A Shielded Loop Antenna
Finding a good, inexpensive antenna for receiving WWVB can be a challenge.
You need decent sensitivity, but noise rejection is also very important.
I put an antenna together many years ago that has served very well, and
this web page finally gets around to documenting it. Note that there's no
reason this design couldn't be scaled to other VLF/LF frequencies; all
you'd need to do is change the amount of tuning capacitance.
This is an electrostatically shielded tuned loop. That's a mouthful, but it's actually very simple to put together. Its advantage is that the shielding makes the antenna much less susceptible to local noise.
This antenna is easy to assemble, and because of the very low frequencies (and therefore long wavelengths) involved, there's nothing critical in the assembly technique. It's best to have some test equipment available to tune the loop, but in a pinch you can get by just with your WWVB receiver.
(Note: there should be some pictures of the antenna here. Soon...)
- 100 foot (30 meter) length of RG-58 or RG-59 type coaxial cable; the quality is totally non-critical, so use the cheapest stuff you can find
- 10 foot (3 meter) length of stranded, insulated wire, 14 to 20 gauge
- Coax connector of your choice (a panel-mount type may be easiest to use)
- Variable capacitor with the largest capacitance you can find; plate spacing is not critical
- Silver Mica capacitors in various values, totaling about 25,000pF
- 2 3-foot (1 meter) pieces of wood slat, or other non-metallic material to make a cross-shaped mounting frame
- Basic tools: wire cutters, sharp knife, soldering iron, etc.
- Unspool the coax cable (any cheap brand will do) and find its exact center. Using a knife and wire cutters, remove the outer jacket and the braided shield at that point for about 1/2 inch (1.3cm). Leave the center conductor and its insulation intact! Use heat-shrink tubing or, lacking that, electrical tape to cover the cut.
- Trim both ends of the cable so that the shield is folded back and made into a pigtail about 2 inches (5cm) from the end. Trim the insulation from the center conductor at each end so that about 1/2 inch (1.3cm) of bare wire is exposed.
- Wind the cable into a coil of about 3 foot (1 meter) diameter. Arrange the coil so that the two ends of the cable are near each other.
- Mount the cable on a non-metallic frame. You can use two pieces of wood mounted at right angles, or two pieces of PVC pipe, or whatever suits your fancy as long as the frame is not metal.
- The tuning capacitors are soldered between the two ends of the cable center conductor. The total capacitance required will be something between 20,000 and 25,000pF. You'll never find a variable capacitor with a range anything like that, so you will end up with a smaller variable cap (if you're lucky, with maybe 1000 to 2000pF maximum capacitance) and several silver-mica capacitors connected in parallel with it. The exact capacitance will be determined when you tune the antenna; plan on the fact that you will be adding or removing silver-mica caps to set the tuning range.
- Thread the insulated wire into the coil of cable to create a single-turn loop paralleling the cable, with the ends of the wire near the ends of the cable. Cut off any excess wire. This wire forms a coupling loop that transforms the very high impedance of the main loop to a lower and more usable value.
- Solder one end of the coupling loop wire to the center pin of the coax connector. Solder the other end of the wire, and the shield pigtails at each end of the coax loop, to the outer part of the connector. Connect an appropriate length of coax cable between the connector and your receiver's antenna input.
The loop is now complete. It picks up signals in a line parallel with the coil, and nulls signals broadside to the coil. The null is much sharper than the peak, so you can rotate the loop slightly to decrease interference. Although it's always good to have any antenna mounted high and in the clear, I have my loop hanging from a beam in my basement ceiling and there's plenty of signal. One thing to consider if you're planning an outside installation is that the loop may detune with temperature changes.
You'll definitely need to tune the loop to peak it to 60kHz. The easiest way is with a signal generator and signal indicator like a low frequency receiver, an oscilloscope, or a sensitive AC voltmeter with bandwidth of at least 60kHz.
You may be able to use your WWVB receiver, but some designs have a very slow reaction time to changes in signal strength which makes adjustment time consuming. Some receivers don't have a way to monitor the strength of the received signal, and that does complicate things. Finally, the bandwidth of many timing receivers is extremely narrow, and unless your signal generator can be set to within fractions of a Hz, the receiver may not lock on it. (Using WWVB itself as the signal source may work, but if the loop is far from resonant you may not be able to hear the signal at all. A signal generator can put out a much stronger local signal.)
The precise details will vary depending on your setup, but if you set the signal generator deliver a healthy output on 60kHz, connect its output to a piece of wire several feet long and set in the vicinity of the loop (but not right on top of it) and hook the receiver/oscilloscope/voltmeter input to the coax connector on the antenna, you should be able to see the signal. Adjust the tuning capacitor through its range, and see if there is a peak in the signal. If there is, you're done.
If, as is much more likely, there isn't a peak, you'll need to add or subtract fixed capacitors until you get one. If there's no peak but the signal is stronger with the variable capacitor's plates fully meshed, there needs to be more capacitance. If the signal is stronger with the plates away from each other, then you'll need to remove capacitance.
That's it. Good luck!