VLF RECEIVING ANTENNA

Building a basic antenna for VLF/ELF is different from standard radio because you aren't trying to capture a specific frequency, but rather a wide range of "audio-like" electromagnetic waves. Since these waves have wavelengths of thousands of kilometers, you can't build a "full-size" antenna. Instead, you build a miniature probe that captures the electric field (E-Field) or the magnetic field (H-Field).
Here are the two most common DIY methods:



The "Long Wire" (E-Field) Antenna


This is the easiest to set up if you are in a quiet, rural area.


The Build

Take a piece of insulated copper wire (10 to 30 meters long). String it as high as possible between two trees using plastic insulators (like PVC pipe pieces) so the wire doesn't touch the branches.


The Connection

Connect one end of the wire to the center pin of an audio jack (3.5mm). Connect the ground of the jack to a metal rod driven into the earth (ground stake).


Safety

Never use this during a thunderstorm. The wire will act as a lightning rod.



An example of long wire antenna



The "Loop" (H-Field) Antenna


This is better if you have some local interference (like power line hum) because it is directional.


The Build

Create a large wooden frame (cross-shaped or square, about 1 meter wide). Wind 50 to 100 turns of thin magnet wire around the perimeter.


The Connection

The two ends of the wire go directly to the "Tip" and "Sleeve" of your audio input.


Pro-Tip

You can point the face of the loop towards the signal source and rotate it to "null out" (cancel) noise from nearby power lines.



Simple diagram of a loop antenna



The Secret Ingredient: The Pre-Amp

The signals captured by these antennas are extremely weak. You cannot just plug them into a speaker.


Direct to PC

You can plug the antenna into the Microphone Input of a laptop (using a battery-powered laptop helps reduce 50/60Hz noise from the wall outlet).


Dedicated Pre-Amp

Most enthusiasts use a simple JFET-based or operational amplifier based preamplifier to boost the signal before it reaches the computer.



General diagram of an operational pre-amplifier



Important: The Location


If you try this in your living room, all you will hear is a loud "BUZZZZ" from your house wiring. You must go to a park, a field, or a forest, at least 100-200 meters away from any power lines.



Stay away from high-voltage lines



(((( 📡 IZ0GIF 📡 ))))