VLF/ULF/ELF Project

Preamp Schematic.

Amplifier Test.

A test was conducted by inserting the amplifier between a 30-turn loop antenna and an SDR-IQ. The antenna was inside, adjacent to the laptop computer. The results Without Amplifier and With Amplifier.

Moving the loop outside, 80 foot from the house, resulted in With Amplifier. Much of the noise has been eliminated.

The system is only grounded via the laptop AC cable, a poor ground at best. Attaching a ground rod to the coax shield at the antenna resulted in lots of hash and monitor noise. Ground loop currents?

For the tests the amplifier was powered with two 9.6V NIMH 2200mAh batteries. The AD620 amplifier current loads are +Vs 0.92ma and -Vs 0.97ma, well within the range of reasonable battery operation.

The amplifier was connected to an SDR-IQ with RG-58 coax.

Screen at Rialto Beach WA 04052009 2252Z

I took the 21 turn loop to Rialto Beach, WA (47 55N 124 38W) and had much better results as compared to in town. The display has some horisonal lines that sounded like lightning at 22:52:27Z.

Took the loop to Tahuya State Forrest today. Seems to have little 60Hz. Captured RDL on 21.1KHz.

PROPOSED PROJECT:

Description

The project will use two, orthogonal loop antennas on a 12 inch x 12 inch x 36 inch PVC pipe frame a pre-amplifier/line driver, CAT-5e cable and a line receiver connected to an SDR14-IQ receiver and/or the audio card of a notebook PC.

Loop Antennas

The idea for the frame comes from various web pages.
The number of turns in the loops has not been determined.

Pre-Amplifier, Line Driver, and Line Receiver and Cable.

Placing the loops some 1000 feet from the receiver and computers may eliminate the feed-thru of the monitor horizonal signals and other laptop emissions. To achieve this distance, CAT-5e cable is used. The specification for the Belden cable shows the attenuation to be on the order of 2db for 100 meters. CAT-5e cable is relatively inexpensive for 1000 foot spools as compared to coax. To use the CAT-5e cable, an isolation and differential driver amplifier is required. To avoid intermodulation signals low gain is used in the preamplifier.

The Preamplifier Schematic shows the connection between the loop and the receiver.

OP275 opamps were chosen for the first design of the preamplifier/driver and line receiver. The OP275 contains two opamps in a single 8-pin package. The specification of the opamp is found on the Analog.com web pages. This opamp has unity gain at 9MHz and is unity gain stable. The opamp has relatively low noise and a high input impedance.

Spice simulation of the schematic components shows a relatively flat response from DC to some 200KHz. The spice model for the op275 was obtained from Analog.com.

SDR14-IQ

The SDR14-IQ will be used along with Spectravue and other software to capture signals. Ultimate data collection will be performed on a Linux system.

Yeasu VR-5000 Receiver

IQ Detector and ADC with Atmel ATmega128 Processor

Audio Card

Tektronix TDS2014 Oscilloscope

The Oscilloscope will be used for time based measurements of signal phases as related to the GPS 1PPS signal.

The TDS 2014 also has a FFT function.

Garmin GPS-18

Initially, the Garmin GPS-18 will be used for time stamps as well as for the GPS 1PPS signal for phase measurements. The rise time of this signal is poor as compared to other sources. However, it is easy to use and will be use for initial setup and preliminary measurements.

Synergy Systems GPS Time Base

The Synergy GPS Time Base provides a much more stable source for the GPS 1PPS signal.