WWVB Loopstick Antenna & C-Max Receiver
© Brooke Clarke,2003 - 2007 7.5" rod |
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 2.5" rod |
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I wanted to make a loop antenna for receiving WWVB based on the Amidon R33-050-0750 Ferrite rod. This uses the type 33 material and is 1/2" diameter by 7.5" long. After trying a number of different windings I think the current version may be optimum for WWVB at 60 kHz. It's bank wound using 2 layers of 5x36x44 Litz wire.
Bank Winding
The following explanation of "Bank Winding" is for a 4 layer coil.
"Bank Wound" means that you start the winding at the left end, then make 4 turns then wind backwards for 3 turns then forward for 2 turns and then backward for one turn. Now there are 4 layers of wire all at the left end. This process is repeated until the length of the rod is covered. The important fact about this type of winding is that the electrical distance from any one turn to it's immediate neighboring turn is is just a few turns. If the coil was wound a layer at a time, then the electrical distance between turns may be hundreds of turns. The self capacity of the completed coil depends on both the capacity between adjacent turns and also the voltage between the turns.
If you look at the equivalent circuit for a single layer coil where the turns are spaced about one wire diameter it can easily be seen the the total coil voltage is divided so that 1/N of the voltage is across each turn-to-turn capacitance. That's to say the capactance is very closd to the capactance of the first to the last turn. But if you added a second layer then the two coil terminals are at the same end and the turn-to-turn capacitance for the turns adjacent to the two terminals has 100% of the coil voltage across it. The next turns away from the terminals has (N-1)/N of the full coil voltage, etc. This results in a huge self capacity.
Litz Wire
There are a number of effects that cause the A.C. resistance of a wire to be higher than the D.C. resistance.
Litzenhardt wire is made up of a number of independent conductors that are insulated from each other. There are a large number ways that the strands can be arranged.Skin Effect
In a straight wire, with no other wires near by, as the frequency increases the electrons carrying the current move away from each other and so end up more and more near the surface of the wire and less and less along the centerline.
Proximity Effect
Is similar to Skin Effect, but is caused by the interaction between wires that are near to each other.
The wire I used is made from AWG number 44 wires that are in bundles of 36 wires and there are 5 of these bundles in the final wire. If you needed a cable with 180 seperate wires you could use this Litz wire that way. Since a single # 44 wire has a diameter of 0.00198" the total area of the 180 wires is equivalent to a solid wire of diameter 0.02656", or between AWG 21 and 22.
The problem is the insulation takes up space that could be used for copper. The result is that Litz wire typically only works well below a few MHz. It's interesting that at mains power frequencies (20, 50 or 60 Hz) the largest size single strand wire you can buy is determined by skin effect. If a load requires more square mills of copper area then you need to use multiple insulated conductors.
MWS - Litz tool - input the AWG size to get the cross sectional area and it tells you the available strand count and wire sizes.
The following plots were made using the HP (Agilent) 4395A in the Network Analyzer mode and using the Z Transform technique where the unknown device is placed in series with the center conductor.
Imaginary Part of Impedance Plot
The marker shows 1.004 k Ohms at 32.960891 kHz. The network analyzer has already separated the real and imaginary parts so this is pure inductance.
L = X / (2 * PI * F) = 4.83 mH
Real Part of Impedance Plot
You can see that at 100 Hz the real part of the impedance is about 1 Ohm. i.e. the DC copper resistance. The marker shows the resistance has climbed to 29.325 Ohms at 60 kHz.
Without making a plot of "Q" vs. frequency and analyzing the slope it's hard to say what's causing the resistance. It may be skin effect, proximity effect, or dielectric losses or some combination of these.
Resonated with Mica caps
Marker shows 94.308 dB at 62.485345 kHz.
Ohms = 10 ^ (dB/20) = 51, 927 Ohms.
The shunt capacitance is given by:
C = 1 / [ L * (2 * PI * F)^2] = 1.34 nF
The HP 4332 LCR meter measured the caps at 1.35 nF.
The loading capacitance value was chosen so that when the coil, caps and a small variable cap and the input capacitance of an amplifier are connected the resonance point can be brought right on 60.0 kHz.
The Q = XL / R = 51,927 / 32.96 = 1,575
A similar plot of just the coil shows that it's self resonance frequency is very close to 500 kHz. That's good in that you don't want to run a high Q coil near it's self reasonant frequency and here we are well away from it.
This is the 05309 hits since May 07 2007 time this page has been accessed since 22 Sep. 2003.
