I am looking to transmit ELF through VLF using either just a +20dB signal generator or less than 10W amplifier over a distance of no more than 2 miles from a very small form factor. In the past, I have only worked with 3MHz - 50GHz and am new to the kHz range and below on how it can propagate over an antenna. The ELF and VLF transmitters I have found are facilities that can communicate around the globe, but I have found no one who talks about short range transmission with these bands. I have looked into electronically short waves and wave division but to transmit may not be possible at such a low power, especially over such a small profile?
We are testing a superconductor receiver that can see far below the noise floor so it's sensitivity is very high and the signal only needs to be seen, with a possibility of a signal carrying data. We can do inline all day, but need to test over the air from a distance which has yet to be determined but we are looking at a mile to two mile separation. Is this even possible in a small form factor (i.e.: handheld antenna to 30ft vertical) or will the wave dissipate very quickly over the air?
For your application, it sounds like you should look into a loop antenna. A traditional electric antenna at these frequencies would be a monopole with the Earth as a ground plane (creating a dipole image) to produce vertically polarized electric field. Ideally the wire needs to be a quarter wavelength in order to create the most efficient resonant monopole. That's huge at the frequencies you're talking about, and anything of the size you're talking about would be terribly inefficient and not want to radiate well.
By going to a loop antenna, you're creating a magnetic dipole and the impedance of the loop can be tuned to match the amplifier you're driving it with. A horizontal magnetic dipole (plane of the loop vertical, normal to the plane horizontal) will radiate a vertically polarized electric field. The difference is that it's directional in the far field, with the maximum signal in the plane of the loop. That also means you're radiating straight UP as well, since the pattern is that of a horizontal dipole. The nulls are through the loop.
Note however that in the NEAR field, the strongest (magnetic) signal is along the line through the axis of the loop, since you essentially just have an inductive coupler.
For your application, it sounds like you should look into a loop antenna. A traditional electric antenna at these frequencies would be a monopole with the Earth as a ground plane (creating a dipole image) to produce vertically polarized electric field. Ideally the wire needs to be a quarter wavelength in order to create the most efficient resonant monopole. That's huge at the frequencies you're talking about, and anything of the size you're talking about would be terribly inefficient and not want to radiate well.
By going to a loop antenna, you're creating a magnetic dipole and the impedance of the loop can be tuned to match the amplifier you're driving it with. A horizontal magnetic dipole (plane of the loop vertical, normal to the plane horizontal) will radiate a vertically polarized electric field. The difference is that it's directional in the far field, with the maximum signal in the plane of the loop. That also means you're radiating straight UP as well, since the pattern is that of a horizontal dipole. The nulls are through the loop.
Note however that in the NEAR field, the strongest (magnetic) signal is along the line through the axis of the loop, since you essentially just have an inductive coupler.
The distance between TX and RX is smaller than the wavelength of VLF (a few kilometers), so it is a near-field transmission. It has substantially different characteristics than far-field transmission. For example, there is a measurable feedback effect from the receiver to the transmitter, which is exploited in RFID: You can read on the TX whether the receiver is switched on or off and can use these properties for passive data transmission. This works even at very low transmission power. Probably magnetic antennas are more suitable than electric (shortened) dipole antennas. There is no 1/r dependence of the field strength, the E around M fields are not necessarily orthogonal.
I agree with Herbert Weidner . Loop antennas with inductive coupling would be a good solution. You could use a very large horizontal loop, and the receiver could be in the same plane. In the near field the equations for magnetic field strength are the same as the DC ones and in the plane of the coil and along the axis (if you use a small vertical coil) are simple and easy to use to predict coupling.
Malcolm White Herbert Weidner Michael Foegelle
thank you for your responses! this has more verified what we had already researched and your expertise has helped greatly! we have a loop antenna in mind we were looking at that can handle 300W but this has only been near field. Our issue is that we need to prove the system with data transmission.... which my understanding is that the processing a lone could take hours for a basic message. We have worldwide known sources... but to control the experiment, we want to control the data. The system sees close to the earth noise floor but I’m unsure how far we can transmit on a magnetic signal, or if that is even possible without fully propagating a signal using 1000+ miles of copper.
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