The material below has been contributed by several active SID observers and assembled by Michael Hill in response to requests for information regarding assembly and tuning of antennas used with the Gyrator II receiver. Requests for additional information can be directed to him at the following addresses:
Email: firstname.lastname@example.org Postal Mail: 114 Prospect St., Marlborough, MA
Successful monitoring of solar flare induced SIDs involves three basic hardware components: the antenna, the receiver and a recording device. Most regular observers build the first two and, more often than not, build all three. The antenna is the simplest to build but probably the hardest to understand since the functioning of an antenna is difficult to measure and quantify with the limited test equipment that we may have available to us. It is best then, in the interest of moving forward with the construction of a SID Monitoring Station, to follow the successful antenna designs already in use by current observers. This article is a summary of some of those designs with a description of how to build and how to use each antenna described.
Antennas have been described a number of times in past AAVSO Solar Bulletin/ SID Technical Reports and are covered in detail in many other publications. If one has access to these they are a valuable source of information on the art of antenna construction. This purpose of this compilation, however, is to group together some of the designs successfully in use by our observers today. By providing this with the receiver and A/D converter descriptions that are part of the SID Equipment List, the entire construction of a SID Monitoring station may be based on information from a single source.
VLF Antennas Arthur J. Stokes N8BN (A62)
Many types of antennas have been used for VLF monitoring. The listing here describes those that have generally been used:
1) A long-wire antenna. Usually these are erected between the receiver location and a conveniently placed tree or structure. As a general rule, the longer the wire, the better. A coax line brings the signal to the receiver. A ground connection of the PC board to a ground stake or cold water pipe or the ground of an AC house plug is needed for this type of antenna to complete the circuit from antenna to ground.
2) Vertical antennas of various descriptions, such as the “Maag antenna”, which consists of twenty feet of three-inch down spout mounted and braced with guy wires. A nine-foot CB whip has also been used. The gutters and downspout of a house have also been used. Another monitor has used a 24-foot vertical length of one inch aluminum tubing mounted on an insulated post. The ground is necessary for these antennas.
3) A third type of antenna is the resonant loop. These can be of various sizes, from a ten-inch square to thirty inches. A convenient size is about sixteen inches on a side, wound with about 100 to 150 turns of #26 enameled copper wire. Wood crosspieces form the frame of the loop with V notches cut in the ends. The proper value of capacitance is connected across the ends of the loop to resonate it at the frequency of the station signal. This is most accurately done with a signal generator and an oscilloscope. Capacitance values generally run from 3000 to 5000 pf. These loops can tune quite sharply and should be rotated to align the plane of the loop toward the station being monitored. A short length of coax or twisted wire pair connects the loop to the receiver.
A small loop that I have recently been using with good results utilizes two 15 inch-long wood crosspieces that are cut from 3/4 inch square stock. Notches are cut in the center of each piece, 3/4 inch wide and 3/8 inch deep. Vee notches are cut 1/2 inch deep in the end of each piece. The two pieces are then assembled in the form of a cross, using glue for attachment. The frame is then wound with 200 turns of #26 enameled wire. Both ends are terminated at one of the arms. A two terminal tie point is mounted on the arm to serve as a connection point. This loop was resonated to 24.0 kHz for the NAA frequency. Polypropylene capacitors were used to give the loop a high Q. A twisted pair of wires was also connected to the terminal point, along with the capacitors.
The value of capacitors to resonate at 24.0 kHz was 1320 pf. This completed loop had a Q of 37 and a half power bandwidth of 650 hertz. This narrow bandwidth helps to reject unwanted signals. Ceramic capacitors from Radio Shack may be used in place of the polypropylene type with a somewhat wider bandwidth.
The loop was mounted on a wood base three inches by six inches. A very good signal was received from Cutler, Maine at my location in Northern Ohio.
