Figure 1:
Top:  The rooftop-mounted 60 kHz shielded loop antenna.  Inside the pipe - in which there is an insulated gap at the top - are 4 turns of wire.
Bottom:  Inside the outdoor box to which the loop is mounted showing the amplifier circuitry.  The four capacitors visible at the lower-left corner of the perfboard are used to broadly resonate the receive loop.
Click on an image for a larger version.

In the late 1990's a friend of mine came to me with a problem:

In the office building in which he worked (a city government building near downtown Salt Lake City) there was a conference room with a pair of "Atomic Clock" receivers on a middle floor.  Being that this was a modern building of steel and reinforced concrete construction, and because it was full of electronic devices such as computers, fluorescent lights, etc. neither of these receivers ever managed to get a good enough signal to synchronize themselves to the 60 kHz time signal being transmitted from Fort Collins, Colorado by the NIST station, WWVB even though the signal from that station here in the Salt Lake area is quite strong.

What to do?

The solution was to bring a signal into the room from outside, a project that would involve a receive antenna placed in a location that did have a good signal - such as the roof of the building - and then convey it to the conference room and somehow couple it to the clocks in question.

That seemed simple enough!

The design called for a three-part system:

• An outdoor shielded loop with an integral amplifier to receive the signal and amplify it before sending it down a coaxial cable.

• An indoor box to feed power to the outdoor box and further-amplify the signal.

• One (or more) loops to be placed near the indoor clocks to couple into them the now-amplified signal.
  

Comments:

• This project was intended to receive the 60 kHz signal from the U.S. based WWVB transmitter.  As described, it could also be used for the UK-based 60 kHz MSF or the Japanese 60 kHz JJY signal.

• By adjusting the resonant frequencies of the loops, there's also no reason why it could not also be used for the Japanese 40 kHz JJY or the German 77.5 kHz DFC77 signal as well!
  

Shielded Loop:

Working with him, I sketched out the design for a shielded loop and the result was that visible in Figure 1.  Since he was handy with a welder and had a lot of pieces around it was a natural to make it out of scrap pieces of 1/2" electrical conduit and using a conduit bender he fashioned a "square loop" that was about 18" (45cm) on a side as shown in the picture.

A shielded loop has several advantages over a simple electrical whip antenna:

• Being shielded, it is resistant to "near-field" interference.  This sort of loop responds only to the "H" (magnetic) component of the signal whereas it is often the case that nearby noise sources emit most strongly the "E" (electric) component.  This can provide a degree of rejection of some types of noise sources.

• A loop antenna has a "figure 8" pattern and thus has two nulls and a degree of directionality.  If there is a particularly egregious noise source, it may be possible to rotate the antenna to null it out - provided, of course, that the noise source isn't in the same direction as the desired signal!

What is required to construct a shielded loop is simply to have the wire making up the conductors of the loop run inside a metal tube that is grounded and, therefore, shielded.  In order for this to work however, there needs to be a gap in the shield or else it will simply "short out" the impinging magnetic field from the desired signal and for reasons mechanical, electrical and symmetrical, it is best that this gap be right in the middle of the loop.

Describing the loop:

Again, this is a shielded loop which means that the wiring is contained within the tubing itself.  When the loop was made with a conduit bender, the two ends of the loop were brought close together, but not touching, and this gap is covered by the black tape/tubing seen at the top of the loop in the picture!

At the bottom of the loop was welded a small plate of steel as can be seen near the left side in the bottom picture of Figure 1.  This plate was welded in the middle of the loop, opposite the gap at the top and such that when the plate was bolted to the side of the utility box, it held the loop upright.

Important notes about welding metallic conduit:

Electrical conduit typically has zinc plating and welding to it can release a hazardous substance!

Appropriate precautions with regard to proper ventilation must be taken!

Once the plate was welded to the conduit, a small hole (about 3/8" or 10mm diameter) was drilled through the side of the plate and into one side of the metal loop to allow access for the wires inside the loop.  Because it was welded, the connection was weather-tight so that this plate could be bolted to the side of the utility box and the wires in the loop passed through a matching hole drilled in the side of this same utility box:  You can just see the wires coming through the hole on the bottom picture of Figure 1.

