Mechanical Antenna - Page 1
Forum section: electronics / rf-microwave

author: Manul
author_rank: Super Contributor
timestamp: July 26, 2023, 02:44:20 pm
content: Recently I got interested in cave exploring. So this year I was crawling through a cave and my EE (expectedly) kicked in. A beacon or a communication device would certainly be an exciting thing (especialy if DIY). Not sure if I have time to actually proceed with building it, so as for now it's just a casual thought.I've noticed many recent articles about "mechanical" antennas. Mostly piezoelectric resonator based (VLF, LF). And rotating permanent magnet "antennas" (although these are realistically ELF to ULF band only).I have a feeling that these mechanical antennas will not scale well. It is maybe more compact and power efficient at small scale, but if you want 300 meters distance, it is still better to use magnetic loop or long wires and just ramp up the power.Have any of you ever experimented with piezoelectric antennas or rotating magnets (or anything else of that sort)?

author: Infraviolet
author_rank: Super Contributor
timestamp: July 29, 2023, 03:19:10 pm
content: For caving use you might do betetr with some sort of audio system? Modulate data in to an audio burst format, let it reflect its way along the passages. Even that would be tricky, different frequencies present in the fourier transform of a digital burst of data might reflect differently or be more absorbed in to the rock during reflection, and distort the data after enough reflections. A sensitive microphone at the other end could pick the sound up, and in a typical cave the audio noise floor is going to be pretty low anyway, just a bit of water dripping I'd guess.

author: szoftveres
author_rank: Regular Contributor
timestamp: July 29, 2023, 04:32:23 pm
content: This acoustic tool uses an idea similar to time domain reflectometry in VNAs:https://youtu.be/-xpVZQP4hEoAt 1:20 you can hear the tones it's playing, inverse fourier transform converts this frequency "sweep" to TDR, just like on a VNA.

author: Manul
author_rank: Super Contributor
timestamp: July 29, 2023, 04:34:22 pm
content: Well, sound propagation (unpredictability and lack of) in caves was one of the biggest surprises. In some places I was not able to shout to others at just 10's of meters distance. You stop, even hold your breath to lower noise floor, listen and nothing, nil, zero. One would think, that rock surfaces should reflect sound with low loss, but it seems not really, or at least, not always. It must be something to do with rough surfaces and narrow passages going to bigger ones and back to narrow.I was thinking RF, below 50kHz or so. These low frequencies should propagate quite reliably. Some system I've seen on the internet used 80kHz if I remember correctly. Well, it's quite an interesting project.The thing is that it is hard to radiate low frequencies efficiently, the wavelength is so huge. But I've found lot's of research from the recent years about antennas which use electric or magnetic dipoles and rotate or vibrate them mechanically. That is something new to me, hence this thread.For starters, take a look at these links:https://www.sciencedirect.com/science/article/pii/S2589004222021058#bib98https://www.nature.com/articles/s41378-022-00395-xGenerally, as I understand, it is more efficient to rotate or vibrate an electric charge, not a permament magnet. Some research done about using electret material, some just parallel capacitor plates or resonating piezoelectric material. All this seems a bit esoteric, but in principle quite simple. Seems the interest into these is rising. I wonder if someone tried something of that sort.

author: Manul
author_rank: Super Contributor
timestamp: July 29, 2023, 05:18:54 pm
content: Quote from: szoftveres on July 29, 2023, 04:32:23 pmThat's a cool device, it must be really hard to inspect pipes in any other ways. But I feel that sound in caves is unpredictable. I've been in places where I felt like being inside a muffler full of cotton wool. I don't know how else to describe it. Sounds were just dying like going into invisible dummy load.

author: coppercone2
author_rank: Super Contributor
timestamp: July 29, 2023, 05:35:55 pm
content: there is also the rotating electret for whatever that is worthI imagine a 1/4 inch pneumatic rotary tool would be a fine test transmitter for anything like this. Fixing the magnet to the shaft does not need to be a difficult endevor if you make a wooden box for it to rotate in so it contains the projectile if your bond to the high RPM magnet fails. I imagine just drilling a hole in a block of wood would be fine.