A Tuned Outdoor Loop Casper Hossfield (A-5)
I use an outdoor loop to eliminate much of the interference that indoor loops pick up from 120-volt house wiring. Multi-conductor cable with a plastic sheath is sufficiently well insulated to withstand being outdoors and loops I made 20 years ago are still OK. The total length of the wires in the cable when connected in series should be about 300 feet. Home Depot sells 7-conductor thermostat cable, which is a good choice. Wind it into a loop small enough to be self-supporting and tape it together with black electrical tape.
Another good choice that I use is 15-conductor cable, which can be ordered from “Sky Craft Parts and Surplus” (407 628 5634). Order 21-feet of 15-conductor #20 AWG cable, part # CJN at $0.47 per foot to meet their $10 minimum. Wind this into a 5-turn coil and connect the color-coded wires in series. Hold the coil in place with black electrical tape until you glue it with cyanoacrylate (Krazy Glue). One drop in four places between turns will make it into a nice looking self-supporting loop after you remove the tape. The cyanoacrylate hardens in seconds and the loop can be used indoors or outdoors. If you put the loop outdoors and attach one end to a ground rod it will eliminate a lot of interference. Ground the end that goes to the sheath of coax that should be used to connect the loop to the receiver The ground rod need not be driven more than a foot or two since it is not part of the tuned circuit. Make it the ground for the receiver through the coax’s BNC connector. This eliminates interference that can be picked up by using the neutral 120-volt house ground. Tune the loop the same way Mike Hill describes (see “A Tuned Indoor Loop” below) but put the loop tuner indoors if the loop is outdoors and connected with coax. If you use a Radio Shack 8-pole DIP switch to select the tuning capacitors the tuner is small enough to be inside the receiver. This eliminates a break in the coax for the tuner and therefore requires only one BNC connector for the antenna.
The loop does not have to be free and clear outdoors. Set it on the ground or hide it in shrubbery and it will work fine. Remember that the US Navy submarines receive the 24 kHz signal with an antenna submerged in the ocean’s salt water. If you can place the antenna as little as 10-feet from the house it will eliminate a lot of interference radiated by 120-volt house wiring. Its level falls off as the fourth power of distance, so farther away is much better. If you have an oscilloscope, connect it to the RF output of the receiver and sweep at a multiple of 60 Hz and you will see the power line interference spikes riding on the 24 kHz signal. They are the reason you don’t get a nice clean trace. It pays to eliminate them as much as possible.
A Tuned Indoor Loop Michael Hill (A87)
My antenna is a tuned loop antenna as described in Peter Taylor’s book “Observing the Sun”. A loop antenna works by the fact that as the propagating signal cuts across the wires of the loop, it induces a current into the wires. The antenna is responding to the magnetic field of the electromagnetic signal. The many turns of the loop act as a large inductor, and by connecting the proper capacitor to this inductor, you create a tuned LC Circuit that resonates at the frequency of the signal that it is receiving. This resonating (and therefore amplified signal) is what is fed into the Gyrator II SID Receiver. The tuning capacitor will be in the range from 4600pf to 900pf for a frequency range of 15 to 30 KHz.
The antenna form is made of wood. A one-inch dowel is set into a hole bored into a square 2” thick block. Near the top of this dowel, drill a hole for a smaller ¼” dowel pin. This is the antenna post. The antenna form is made of two 1/2” square crossbars. These must be some sort of hardwood and should be 24” long and joined in their midpoints to form a cross. Before joining them cut notches at each end. Remember that these notches are for the wire wound around the periphery of the square formed by the ends of the dowels,. so make sure they are notched in the correct orientation. Join the two crossbars by cutting 1/2" x 1/4" deep notches at the center of each so that they fit into each other. After gluing the pieces together at the midpoint, drill a 1/4" dowel hole in the center of the cross. A 1/4" dowel pin is pressed through this hole and through the antenna post to support it.
The antenna winding is made of #26 coated copper wire (like the wire you see in transformer). If you cannot get this then a roll of standard plastic coated wire will do. In this case, I would suggest using #28 . You must wind 125 turns of the wire around the periphery of the cross. This needs some sort of setup where the antenna form is on a rotating spindle with a second spindle about 4 feet away onto which the roll of wire is placed. Drill two small holes just big enough for the wire at the end of one of the cross bars and thread one end of the wire through one of them. Tie the wire off leaving 6-12” of the wire end. Rotate the antenna on the spindle and guide the wire into each of the notches as it wraps itself around the antenna form. After 124 more turns tie off another 6-12” length of the wire and then scrape the insulation off each end of the wire.