The utility box itself was obtained at a hardware store and it is one of those cast aluminum waterproof boxes used for mounting outdoor outlets and a blank cover plate (with gasket) was used to cover the open side.  These boxes typically come with screw-in plugs to seal the unused holes and these plugs were used along with RTV (silicon rubber) sealer to maintain a water tight seal.  As can be seen from the picture, some clamps were attached to the side of the box to allow mounting to a rooftop pipe and the bolts penetrating the side of this box were also made watertight with RTV.

Note:

"Silicone II" (tm) was used to waterproof these connections.  This is the type of RTV (silicone seal) that does not smell of vinegar and thus, it doesn't have the potential of corroding electrical components.  If you do use "standard" silicone - that is, the RTV has the odor of vinegar - make certain that it cures for a few days and that the odor has worn off before sealing the box.

For this installation, the metal loop conductor was welded to a plate, but it is possible to use compression connectors to attach the electrical conduit directly to the box without welding - something that can simplify construction and allow very easy fishing of the wires inside the loop.

If you do this you must guarantee a very reliable electrical connection of the pipes comprising the shielded loop to the metal box containing the circuit or else it will not work properly!  Note that some types of compression connectors use rubber or plastic gaskets/grommets so that you cannot guarantee that the compression connector itself will provide a reliable electrical connection so additional clamps/wires may be required to provide the connection!  If you solder, be careful to avoid melting the insulation of the wire inside!

Figure 2:
Schematic of the outdoor and indoor amplifier circuitry.  "Version 1" of the indoor circuit uses the LM7171 high-speed, high-power op amp while "Version 2" uses the more commonly-available LM386 audio amplifier.
Click on an image for a larger version.

• Connect an oscilloscope across the terminals of the loop.

• Connect a 100-220 ohm resistor across the terminals of the loop in parallel with the oscilloscope - value not critical.

• Connect a signal generator across the loop with a 1k resistor in series between the "hot" side of the loop and the signal generator.  Most audio signal generators will cover at least through 100 kHz.

• Turn up the signal generator near maximum output and adjust the oscilloscope until you see the signal from the generator.

• For a starting point, connect four 0.1 uF capacitors in parallel and places them in parallel across the loop.
• Use only plastic-based capacitors (polycarbonate, Mylar, polystyrene, polyester, etc.)
• Do not use ceramic capacitors as they may be both too lossy and temperature-unstable!

• Adjust the frequency of the signal generator up and down from, say, 20 kHz to 300 kHz or so.  You should see a definite, very broad peak in the amplitude of the signal on the oscilloscope as you tune through its resonance.

• If the broad peak is higher than 60 kHz, add more capacitance and it is lower than 60 kHz, remove some capacitance.

• Because the peak is so broad, it's not necessary to get the peak exactly on frequency - just "sort of" close (+/-10%) will be fine as the "Q" (sharpness) of this resonance is quite low.

• If you are going to use antenna this for 40 kHz JJY or 77.5 kHz DCF77 signals, simply resonate the loop at the appropriate frequency.  If you need to use it for both the 40 and 60 kHz JJY signals, simply resonate the loop on frequency of the weaker of the two time signals.  If the two JJY signals are of similar strength, resonate at 47-49 kHz, a point geometrically midway between the two.
  

Signal cable:

As noted in the text above, inexpensive TV coaxial cable (RG-6 type) was used both to bring the amplified signal inside and to convey the DC power to the outdoor unit and "F" type connector were used as they are designed for this type of cable and they are both cheap and readily available.  It's worth noting that the impedance of the coaxial cable used (75 ohms for RG-6 and similar TV cables - even RG-59, if you have some to get rid of!) is pretty much irrelevant at this frequency so it's worth using whatever you happen to have available.

Note that since this cable is carrying DC, it must be well protected from moisture.

If you look at Figure 1 you'll note that two right-angle F-type adapters were used to connect the coax to the circuit board.  This necessity was somewhat of an accident when it was discovered that a direct connection to the cable wasn't possible within the confines of the small box!  A bit better planning would have prevented the need of these extra adapters.  Alternatively, a chassis-mount F-type connector could have been mounted directly to the box using RTV and/or tape to seal it against moisture.

Figure 3:
Top:  The exterior of the indoor amplifier/power coupler unit.
Middle:  The component side of the indoor unit circuit board.
Bottom:  The bottom side of the indoor unit circuit boar.
Click on an image for a larger version.

