The loop antenna for a VLF receiver does not have to be free and clear. VLF radio waves are subject to a principal of optics known as Brewster scattering which allows them to penetrate a small fraction of a wavelength into a conducting medium. All electromagnetic waves obey this principle. This is why the Navy uses VLF radio frequencies to communicate with submerged submarines. Their long wavelength scatters into salt water deep enough to be picked up by an underwater antenna trailed just below the surface. They also have no difficulty reaching your loop antenna hidden among trees and shrubbery and sitting right on the ground. The plane of the loop is the direction of maximum signal so orient the loop so the signal you want to record is in the plane of the loop. There is a sharp null in the direction perpendicular to the plane of the loop. The maximum is much broader than the null so an unwanted signal on a nearby frequency can nulled out while favoring the wanted signal.
The diagram (Fundamental Circuit Diagram of VLF Receiver to Detect Solar Flares and Gamma Ray Bursts…Page: 15) shows resistors that amplify 900 X. Here we change R-4 to 22K to give amplification of ~150X. If you use too much amplification it will saturate the amplifier and draw a straight line that cannot show sunrise and sunset patterns and record SIDs. The TL082 is a dual opamp. Each opamp is a separate amplifier whose amplification is equal to the ratio of the resistors connected to its inverting input (pins 2 and 6). When they are 100K and 3.3K each stage amplifies 30 X for a total of 900 X. If you change R-4 to 22K the total is 30 X ~5 = 150. If R4 is 10K it is 30 X 10 = 300. 33K will give 90 X. You can also draw a straight line if the amplifier oscillates and saturates the amplifier. Avoid feedback oscillation by suspending the amplifier board half way between the periphery of the loop and the center of the frame. Suspend it on the input wires and the 4-wire telephone cable which should come out perpendicular from the center of the loop for a distance of about two meters. Do not let the amplifier touch the wooden frame. If the output cable passes close to the loop the amplified signal it carries can couple back into the loop and cause feedback oscillation that draws a straight line that also will not show sunrise and sunset patterns nor record SIDs. A signal generator can also saturate the amplifier if it is coupled too tightly to the receiver. Always avoid straight lines as you tune your receiver to the station you are looking for.
If all else fails and you can’t get your receiver tuned to the signal you want you may do better with a signal generator you can build from about $12 worth of Radio Shack parts. This is a Wein-bridge oscillator that produces a true sine wave with good stability and is easy to control. It will need to be set to the frequency you want to tune to with a frequency counter. If you do not have a frequency counter or a friend who can set it, you can send it to us and we’ll be glad to set it for you. It is small and very light and can be mailed for 37 cents or anywhere in the world by airmail for 80 cents. There is one correction that needs to be made to the circuit diagram (shown in page…15). A 10 mfd electrolytic capacitor should be connected across the 5k variable resistor that controls the output level to the recorder.