A Simple, Easy to Build VLF Receiver to Detect Solar Flares and Gamma Ray Bursts - From the October 2002 Solar Bulletin
Here is an updated version of the simple VLF receiver first described in the April SID Supplement of the Solar Bulletin. You can build it in one afternoon from about $10 worth of Radio Shack parts. The loop antenna that goes with it will take another day to build from stuff you can buy at Home Depot or a similar store. 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. I meet the first requirements by building a hexagonal loop antenna that measures 1.5 meters (59 inches) across the diagonals and winding it with 24 turns of # 14 stranded copper wire. I meet 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. I 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 if 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. The chart below shows an interference-free recording of the signal from NAA in Cutler, Maine, USA transmitting on 24 kHz that I made in Orlando, Florida, USA with the loop antenna 8 meters (27 feet) from the nearest 60 Hz electrical wiring. Below the chart is a simplified diagram of how to hook the parts of the receiver together. Below that is a parts list. It is not necessary to put the receiver in a weatherproof box. Two things are necessary to protect the receiver from the weather. First, all soldering flux must be removed from the circuit board with flux remover, something Radio Shack no longer carries. Some other radio and electronics stores do carry it. It is usually a mixture of Ethyl either, Acetone and alcohol. Probably nail-polish remover would work. After removing the flux, candle wax is dripped on the amplifier board to cover it completely and protect it from rain. The tuning capacitors for the loop are not mounted on the board. They are connected directly across the loop leads and do not need to be protected.
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 lice 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 from Home Depot or any lumber supply house. 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. I 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 you use so you should buy the wire first. Then you can measure how wide 24 turns will be. 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. I 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 60 degree angles between them 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. You should buy some extra because I have not included the thickness of the wire in my calculations. Before you start to wind the wire on the frame you should 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. Cut two pieces about 2-feet long to clamp the diagonals while you wind the wire on the frame. Clamp these in place with C-clamps. 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. Glue them in place too with the epoxy.
I tuned my 1.5 meter loop with 24-turns of # 14 wire with a precision decade capacitor bank made by Cornell-Dubilier to determine the actual capacities 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 (no call letters) 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
You can make a tuner to find these stations from two Radio Shack 8-position DIP switches. These consist of eight little singlepole, single-throw switches side by side that mount on a printed circuit board. Radio Shack only carries the capacitors you need far this tuner in ceramic dielectric so you should use their ceramic capacitors. Below are the capacities for the sixteen capacitors you will need. Mount each switch on a little circuit board and connect one capacitor to each switch so when all 8 switches are in the an position all eight capacitors are connected in parallel. Leave the leads long when you solder them into the tuner so later when you have determined the combination of capacitors that tune to the station of your choice you can unsolder them with long enough leads to span the distance between the ends of your loop.
Switch Number One:
Position # 1.....100 pfd
Switch Number Two:
Position # 1...0.001 mfd
These two tuners should make it possible to find your station without an oscilloscope and signal generator. Connect them temporarily with Alligator clip leads across your Loop. It will take some patience but you can choose a combinations that add up to the values given in Table 1 to get close. Then tune up and down in 100 pfd increments until you peak on a strong signal. Use a multimeter or your recorder to measure signal strength. Record the strong signal you have found for a few days to make sure it shows sunrise and sunset patterns. If it shows these patterns you have successfully tuned your receiver to a suitable signal and it should record solar flares as SESs. Unsolder the selected capacitors from the tuners and solder them across the ends of the loop. Remember the ceramic capacitors you buy from Radio Shack are rated for only 20% accuracy so if you connect the values in the table above to your loop you probably will probably be way off. I connected 20% capacitors adding up to 0.185 mfd across my loop and NAA was nowhere to be found. I added capacitors 100 pf at a time to tune in NAA. Checking later with a signal generator and counter I found the 20% error capacitors tuned to 28 kHz instead of 24 kHz where I wanted to be.
The instructions above for the hexagonal loop antenna receiver are taken from the April SID Supplement. By now experience has shown not everybody is able to build the receiver and get it tuned to a suitable signal to record SIDs. This is especially true if you try to tune to a weak signal. For those of you who might have trouble I am here to help you. Do the best you can but if you can't get it to work please send me an email and I'll be glad to help you get it on the air recording SIDs. I've never seen one yet that I couldn't fix. It is understandable that some might have trouble because we no longer live in a world where people build homemade radios. Amateur scientists no longer build their own homemade instruments. This is regrettable because there is much pleasure doing science with something you built yourself and that is your very own thing. Also you are very apt to have a much better understanding of how an instrument works if you built it yourself. But the way things are is the way they are and will be into the future. It is all for the good and we really do live in a better world today despite what some people will tell you. Even though things are better and commercially available scientific instruments are available, you still can't buy a good SID receiver so you will have to build your own. That is what I hope you will do so please give it a try. Gamma ray bursts (GRBs) are cutting edge astrophysics that are detected with high technology satellites that costs many millions of dollars. I'm sure you will find great pleasure in detecting one with a simple homemade radio you can build yourself on the kitchen table for a few dollars. Good luck.
Some Additional Helpful Information
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 shows resistors that amplify 900 X. This was OK recording the 24 kHz NAA signal in Florida but is too much amplification to record NAA from New Jersey. Here I 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. If you think it will help you please send me an email and I'll tell you how to build it. 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 me and I'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 above. A 10 mfd electrolytic capacitor should be connected across the 5k variable resistor that controls the output level to the recorder.
It is possible the reason you can't find a station you are looking for is it might be off the air. The Navy VLF stations all usually have a scheduled maintenance day each week and occasionally they are down for extended periods. NPM in Hawaii is presently down and has been for months. To find out about such things, you can subscribe to the Solar Observing and SID Monitoring Forum (http://www.aavso.org/forums/variable-stars/solar-observing-and-sid-monitoring) and you will be able to keep up with what is happening in the SID world. You are also welcome to post questions to help you get your receiver built and working and on the air to record solar flares and gamma ray bursts. There are experienced SID observers who will be glad to answer your questions and help you with any problems you may run into.
Good luck. CHH