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DCF77 cross antenna

1 DCF77 cross antenna

The German date and time standard transmitter signal of DCF77 can be received all over Europe, but signal strength is strongly depending from the antenna direction. To get independent from the direction this cross antenna has been developed and tested.

I tested two versions of the antenna. Both consist of two coils on two different ferrite rods. In the first version, those two ferrite rods are mounted in an angle of 90°. In the second version those are mounted in an angle of 45°. By angling, one of the two rods always has reception, no matter what angle the DCF77 transmitter has towards the antenna rods, even if the other rod is completely misaligned. The signal of the sum of both coils is never zero, the misaligned coil just does not add to the total signal.

The second version, with 45°, has been develloped because two horizontal 10 cm rods do not fit into many plastic casings. In small casings (e. g. with 5 cm) the first rod can be placed on the bottom while the second rod can be placed on the top of the casing, both having 45° offset to each other. The nearer the angle of both towards 90° they come, the smaller is the angled amplitude difference.

Directional signal strength of one or two antenna rods That is how the signal strength varies in different angles, for a single rod/coil and for two rods/coils in 90 and 45° direction. In the cases with two rods the reception strength is never zero but varies between 0.5 and 0.7 (90°) resp. 0.92 and 0.35 (45°).

To test the two rods a little further, I have given them different windings. For the 90° version I covered 45% of the 10 cm ferrite rod with copper enameled wire (0.255 mm) in single layer fashion (just because the two rods then can be tied together in a 90° angle without too much interference between the two coils). That meant 110 windings for each coil. In the second version I covered the complete rods with single-layer wire, leaving 0.5 cm on both ends uncovered. That meant approximately 350 windings for each coil.

Tying the two coils together and angling them has interesting effects on the inductivity of the single coils and on their sum. In the 90° case the inductivity of the crossed coils is smaller than the sum of the inductivity of both, while in the 45° case their inductivity increases. If, in the second case, their mounting in a larger distance (e.g. 10 cm between both) is chosen, this effect will be much smaller than with both ends tied together.

1.1 Mounting

Mounting of the 90- and 45-degree antennas The 90° antenna (left picture) is mounted like shown. First the rod is shrinked with a piece of plastic cover to get some distance between the rod and the wire. The 10 cm rod is then covered with 110 windings of 0.255 mm copper wire over 45% of its length and both wire ends are fixed with plastic tape. The two rods are then tied together with two crossed cable ties so that the angle is approximately 90°. The inner ends of the two coils are soldered together.

The 45° antenna is mounted similarly but the whole rods are covered with wire (except 5 mm on both ends). For each coil one needs approximately 13 m of copper wire. Both ends are fixed with a piece of shrink sleeve. Both ferrite rods/coils are mounted on a blank 100-by-100 mm epoxy plate and fixed with cable ties. The near ends of both coils are soldered together.

1.2 Measuring the coils

Measuring the inductivity of the coils The coils have been measured with two different methods:
  1. with a FET and a variable capacitor equipped grid dip meter,
  2. with a CMOS oscillator.

1.2.1 Measuring results with a grid dip meter

With my grid dip meter I installed the coil and varied the FET oscillator with the 2*365 pF variable capacitor. From previous experiments with fixed inductivities a capacity of 200 pF (in full capacity) resp. 23 pF as smallest capacity has been determined.

The two 45° coils oscillated with 217.64 resp. with 221.75 kHz under full capacity, resulting in inductivities of 2.67 resp. 2.58 mH. The sum of both would then be 5.25 mH.

1.2.2 Measuring results with a CMOS oscillator

CMOS oscillator for inductivity measurements This schematic was used to determine the inductivity in a different method. Measuring with this resulted in significantly larger inductivities of 3.87 resp. 3.79 mH, which would result in a sum of 7,66 mH.

Measuring the 45° coils tied together resulted in a significantly higher inductivity: 9.58 mH. That's what you get from nearing the coils in an angle.

1.3 Buffer stage

RF entry stage DCF77 cross antenna buffer stage and AFC This is the schematic for the buffer stage (the 45° version). The antenna circuit is formed with the stacked coils and a capacitor of 330 pF. The signal goes to the gate of a N-FET (any N-FET type can be used). The drain and the source of the N-FET are connected to two resistors of 1k (to the filtered operating voltage and to ground, the HF is coupled with two 1nF capacitors (ZC = 2.5 kΩ) to the amplifier stages (symmetric output).

