The loop antenna for a VLF receiver :Some informations

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.

Hexagonal frame for the loop antenna receiver

Below are instructions for making a hexagonal frame for the loop antenna receiver. If you follow these directions you will have a nice looking single layer loop that will have no zigzags but a less carefully made frame will probably work just as well provided you maintain the dimensions fairly close.


The drawing above shows how the hexagonal frame is shaped slice a paddle wheel with the six’/4 inch plywood paddles mounted all facing in the same direction on the ends of three diagonals. The diagonals are 1″ X 2″ nominal, actual size 5/8″ X.1 3/8″, wood . Each diagonal is cut to be exactly 57 inches long. A hole is drilled in the centers of the three diagonals and they are fastened together with a bolt. We used 5/16-18 threaded brass rod and brass nuts and washers to prevent rust. The plywood paddles are 6-inches long and the width of 24 turns of whatever size wire. The paddle drawings show how the three countersunk holes for the mounting screws are centered on two paddles, offset 5/8″ to the right on two paddles and offset 5/8″ to the left on the other two. These offsets produce a hexagonal frame that will lay flat and the winding will not zigzag. We used #10 brass flat head wood screws 3/4″ long to mount the paddles on the diagonals. The paddies should extend exactly one inch beyond the ends of the 57 inch long diagonals so the distance between the ends is exactly 59 inches which is 1.5 meters.

When the diagonals are set with 30 degree angles between than the distance from each paddle to the next will be 0.75 meters and the length of a turn will be 6 X 0.75 = 4.5 meters. 24 turns will then require 4.5 X 24 = 108 meters or 354 feet of wire. This is a little more than the 300 feet usually recommended but it should work OK. Before we start to wind the wire on the

frame we clamp the diagonals so they won’t move. The center bolt cannot be made tight enough to do this. The wood for the diagonals comes in 8foot lengths so you will have some left over. We cut two pieces about 2-feet long to clamp the diagonals while winding the wire on the frame. Clamp these in place with C-clamps. We glue the finished winding to each paddle with 5-minute epoxy glue before removing the clamps. Now the diagonals can’t slip and the wire won’t slide off the edges of the paddles. The ends of the winding should pass through small holes both in the same paddle and extend out a few inches.

It was tuned by 1.5 meter loop with 24-turns of # 14 wires with a precision decade capacitor bank made by Cornell-Dubilier to determine the actual capacitances needed. Here are accurate values for the capacitors needed to tune to some popular VLF stations.

60 kHz, WWVB Fort Coffins, Colorado, USA. ……0.002 mfd
25.2 kHz La Mourie, North Dakota, USA …0.0175 mfd
NAA 24 kHz Cutler, Maine, USA ……0.0185 mfd
37.5 kHz, NRK Grindavik Iceland ……0.008 mfd
24.8 kHz, Tim Creek, WA, USA …….. 0.0178 mfd
21.4 kHz, NPM Hawaii, USA……….. 0.023 mfd

We can make a tuner to find these stations from two Radio Shack 8-position DIP switches. These consist of eight little single pole, single-throw switches side by side that mount on a Printed Circuit Board. Radio Shack only carries the capacitors we need for this tuner in ceramic dielectric so you should use their ceramic capacitors. Below are the capacitances for the sixteen capacitors we will need. Mount each switch on a little circuit board and connect the capacitor to each switch so when all 8 switches are in the an position all eight capacitors are connected in parallel.

*      Switch Number One:
Position # 1…..100 pfd
            2…..100 pfd
            3…..100 pfd
            4…..100 pfd
            5…..100 pfd
            6…..470 pfd
            7…..470 pfd

*      Switch Number Two:
Position # 1…0.001 mfd
            2…0.001 mfd
            3…0.001 mfd
            4…0.0047 mfd
            5…0.0047 mfd
            6….0.01 mfd
            7….0.01 mfd
            8….0.01 mfd

These two tuners should make it possible to find the station without an oscilloscope and signal generator. We connect them temporarily with Alligator clip leads across the Loop. Then tune up and down in 100 pfd increments until we peak on a strong signal. We used a multimeter or our recorder to measure signal strength. We recorded the strong signal found for a few days to make sure it shows sunrise and sunset patterns. If it shows these patterns we have successfully tuned our receiver to a suitable signal and it should record solar flares as SESs. We unsolder the selected capacitors from the tuners and solder them across the ends of the loop. The ceramic capacitors are rated for only 20% accuracy so we connect the values in the table above to our loop. We connected 20% capacitors adding up to 0.185 mfd across the loop.



