Re: LF: VLF Small magnetic antenna for Tx

[John Rabson <john.rabson07@gmail.com>]

Luis,

I think this system would be of interest to other caving groups, so I am copying your posting to the editors of the Cave Radio & Electronics Journal in the UK.

73 John F5VLF

On 1 Feb 2018, at 19:58, VIGILANT Luis Fernández <luis@vigilant.es> wrote:

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