- The construction of this antenna is intended to be inexpensive, fast, and effective. It will actually take more time to mark up this article then it did to construct the antenna. I have found this large loop construction to be very capable in receiving WWVB in California. As long as the practices used here are regarded, you should have no troubles picking up the station from most of the US.
- The Materials
- The materials for this antenna are as simple as I could make them. The rim is a 24 inches, wide tire type. I went to a local bicycle shop and got this for free as it was twisted and old. Surplus wire was used to wind the coil. I used 22 gage wire with two layers of insulation. It is plastic insolated with an additional layer of cotton. If you use a wire with a relatively thick insulation, you will be able to get more turns on your form. This keeps the inter-turn capacitance down. A wad of thin foam packing material. Additionally is a small block of wood, screws, small steel can, and aluminum foil.
- The Construction
- Use a hack saw and cut the bicycle rim creating a gap in the loop. This must be done to eliminate the shorted turn the rim would create. Using a couple of screws, fix a small block of wood across the gap to hold the rim ridged. Nick the rim with a file and solder a wire to it for the ground. Fold some packing foam and lay it into the rim. This is to keep a gap between the turns of your coil and the rim. You will be able to get more turns on the rim by keeping the capacitance between the rim and the coil down. Leave only enough of the foam projecting from your rim so you can fold it over your coil to just cover the turns without covering the outside of the rim.
Leaving a foot or two of wire out of the rim start wrapping your wire. The wire will hold the packing material down nicely as you go along. You should be able to get well over a hundred turns of wire on the rim before you start checking for self resonance.
If you think you are getting close to resonance, it is time to check. You will need an oscilloscope and an audio oscillator that will go to 60Khz. Scrape the wire just enough to make a connection with an alligator clip. Ground this connection to the rim and the ground of the scope probe. You must ground the rim to your circuit or you won't make an accurate measurement of the finished product. You really should use a 10:1 probe so as not to load the coil too much. Connect the scope probe to the end of the wire that is the start of the turns. Note: If you connect to the starting turn of the coil to ground, the wire from the supply reel will load the circuit and give you an erroneous measurement. Place a small turn of wire from the audio generator inside the rim. Watch the scope as you vary the oscillator frequency. You will see the generator output peak on the scope when you are at the resonant frequency of the coil. Add turns to the coil and check until you get to resonance of between 65Khz and 70Khz. Don't try to get to resonance as when you are done with the construction you will not be able to adjust the frequency up without removing turns.
When you are done with the windings, wrap the aluminum foil around the rim as shown in the top picture and check your frequency again. It will drop a little, but if the foam insulation is thick enough, it won't be much. Be sure you don't bridge the gap in the rim and create a shorted turn.
- The Pre Amp
All that is left is the construction of the pre-amp and the mounting. The pre amp is a simple FET amplifier.
Use a small piece of PC board and scab the parts onto it. There are just too many/few parts to do it any other way. The board will make it easy to attach all your grounds. C2 is at least 50uf. The resistor around 2k. The FET I used is a 2N5951. Any general purpose FET will work. Select a silver mica, or use a small trimmer for C1 and bring the resonance of the coil to 60Khz.
The feed line can be anything that is shielded. As a matching section, it's impedance will have little effect on the several K ohm output of the pre-amp. I used some audio cable on the first one I built many years ago. I had surplus 75 ohm and 'F' fittings for a simple installation this time. 'F' splices are also very easy to put shrink over for weather proofing. If you can't get a piece of cable from an old TV with copper shielding, you will have to mount a female 'F' fitting to the board to start your feed. You can also do your stub with RG-58 and an 'F' fitting. A mismatch at this frequency is meaningless.
Slip a plastic bag over the pre-amp before you place it in the can. This will keep moist air from it as well as insolate it from the inside of the can. Attach a strain relief to the block of wood. If you don't have a commercial made one, use a piece of solid number 12 and make several wraps around your feed line.
Nov 23, 1997. I have done a preliminary documentation to the receiver processor. You can find it through this link:
[The WWVB Receiving Processor]
The green lamp indicates lock. The amber is the demodulated time code. The output is 1Mhz, derived from a 6Mhz VCXO (Voltage Controlled Crystal Oscillator).
Thanks for reading and write if you'd like, Dan. firstname.lastname@example.org
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