the Asia Pacific.
Something curious though.
I noticed that there were no reports on 80m and only a few on 40m. Pretty evenly split are reports on 15m and 10m.
This was odd to me, since I would have told you that the antenna is a 40m mono-band antenna. You might recall, built by Walter VK6BCP (SK), I was given several antennas that are essentially helically wound verticals. I've used these ever since.
I was adamant that I'd changed my antenna to 40m.
After scratching my head for a bit, I decided that measuring might be a good idea. I discovered that the SWR, the Standing Wave Ratio, a traditional measurement of antenna effectiveness, on 80m is infinite, it's high on 40m, over 3 to 1 on 15m, just on 3 to 1 on 10m and similar on 2m. In other words, this is probably a 15m antenna.
Now here's some things to observe. Based on reports across Australia, this antenna still radiates my 200 mW beacon on 40m, even though the SWR is 17.5 to 1, yes, you heard that right.
This by way of noticing that the SWR has a relationship with how your antenna functions, but you might know, a dummy load has a perfect SWR of 1:1, in other words, the SWR is one way of characterising your antenna, but clearly it's not the final word on the ability of your antenna to get on-air and make noise. I've said it before, any antenna is better than no antenna.
This left me with a growing sense of unease. Several of my measurements are missing, I wasn't using the antenna I thought I was, I don't recall exactly when I switched over from my 10 mW antenna experiment to my 200 mW dummy load, and no doubt there's other things that are lost, like local thunderstorms that encouraged me to disconnect my external receive antenna, or when I rebooted a frozen computer that wasn't decoding anything, or when the internet was down, not reporting to WSPRnet.org.
Some of these things I can reconstruct. For example, I can roughly see in the logs when my beacon was reconfigured to 200 mW, but was it still connected to an antenna, or did I immediately connect the dummy load?
I came to realise, probably again, that I should keep better notes beyond those that make it into my weekly efforts here.
So, I built a "lab-notes" tool that will help with that. It's on my VK6FLAB GitHub page and consists of a single bash script that takes care of business. The notes are also stored on GitHub, so you can follow along. Feedback welcome.
Admittedly, this still requires that I take notes, but at least one friction point has been removed, namely a place to make such notes. Feel free to make your own.
In the meantime I'm going to figure out if I have an 80m antenna lying around and if I can use it with my beacon across multiple amateur bands.
I'll make a note when I do.
I'm Onno VK6FLAB