Indoor amplifier and power inserter:

See figure 3 for pictures of the indoor amplifier/power inserter.

Inside, there is another box that further-boosts the receives signals and puts the DC power for the outdoor amplifier on the coaxial cable.  This consists of a "strong" op amp that is capable of feeding one or more loops that would be placed near-ish the clocks needing the strong signal and in this particular case, I used an LM7171 high-speed operational amplifier.

The LM7171, made by National (now Texas Instruments) is capable of driving several low-impedance loads and at 60 kHz it can have considerable gain - a perfect device for our purposes.  As can be seen from the diagram of Figure 2 U201 is wired as a conventional non-inverting amplifier using R204 and R203 - the latter being adjustable - up to a setting of around 10-fold (e.g. 20dB.)  R207 and R205 are used to provide a mid-supply voltage reference and C207 blocks the DC from appearing on the output loops while R208 and R209 set their source impedance.

Of particular note is L201 and D201.  As is the case with L102, L201's value isn't particularly important and about any inductor of 1 millihenry or higher is adequate, but note the inclusion of D201:  This Zener diode suppresses a voltage spike from L201 should the connections at the RF input/DC output (J201) be accidentally shorted and without it and series resistor R201, it's possible that the amplifier (U201) will be destroyed!

One problem with F-type connectors is that it's quite easy to accidentally short them out when connecting them so it is strongly recommended that one not omit D201, above.  It's also recommended that L201 be somewhat more substantial than a tiny, molded choke as this type can be destroyed almost instantly if the output is accidentally shorted.  What worse is that it's often the case that these small, molded inductors do not fail open-circuit, but rather shorted:  The outdoor amplifier may still get power, but without the inductance the signal is effectively "shorted" out and not only that, any noise that might be coming from the power supply and the AC mains can be inadvertently coupled into the circuit and severely degrade performance!

The 12 volt power supply isn't particularly critical.  Ideally, this would be a regulated +12 volt supply but it was found that given the modest current requirements of the entire circuit (under 100 mA) a simple unregulated "wall wart" with a voltage range of 11-15 volts was used and found to be entirely adequate.

As can be seen in Figure 3 the circuitry was built in a piece of phenolic prototype board.  Three chassis-mount F-type connectors were attached to the bottom (non-component side) of the board and these were attached to the front of the plastic utility box containing the amplifier, holding the board in place with the screws.  Not shown is the 12 volt, garden-variety NON-switching wall-type power adapter.

Comments:

• The LM7171 is available from DigiKey at the time of writing, but it is not a device that is particularly common.  While it has the same pin-out as the venerable 741, that device will not work in this application due to its rather low gain bandwidth and (more importantly) it's not capable of properly driving the output loop coupling into the receiver.

• It should be possible to substitute a more common LM386 audio amplifier chip - see the description of an alternate "indoor amplifier" using the LM386 near the bottom of this web page.
  

• Do NOT use a switching supply to power this unit.  Nowadays, it is more difficult to find a "wall wart" that is NOT a switching power supply and because this system operates in the 40-80 kHz range, it is important that a switching type wall wart NOT be used as it is likely to produce interference that is on or near the frequency of the time station!  It is best that one uses an old-fashioned transformer type power supply!  If you can, try to avoid plugging the wall wart for this antenna into an outlet that is near - or on the same circuit as a switching supply, too.
  

Coupling loop(s):

In the original installation there were two clocks in the conference room that needed a signal to synchronize the time, so it was necessary to construct two coupling loops.  These loops were made from a piece of 4-conductor telephone cable and were about 18 inches (45cm) in diameter and the four conductors were connected such that they formed a 4-turn loop and at the feed point of each loop was soldered a chassis-type F connector to allow connection to the RG-6 cable from the indoor amplifier.  While not strictly necessary, this coil was also resonated to the intended frequency (60 kHz) with capacitors using a signal generator and oscilloscope as described above.  In retrospect, this step was likely overkill as the coils seemed to radiate a very strong signal to the clocks even when they were placed well over 3 feet (1 meter) distant!

