Q/A on SID Antenna and Receiver Construction and Operation

Q. Where can I get #26 enameled wire?
A. You can get the wire from Sky Craft Surplus & Parts in Winter Park, FL. Their phone number is 407 628 5634. They do not have an 800 number. They have a minimum of $10.00 that they will ship with UPS so just order ten dollars worth of wire and you will have enough for several loop antennas. They will probably have #26 but if not you can go with the next lowest number #25, #24, etc.

Q. How do you tune a Loop Antenna? Where do you connect the signal generator and scope?
A. You can couple the signal generator to the loop antenna by simply running a single insulated wire from the output of the signal generator to the loop. Do not connect it directly to the loop. Just wrap the end of the wire around a section of the loop several times and it will then be coupled loosely to the loop but enough to produce a signal on the loop that you can see on the oscilloscope. Set the signal generator for its highest voltage output and the scope for one of its more sensitive settings. Connect the two ends of the loop to the input of the oscilloscope. Sweep the signal generator through about 15 to 50 Khz range.

Now you are ready to tune the loop but you will need a selection of fixed capacitors to tune it. You can get these at Radio Shack but a better way might to be call Mouser Electronics at 1 800 346 6873 and order their kit # PS-KT2 The kit contains 12 each of 12 sizes from .001 mfd to .01 mfd, 120 pieces in all and they are Polystyrene-dialectric capacitors which are high-Q and much better than stuff from Radio Shack. The kit also contains the other capacitors you will need to build the Gyrator receiver. If you plan on building several receivers the kit will save you money in the end. With the loop connected to the receiver and about .01 mfd capacity across the loop, tune the signal generator through a range of 15 to 40 kHz while watching the scope. When the signal is at a maximum level on the scope, you are at the current resonance frequency of the loop antenna. Now add or subtract capacitance and retune the signal generator to a new resonant frequency. Keep doing this until the resonant frequency is the same as the frequency of the station you wish to monitor. For instance if you were going to monitor NAA in Cutler, Maine, the resonant frequency would be 24.0 Khz. Whatever capacitors have been selected should then be soldered across the two ends of the loop wire.

Now that you have the loop tuned, turn off the signal generator and set the scope to its highest sensitivity. You should be able to see the signal from the station you are monitoring. If you don't, rotate the loop until the plane of the antenna is pointing in the same direction as the station you are monitoring. You should be able to see the signal rise and fall as you turn the antenna. If you don't see anything and are sure that the loop is tuned correctly then you may not be able to pick up the signal from the station you have selected due to a low output power level and your distance from the station. Also be aware that some stations go off the air one day a week for maintenance. So try again on another day before giving up on that station.

Q. How can I check to be sure the receiver is working?
Turn the receiver on and connect the scope to a point on the printed circuit board that is just before the rectifier diode. Connect the tuned loop antenna to the receiver input and position it where you were getting best reception when tuning the antenna by itself. Slowly turn the receiver dial until you see the signal show up on the scope and position the dial for the maximum signal amplitude. The tuning of the receiver is very sharp so turn the dial very slowly or you may miss the signal. If you can tune the signal at this point in the circuit then you should repeat the process while monitoring the DC output of the receiver. At the same setting of the tuning dial that you found before, you should get a corresponding peak in DC signal level. Set the gain so that at midday you get a DC level of about 2.5 to 3.0 volts.