The capacitance needed to tune across the range covered by stations monitored for SID detection varies from 900pf to 4600pf. If one cannot obtain a variable capacitor of this magnitude then a simple solution is to build a capacitance switch box which contains 6 or 7 fixed capacitors that are placed into the circuit with a switch. I built one with six capacitors but would recommend seven. I have the following values in my tuning box: 50pf, 100pf, 200pf, 470pf, .01uf, .01uf (.02uf should be added)
These, used in combination, allow a selection of any value with a tolerance of ±50pf. This is sufficient to tune close enough to the resonant point of any frequency to allow adequate reception. I measured the capacitance required to tune from 15 KHz to 30 KHz using an RF frequency generator and an oscilloscope. I connected an un-tuned loop to the RF generator and aimed it at the antenna under test, which was subsequently connected to the scope. After setting the RF Generator, I adjusted the capacitance until I got the strongest signal level. The results are tabulated below.
|Frequency (KHz)||Capacitance (pf)|
The capacitors are connected across the ends of the loop of wire. I used a 4 position terminal strip to allow me to tie together the wire antenna leads, the capacitor box, and a BNC connector which is mounted on a metal flange at the base of the antenna. I run the signal to the receiver with a shielded coax cable.
The loop antenna must be positioned correctly for proper operation. The strongest reception to a signal will be when the plane of the antenna is parallel to the propogation path of the signal being monitored. After determining the direction of the station from your location you can orient the antenna properly for the best reception.
Loop Antenna with Inductive Coupling Guglielmo DiFillipo (A93)
I have built a loop antenna on wooden cross arms, 4 cm square, 75 cm long on which I have wrapped 140 turns of 0.25 mm wire and tuned with a capacitor of 2000pf (for around 20 kHz).
The capacitor of 2000 pf disturbed the tuning of the Gyrator II. As a result, it has been necessary to join the antenna in an inductive way to the Gyrator II with 18 ± 22 turns of 0.25 mm wire, wrapped above the primary wrapping of 140 turns.
The coaxial cable (75 ohms) the Gyrator II connects to the secondary winding (18 ± 22 turns) of the antenna. I don’t believe there is another way to connect an antenna to the Gyrator II, without modifying the frequency of his oscillator.
I hope that the description is satisfactory. As you know, I know very little of the English language.
Final Note Michael Hill (A87)
While experimenting with a new setup, I got to thinking about the fact that all long wire antenna designs mention grounding as a requirement. Since my data traces made on a Rustrak recorder and on my computer always tended to be a bit on the noisy side, I wondered if grounding my system would be of any help. I therefore grounded the ground plane of the Gyrator II receiver PC board to the house ground at the AC outlet. The two ends of the antenna loop connect to the tuner input and the ground plane of the receiver. I felt that having the PC board connect to the house ground rather than be electrically floating would provide a more stable signal. The result was in fact a much more stable and clean signal, which was nice to see. I asked another observer, Cap Hossfield, why all descriptions of loop antenna systems did not include this as a requirement, and he reminded me that not all house grounds are the same. Some might have noise riding on them for other reasons, and connecting to them could actually make the signal noisier.
This all points to two things: First of all, for any design idea you decide to follow, don’t hesitate to try things to make it better, and second of all, don’t think that what you see is the only way to do something. This is an area where there are many factors that affect signal strength and receiver performance. If you don’t get a good clean signal right away or don’t get any signal at all, don’t despair. Look at what you have done and try to see what needs to be done or what needs to be improved. Trial and error and an investigation into antenna operation and design will point you to the way of good reception and, in doing so, will gain for you a firmer understanding of antennas, signals and reception. It’s all part of the fun of doing radio science with the Sun.