Note that the 90° version needs a larger capacitor of 2.7 nF due to the smaller inductivity of the coils.

The buffer stage with the FET is necessary to protect the sensitive properties of the LC resonance circuit. The large coil above has an inductivity of 9.58 mH. That means that the coil has, at 77.5 kHz, an inductive reactance of ZL = 2 * π * f * L of 4,66 kΩ. If the coil is in resonance with the capacitor, the reactance of the LC circuit is by a factor of Quality larger than this, the circuit has more than 466 kΩ. That means that the resonance curve is very narrow, suppresses nearby noise sources and the sensitivity is very high.

Hence, it would be not a good idea to attach a stage with a lower resistance to it. This would seriously drop the LC circuit's high sensitivity and would broaden the resonance curve. The FET stage does not amplify, but only keeps the high entry resistance and provides a reduction of the resistance at its output. The high quality of the LC circuit on its input is protected and kept.

1.4 Frequency adjustment

The resonance frequency of the cross antenna and the 330pF capacitor can differ slightly (temperature, iron in the near field, etc.). Therefore two varactor diodes are attached to the antenna circuit, both in reverse direction (anti parallel). I have used two of the three diodes in a TOKO KV1235Z, use of other types such as BB112 (double diode) is possible. The diodes should at least have 100 pF at 0.7V (medium wave types).

Depending from the AFC voltage (0 to 5 V) half of the capacity of the regulating varactor lies parallel to the antenna circuit. This allows for a sensitive regulation of the resonance frequency and adjustment to 77.5 kHz. You can use a potentiometer, a trim resistor or a digital PWM to adjust that. Because the varactor diodes are operated in reverse direction, no current is drawn from the diodes.

Capacity diode KV1235Z This is approximately the capacity of the KV1235Z diode versus the reverse voltage applied. As the original curve in the available datasheets looks a little bit weird, I have interpolated it with a polynome (see the calculation sheet "FET-RX" in the LibreOffice Calc file. So do not expect this to be correct.

Capacity diode and AFC region This is the capacity of the varactor diode and the resonance frequency of the cross antenna versus the AFC voltage applied. Two combinations are considered here: The range that the combination of the large coil and the small fixed cap allow is fine, but the small coil with the large fixed cap covers only a very small range. Note that two of those varicap diodes are anti-parallel, so their capacity is halved.

Three varicaps parallel In this case we can apply the varicaps a little different to enlarge the range: we put three of them in parallel and reduce the fixed cap to 2.2 nF. The orange curve in the diagram shows that the range is now comparable to that with the large coil.

1.5 Properties of the cross antenna

By adding two coils with only one capacitor in the antenna circuit a phenomenon occurs that has to be accounted for in frequency adjustment: both coils have a combined inductivity as if they were one but each coil has its own additionally. This is approximately half of the combined inductivity and produces its own resonance. If the capacitor is larger, this second resonance can be reached. In the 90° case this second resonance cannot be reached because the varactor diodes do not have enough capacity. But in the 45° case, with a large inductivity and a smaller capacitor, the varactor diodes can well reach this second resonance points. In order to not stick to this second resonance point (with its single direction property) the voltage of the varactors should always start from +5V downwards, even if the signal strength is larger with the larger capacitor (e. g. when one of the coils is in perfect direction towards the transmitter).

This second resonance could have been avoided if both coils get their own (larger) capacitor. But that would make frequency adjustment via AFC more complicated because one needs two PWMs for AFC or a small fixed capacitor exactly compensating the difference of the inductivity of each coil.

Practice has shown that this antenna is very selective. While my energy saving lamp transmits at roughly 80 kHz, and with that strong signal confusing commercially available DCF77 receivers so that they do not work in less than 50 cm distance to the lamp, the cross antenna is not sensible for that. Whether can not be determined exactly, but this effect alone is a good argument for having a home-brewed receiver instead of the cheap mass ware.

The cross antenna is very insensitive to direction changes. The amplitude drop down to 0.35, when in maximum misalignment, is simply compensated by a small change in the AGC voltage that regulates the gain of the receiver.

Because I do not own a mechanical compass (and the one in my Android mobile is a useless equipment here because the term "North" is a very wide field for that equipment) I am not able to provide exact directional data on the cross two antenna versions. Sorry for that.

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