Design of VLF Receiver to Detect Solar Flares and Gamma Ray Bursts

Here is an updated version of the simple VLF receiver first described in the April SID Supplement of the Solar Bulletin. The design is based on the principle known to all amateur radio operators that the most important part of a transmitting or receiving system is a good antenna and a well matched transmission line.

We met the first requirements by building a hexagonal loop antenna that measures around 1meter (59 inches) across the diagonals and winding it with 24 turns of # 14 stranded copper wire & the second requirement by eliminating the transmission line altogether. The receiver is built right on the loop antenna so there is no need for a transmission line between the antenna and the receiver. After the signal is amplified 900 times it is sent over a transmission line consisting of ordinary 4-wire telephone wire to a recorder driver. There is no need to match this transmission line to the recorder driver because the signal has already been amplified 900 X. There is plenty of signal to make up for any lost on the transmission line. We call this a “Loop Antenna Receiver” because the loop is the receiver. It is the LC resonant circuit for the receiver and owes its success to being a large high-Q loop with much greater aperture than small

loop antennas usually used with Sudden Ionospheric Disturbance (SID) receivers. The low resistance of the #14 wire gives the loop a high Q, about 400 compared to about 20 for small loop antennas wound with #26 wire that are used by most SID observers. The receiver has a pass band of less than 500 Hz which compares favorably with other SES (Sudden Enhancement Signal) receivers in use today.

This loop antenna receiver is meant to be located outdoors so it can be placed as far as possible from electrical wiring which is the source of most if not all of the interference that plagues SES receivers. Below the chart is a simplified diagram of how to hook the parts of the receiver together. It is not necessary to put the receiver in a weatherproof box.


Fig. Fundamental Circuit Diagram of VLF Receiver to Detect Solar Flares and Gamma Ray Bursts



SID :A more detail

It cannot be said that SID monitoring is not with out its problems for recording solar flares.

*      The biggest problem is that not all solar flares can be recorded via SID recording.  Only solar flares that occur during the daytime for a SID station can be recorded. 

*      Furthermore SIDs can become localized at times where say one person in NJ will record a SID while an observer in Florida may not. This happens when a small area of the ionosphere has been affected, yet the signals path to the person is Florida does not travel via the disturbed area.

*      In addition to this the solar flare must be large enough to cause a SID.

*      One last difficulty for those wanting to report their data to AAVSO is that the data must be accurate within one minute per day.  Unfortunately computer clocks do wander a bit, but we have thought of many ways to deal with this problem.


SID monitoring does offer many benefits that may be of interest to amateur astronomers.

*      As mentioned before this method can be used to monitor solar flares, with the small possibility of monitoring a GRB.

*      The SID method can also be used to record radio outburst from the sun that usually foretell a solar flare with in an hour or so. In addition, if the computer can be set up to graph the data it is receiving, then an observer can become aware of a SID event the instant it takes place.

This alone harbors some very interesting possibilities. Rarely do solar flares release energy in the white light spectrum. But a solar flare does release energy that is visible in the H-

alpha spectrum. Therefore an observer viewing the Sun with a telescope and H-alpha filter (they are not cheap, but NJAA does have one.) should be able to visually observe or photograph the solar flare. Also, as brought to my attention by solar observer Maria Hansen, solar flares sometimes occur within a few hours of each. Therefore if the SID monitoring station records a solar flare during the morning, an observer has a good chance of witnessing another solar flare before the end of the day. It is important to note that we can measure our station’s success using the Internet.  Professional astronomers use satellites to keep tabs on solar flares and GRBs.  Their websites usually include such information as to when a solar flare occurred in universal time and its size.  Information such as this can help us determine if our station is working properly and what it is detecting.

Also, the data produced by the station will need to be analyzed.  In turn this data can be used to fill out monthly reports to be sent to AAVSO.



What is SID

SID stands for Sudden Ionosphere Disturbance, which are usually caused by solar flares.  Members of the American Association of Variable Star Observers (AAVSO) SID program use inexpensive radio equipment to detect and record these quick changes of the ionosphere.  Members then send in reports at the end of the month listing SIDs they have recorded.   Their reports typically contain the time a SID starts, peaks, and ends. The ability to detect solar flares with radio immediately caught our attention. The theory behind the program is not difficult to understand. But constructing the equipment for the SID monitoring station was a little over our head.  So it appeared best to drop the idea. Then it became apparent that a SID monitoring station could be constructed with the help of NJAA research members.  Also the station could be located at the observatory and be used by members wishing to get a taste of amateur research.

