Hi VLF
I’m in contact with a local group of espeleo enthusiast, big enought to count with some electronic tecnicians with them
They have been developing their own equipment at VLF. A transmitter which is carried down the cavity and turned on Tx
Directional receivers using ferrite antennas are used at surface to determine the angles to the transmitter, so to accurately
track a number of sites in the cavity and get a map of it
The transmitter has been even used to send CW connecting by Bluettoth with a rugerized Android phone running a CW keying App
Inverse process at surface provided decode of the CW messages (clear aural copy but not CW OMs in the group)
Not EbNaut planned so far ;-)
Talking with Juan Leandro, who is making most of the design and tests in this equipments, we had a discussion about which
would be the performance of such antenna in open field as VHF transmitter. Obviously we have not tested it yet with my Rx
but it is on schedule. In the meantime I think would be interesting to discuss this in the reflector
The antenna is made by using two ferrite cores from CRT deflection coils with 100 turns of 2.3mm copper wire, which provide
1740 uH. Then a battery of caps providing 9.9nF resonate the antenna at 38400 Hz. For the PA and rest of the circuit you can see
the schematic in this link
https://www.dropbox.com/s/6onj8atfo24v5ko/Summer%201esq.pdf?dl=0
So far the tests conducted in cavity worked very good at 70m trought the rocks. But taking it “out” for test rises some questions:
As this is the “near field” it is supposed to work only at magnetic field due to the small size of the antenna ? So only magnetic field for
underground signals at this distances ?
Can we expect to receive the signal with an electric probe antenna in the near field or the only chance is to be further away ?
And then loosing the signal due to the distance ….
Which would be the radiated power of such antenna ? (may be I would have to ask Juan Leandro for more details about currents)
73 de Luis
EA5DOM