It is recommended that chassis-mount F connectors are used at the receiver coupling loops when using RG-6 or another aluminum-shielded coaxial cable since it isn't possible to solder to the shield directly - plus, having just an ordinary connector at the far end of the coaxial cable makes it much easier to run the cable since there isn't a loop (and the resonating capacitors) hanging off the end!  If you really want to attach the loop directly to the end of aluminum RG-6, the only practical way to do this would be to bare a an inch or two (2-5cm) of aluminum shield, coat it with anti-oxidant electrical grease (or petroleum jelly if that's all that is available) to minimize oxidation, and then tightly wrap bare copper wire onto the aluminum shield.  After making a secure, mechanical connection, use either heat shrinkable tubing (and carefully avoiding melting the foam dielectric center of the coaxial cable) or good-quality electrical tape to help secure and protect the connection.  At this point, one should be able to solder to the wire just connected to the shield and the RG-6's center conductor and make the connection to the loop and resonating capacitors.

Adjustment:

There are only a few possible adjustments with this system involving the receiving/coupling loops and the other being the gain setting (R203) of the indoor amplifier.

• Outdoor loop orientation:  The outdoor loop should be oriented that a line drawn through the plane (flat part) of the loop should be pointing at the transmitter - that is, if you were to stick your arm through the center of the loop, it would be pointing 90 degrees away from the transmitter.  For example, if the transmitter is due east or west of your location, the hole in the center of the loop would be pointing north/south.

• Indoor loop orientation:  In this case, the "flat" part of the loop would be directed toward the clock.  In the case of this particular installation, the room had a false ceiling and the loop was simply laid atop the ceiling tile above the clock.

• Indoor amplifier gain adjustment:  This was found to be non-critical and the gain setting (R203) was simply adjusted to be more-or-less mid-scale.

For the "Version 2" amplifier using the LM386, initially set it to 3/4 of full output.

Comments:

• Because this has a high-gain amplifier, it is possible that if the output loop couples to the receive loop that it may oscillate and would therefore not function properly.  If the transmit and receive loops are placed a reasonable distance apart from each other (more than 30 feet or 10 meters) it is unlikely that there will be a problem - particularly if the coupling loop(s) is/are oriented in a direction 90 degrees off from the receive loop.

• If the system does become unstable, first try to increase the separate of the receive (outdoor) and indoor coupling loops.  If this doesn't work, reduce the gain of indoor amplifier (R203/R303) to minimum and the increase it only if there is too-little signal for the clocks to work reliably.

• If one lives within, say, 600 miles (1000 km) of the transmitter it is reasonable to have a good signal at any time of day or night.  If one lives much farther than this, however, its possible that the signal will be strong enough to work only in the middle of the night - say, 1-4 AM, this being due to normal nighttime propagation.  It may only be at night that there will be enough signal for the clocks to properly synchronize.

• For diagnostic purposes, a longwave receiver capable of picking up the 60 kHz WWVB signal directly was used in testing the described system.  When properly operating, the WWVB signal was extremely strong when the receiver was located within 10 feet (3 meters) of the coupling coil indicating that there was likely far more signal than needed!

• Because the low-frequency signal at 60 kHz has such a long wavelength, it simply won't go very far at the signal levels being output by the amplifier so a coupling loop needs to be placed within a few feet of each clock at most.  If there are several clocks - or even dozens of clocks - one must assure that the signal is properly distributed and that there is a loop within "range" of any clock needing a signal.
  

• Practically speaking, a multi-output video distribution amplifier could be used with each output driving its own loop, but one should be aware that some inexpensive units may tend to oscillate (and therefore not work properly - possibly causing interference on other frequencies) if they aren't presented with a resistive load!

A few comments on parts:

The outdoor amplifier doesn't use any exotic or particularly hard-to-get parts so there is a bit of leeway on what will work.  The transistors themselves are typical garden-variety silicon NPN transistors and practically any type similar to the 2N3904 (or better!) will be fine.

Aside from making sure that the loop itself is resonant at the receive frequency, the only other part that is somewhat critical is L102, the choke that passes the DC to power the amplifier, but blocks the RF.  Since the impedance of the output amplifier is on the order of 50-100 ohms, one should pick the inductor so that its own impedance at the receive frequency should be at least 4 times (10 times is better!) this value to avoid the choke causing loading of the signal.  For this, we should determine the inductive reactance, the formula for which is:

XL = 2 * Pi * F * L

Which we can rewrite to solve for L, as in:

L = XL / (2 * Pi * F)

Where:

XL = Inductive reactance in Ohms
F = Frequency in Hz
L = Inductance in Henries

Since we want at least 400 ohms (that is, 4 times an assumed worst-case 100 ohms output impedance) of XL at 60 kHz, we find that we need at least 1 milliHenry of inductance.  Since this is the lowest inductance that we should use we would actually prefer a choke that was several times higher than this.