The ionosphere is an area high above the earth containing a high concentration of ions. It is important to understand the relationship between radio waves and the ionosphere to realize how these disturbances can be observed. Radio waves with long wavelengths can use the ionosphere as a type of mirror.

High Frequency (HF) radio waves and Very Long Frequency (VLF) radio waves typically bounce off the ionosphere and the earth. Hence these radio signals can literally travel great distances by bouncing from the earth to the ionosphere many times. In brief, a VLF or HF signal transmitted in Europe can be received without difficulty in America. (Note* Typical FM radio signals do not use the ionosphere in such a way, hence we do not hear FM radio stations from very distant transmitters). Many countries, including the U.S., use VLF signals to communicate with their submarines. This is due to the VLF signals excellent ability to travel the earth. Our receiver will most likely use NAA, a US Naval VLF radio station in Maine. NAA is located at 24.0 kHz and puts out a continuous signal.




Precession Half Wave Rectifier

One of the non-linear behaviors that are sometimes required in analog circuits is rectification. Rectification is a process of separating the positive and negative portions of a waveform from each other and selecting from them what part of the signal to retain. In the case of half-wave rectification, we can choose to keep one polarity while discarding the other.


The circuit above accepts an incoming waveform and as usual with op amps, inverts it. However, only the positive-going portions of the output waveform, which correspond to the negative-going portions of the input signal, actually reach the output. The direct feedback diode shunts any negative-going output back to the “-” input directly, preventing it from being reproduced. The slight voltage drop across the diode itself is blocked from the output by the second diode.

The second diode allows positive-going output voltage to reach the output. Furthermore, since the output voltage is taken from beyond the output diode itself, the op amp will necessarily compensate for any non-linear characteristics of the diode itself. As a result, the output voltage is a true and accurate (but inverted) reproduction of the negative portions of the input signal. Thus, this circuit operates as a precision half-wave rectifier. If Rf is equal to Rin as is the usual case, the output voltage will have the same amplitude as the input voltage.


Peak Detector

With little modifications basic precision rectifier can be used also for detecting peak levels of signal. In the following circuit a capacitor keeps the peak voltage level of signal and switch can be used for resetting detected level.



Precision rectifier:Part-1

The  precision rectifier, which is also known as a super diode, is a configuration obtained with an operational amplifier in order to have a circuit behaving like an ideal diode or rectifier. It can be useful for high-precision signal processing

 Basic Circuit



 This circuit has the benefit that the op-amp never goes in saturation, so the only thing affecting its frequency response is the amplification and the gain-bandwidth product.

Similar circuitry can be used to create a precision full-wave rectifier circuit.

 The basic circuit implementing such a feature is shown on the right, where RL can be any load. When the input voltage is negative, there is a negative voltage on the diode, too, so it works like an open circuit, there is no current in the load and the output voltage is zero.


When the input is positive, it is amplified by the operational amplifier and it turns the diode on. There is current in the load and, because of the feedback, the output voltage is equal to the input.


In fact the threshold of the super diode is not actually zero, as it should be for an ideal one, but it equals the threshold of the normal diode divided by the gain of the operational amplifier, that is almost zero.


 This basic configuration has a problem so it is not commonly used: when the input becomes (even slightly) negative, the output of the operational amplifier can easily become greater than the voltage supplied to the op-amp, thus causing saturation. Then, if the input becomes positive again, the op-amp has to get out of the saturation to amplify again. This change takes some time, and this greatly reduces the frequency response of the circuit. 

    Improved Circuit


In this case, when the input is greater than zero, D1 is OFF and D2 is ON, so the output is zero, because one side R2 is connected to the virtual ground, and there is no current through it. When the input is less than zero, D1 is ON and D2 is OFF, and the output is like the input with an amplification of ? R2 / R1. Its transfer characteristic is the following:


This circuit has the benefit that the op-amp never goes in saturation, so the only thing affecting its frequency response is the amplification and the gain-bandwidth product.

Similar circuitry can be used to create a precision full-wave rectifier circuit.