S2S contacts using 2200m band (136 KHz)

On 2023 09 25 Ken, VE6AGR and Rich, VE6LCE climbed to the summit of Lusk N2 (VE6/JF-085) and Ian, VE6IXD ascended Stony Peak (VE6/JF-084). The two summits are only 1.4 Km apart and it was our intention to make a S2S contact using 136 KHz, the 2200m band. Yes, indeed the two summits are less than one wavelength apart at this frequency!
The equipment we were using had been constructed by VE6IXD as part of a project to update cave-to-surface communications. Contacts through the rock require low frequencies to reduce the losses in the conductive earth. However the range of the contacts is rarely more than a kilometer and so it is possible to use a magnetic loop antenna and to operate in the near field of the antenna.
The radio transceivers we used were modified QDX-M radios that were purchased in kit form from QRP Labs. QDX-M monoband 5W Digi transceiver At this time, (mid 2023) the mono-band kit radio was available only for 3 bands, 10, 11 and 12m. Fortunately the frequency sensitive components were all through-the-hole components, and no changes were needed to the surface mount circuitry or to the device firmware. After much winding of wire on various toroids, QDX-M devices working in the 2200m band were successfully implemented.

Image 1 QDX-M modified for operation at 2200m.

The antenna design followed conventional cave radio practise. A wire loop made of 2 turns of extension power cable (a total of 6 turns of 1.3mm dia wire (16AWG)) was supported on a rigid, but collapsible frame 1 m square. This loop was then tuned with 630V polystyrene caps to 137.5 KHz and a 50 Ohm connection to the radio was matched into a 1-turn tap with a pot-core transformer. The antenna can be taken apart and carried in a kitbag.

Image 2 Rich VE6LCE operating the station on the summit of Lusk N2, with Stony Peak in the background.

Image 3 Ian VE6IXD operating the station on the summit of Stony Peak

The QDX-M is a digital only device, and the software of choice for our S2S trip was WSJT-X running FT8. Previous tests using WSPR in the suburbs of Calgary, then later on a quiet rural road, suggested the potential range of our system would be close to 4 Km. Our S2S attempts using FT8 were successful!

Image 4, FT8 contacts on 0.136 MHz between hill tops 1.4 Km apart.

Future work is planned to enable keyboard-to-keyboard text communication. We will be testing JS8Call using laptops, and eventually Olivia using AndFLmsg using Android smartphones as the terminal devices.


Great work!


Very nice. I love VLF because it is the technology used by the founders of our wonderful hobby. And we stand on the shoulders of these pioneers.

Unfortunately I no longer have a qso partner for my portable 472kHz station here in the north of DL.

73 Chris


As a follow-up to this item, a few readers may be interested to know that this same 2200m band setup was recently tested to see if it could support 2-way chat using JS8, in a surface-to-cave context. Results were very successful, with SNR’s as high as +09 DB being recorded through more than 100m of solid rock. FB, Ian!

Cave radios have existed for decades and have penetrated much deeper than this initial test, but the small size, light weight and low cost of the QDX-M makes it a potential breakthrough in utility for cave exploration and cave rescue. More tests to follow.

This was not a SOTA accomplishment per se, although there has been the occasional thread on this reflector suggesting there may be room for a Caves On The Air program :grin:… Interesting possibility: the test cave (called Rat’s Nest) is located in Grotto Mountain, which happens to be SOTA summit VE6/HC-015.. In theory it might be possible to have a 2200m band QSO between the summit of HC-015 and the lowest point in the cave, which is about 165m below the entrance. Pretty stiff trip in both directions, though, for a 6 - point activation and chase …


Very interesting. Can you give me more details about the transformer? Which core material and which turns ratio?

73 Chris

1 Like

Hello Chris,
I used an Amidon pot-core, material 77.

The primary is 10 turns and the secondary 35 turns.

I have several of these cores left over from previous projects and have not purchased from Amidon recently as I found them to be expensive.
Perhaps you can find another supplier.


Thank you for the exact information. Then I already have a clue. I haven’t yet thought of using a loop not only for reception but also for transmission in the medium and long wave range.
At the next opportunity I will test this on 472kHz.The pot-core are hard to come by. I’ll try with common toroids.
73 Chris

1 Like

For enthusiasts of S2S (Subterranean to Summit) contacts, I commend Mt Arthur (ZL3/WC241), where you can make an S2S from Mt Arthur (1800m) to Nettlebed (-1200m) . For those who have bothered to learn morse, and properly waterproof their gear, you can go another 250m down…

If you something less squirmy, Mt Evans (ZL3/TM151), and Harward’s Hole would offer easier access…


Hello Simon, I haven’t personally been to NZ, but a couple of caver friends have been in Nettlebed cave, and had a more exciting time than they planned, due to large quantities of water! They did make it out under their own power, and I am glad that a cave radio was not needed to help rescue them.
Take care,

1 Like


Caves On The Air with an c2c contact (cave to cave) the ultimate goal…




Many lives were saved from life rafts during WW2 with a hand-cranked Gibson-Girl QRP and kite / Hydrogen balloon-lifted antenna on 500 KHz plus 8364 KHz… The ocean was, of course, one helluva ground plane.
An airplane, e.g., a PBY, could use its LF ADF to fly directly to the raft, and pick up survivors. Originally developed by Germany, the life raft transmitter was quickly developed for U.S. aircraft and produced in large quantities.


A much later model life raft radio came with a 55 cubic foot fish-shaped balloon in survival orange. I got one surplus for 100 USD from the maker in Paris, Texas. The 220 cu ft bottle of helium I got elsewhere.

Elliott, K6EL


Ahhhh, yes! Remember El’s antenna support: “When the sun goes down, the balloon goes up!”