The tree aerial has been around for a century. Does it work? Perhaps, but mainly so in the jungle.
Hams like us are used to hanging wire antennas from trees. But can we actually turn a tree into an antenna? Apparently, we can.
One hundred years ago, George Squier published his experiences with a tree aerial. You can read these for yourself, here (page 204) and here. Major General Squier was a pretty amazing soldier, scientist and inventor.
He began his experiments in 1904. You will find his methods were simple and experimental. George simply hammered a nail into a tree and connected this with a wire to his radio gear. Amazingly, his system was capable to sending and receiving low frequency radio signals, often over great distances.
The American military conducted extensive tests with tree antennas in Vietnam and elsewhere. Around this time, a more sophistical method was applied to coupling RF to the tree. Rather than nails, engineers wrapped large magnetic induction coils around the tree, and this worked fine across medium and high frequencies. This device is called a hybrid electromagnetic antenna coupler, or HEMAC.
Tree Aerial – How It Works
I believe that fundamentally, a tree aerial is a liquid antenna, similar to the saltwater antenna. But unlike the liquid monopole, I see a tree aerial as basically a shunt-fed grounded radiator.
Living trees have a small amount of conductivity arising from water and sap content. You will find that a living tree transports water and inorganic ions from the ground through to its branches and leaves. Exciting conduction these ions leads to electromagnetic radiation.
If you want a deep dive into the history of the tree aerial, check out Arboreal Aerials – Or Using Trees as Antennas, by G0GSF. If you are really interested in building a helical coil to couple your radio gear to a live tree, read the HEMAC Patent.