When I built this circuit I happened to find a bifilar choke on the AC input of a discarded switching power supply and each half this choke measured as being 40 mH.  Often, these chokes look like small transformers, but when they are used they are connected so that each half is placed in series with the AC power line and offers "common-mode" filtering of the power supply.  Their inductance typically varies from a few hundred microHenries to 10's of milliHenries and since they are usually dual chokes, one can usually wire both halves in series, and since the inductance is related to the square of the number of turns, this can quadruple the inductance, making a choke with single-winding inductance as low as 300 microhenries potentially usable.  One caution with wiring them in series:  Observe the polarity of the winding because getting it wrong can cause the inductance to cancel out rather then increase!

On the indoor amplifier described below, I didn't have another one of those chokes handy for L201 so I rummaged around and found one on the order of 4.7 mH - a value that we know from the calculations above to be entirely adequate.

An indoor amplifier using an LM386 ("Indoor Amplifier Version 2"):

It may be that the LM7171 is a bit hard to find, but one may use the venerable LM386, instead as depicted in Figure 2 as the "Version 2" amplifier..  This general-purpose audio amplifier, while not really intended to work much above audio frequencies, does have a reasonable amount of gain at 60 kHz (or even 77.5 kHz in the case of DCF77) and can be made to work.  While not as "quiet" in terms of noise as the LM7171, there should be enough signal from the outdoor amplifier unit to overcome its own internal noise generation and adequately drive the coils feeding the clocks.

One point of concern when a significant amount of amplifier gain is used - particularly at higher frequencies - is that some of the output signal may find its way back into the input!  To prevent this, C305 and R301 decouple the LM386's power supply from the power being fed up the coax along with the receive signal.  When wiring this portion of the board, make sure that the ground side of R302 (the gain control), J301 (the input connector), and C305 share a common point ground and that they connect to the rest of the circuit with only ONE wire.  Also important is C308, the power supply bypass capacitor for the LM386 which should be placed very close to U301, the LM386 itself with very short leads connecting pins 4 and 6! 

This amplifier uses the same receiver coupling loops as the other version in Figure 2 and if desired, one could add even more output loops - just make sure that each loop has its own resistor in series (e.g. R304/R305).  One probably wouldn't want to use more than 6 loops and it is possible that with that many loops the amplifier might become unstable and oscillate if the gain is turned up too high.  Also note that with more loops, it becomes more critical to keep the clock coupling loops and the outdoor receive antenna far apart!

Reliability over the years:

Since this system (using the circuit in Figure 2) was installed in around 1998, it has only been out of service twice:

• The first time the unit failed was about a month or so after it was installed.  The repair involved taking the cover off the outdoor unit, allowing the water to pour out and the circuit board to dry.  The drilling of two small (1/16" - 2mm) drain holes in the bottom of the box and the addition of more RTV sealant prevented this from happening again!

• More recently (2012) the unit failed again.  This time the failure was due to the filter capacitors in the "wall wart" going bad and the DC supply becoming badly contaminated with AC ripple.  The "wall wart" was simply replaced with another transformer-type and C206 (the 1000 uF capacitor) was added inside the indoor amplifier box for good measure.
  

The pictures on this web page were taken during this more recent repair, after the unit had been in continuous service for 12-13 years.

Final comments:

I've occasionally been asked "Where can I get a system like this"?

The answer is a bit tricky as any system involving potentially involving radio frequencies is subject to FCC rules.  For personal use, one would take care to avoid doing something that could cause interference and considering the frequencies involved, one could reasonably be assured that a system like this would be generally incapable of doing so - even if it were to malfunction in some bizarre way!

Were a system like this to be actively marketed and sold then it would be incumbent on the manufacturer/seller to assure proper FCC compliance - a process that can be both expensive and arduous!

Any questions about this project or do you want to send email?  If so, go here.

This page last updated 20140225