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:
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:
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.
On the bottom hole was attached a compression-type cable connector of the sort used to make a weather-tight seal for flexible "SO" type power cable: These connectors have a rubber grommet inside, the diameter of which narrows as it is tightened allowing a wide variety of different-diameter cables to be sealed and a small-ish version was used that could be made small enough to seal and secure the RG-6 type coaxial cable that emerged from the bottom of the box.
"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!
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.
Indoor amplifier and power inserter:
See figure 3 for pictures of the indoor amplifier/power
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.
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)
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
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.
For the "Version 2" amplifier using the LM386, initially set
it to 3/4 of full output.
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)
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 pictures on this web page were taken during this more
recent repair, after the unit had been in continuous service
for 12-13 years.
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