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Solder Joints in Transmitting Loop Antennas
On 11/5/2015 12:35 AM, Jeff Liebermann wrote:
On Wed, 4 Nov 2015 23:50:38 -0500, rickman wrote: 2. L1 and L2 were over coupled. I reduced the coupling from 1 to 0.02. I intentionally did NOT overlap the resonant peaks so the tuning is slightly off. It's fairly close to critically coupled. Why is this over coupled? When you couple together two tuned circuits, over coupling will result in an overly broad peak (low Q) while under coupling will result in low output. The degree of coupling also has some effect on whether you see one or two peaks in case you really do want a broadband design. For a 60 KHz loop, you want it as narrow as possible, even if it means some additional loss. For a power xformer, you always want as much coupling as possible with as little stray fields leaving the transformer. However, for tuned circuits, you want whatever coupling gives you the desired bandwidth. Different goals, I guess. How do you control the coupling in the real circuit? I was planning to use a current transformer which I assume would be strongly coupled. Of course, I was minimizing C2 which resulted in a high frequency second peak far above the 60 kHz peak. I don't recall seeing a poor Q in the circuit. Q is useful to minimize any nearby interference, but otherwise my concern is max signal strength to get enough signal to be detected by the crude FPGA comparator input. 3. Adjusted C1 and C2 for 60 KHz tuning. 4. Change frequency axis (.ac) parameters. I like to have a major tick at the frequency I am interested in, 60 kHz in this case. So, add it. I spent about 15 minutes (mostly tuning L1 and L2) making the changes and left out all kinds of goodies that would be nice. Title block info, formatting L3/L4 to look like an xformer, etc. I also didn't do a sanity check on any of the components. However, in this case I can't help. I don't know how to add a frequency marker and couldn't find any clues with Google. You have to add a cursor which reads out in a small window, (and may not show up in screen captures, can't recall) or you can do a measurement... which you removed. 5. I got lazy and didn't add the usual title block stuff. 6. There are no values for Rs which needs to be considered. What is Rs, the loss resistance? Yes. Hmmm, this must have been an older copy, Yep, it appears to be missing some things. I am sure I included that, possibly in one of the coils since that is what it is from. I'm not sure I included radiation resistance as I barely knew what that was. I recall someone said it should be in there and gave me a rough value which was very small. I now understand it better and the calculated number is 2.669E-010 ohms, so obviously it can be totally ignored. L2 has Rs=7 ohms. L3 has Rs=0.325 ohms. I think both are rather high for a 60 KHz loop. The other coils have no value for Rs. Uh, high or not, that is the circuit I was simulating. 50 feet of RG-6 coax, solid copper inner conductor and a current transformer I don't have a part number for off the top of my head. I was looking for the turns ratio to give the optimum output voltage from the current transformer giving the load circuit. I'm not sure the simulation would provide that given the strong dependance on Q which can be affected by many unplanned effects. I have already built a frame for 8 turns of coax, but am thinking more would be better to increase the voltage and Q. BTW, it is hard to get much lower on the resistance (skin depth = 0.266 mm) so the Q is about as high as you can get unless you use *much* bigger wire or tubing or add lots more turns. Since adding turns boosts the signal strength I think that is better than the more exotic types of conductors that are required for transmitting loops. Remember, the absolute resistance isn't important, it's the ratio of inductive impedance to resistance. When I do an antenna, I usually have the design running in 4NEC2, which provides me with various parameters including radiation resistance, efficiency, etc. I don't know what a sane number would be for a 60 KHz loop, but can probably find a WWVB antenna model that would give a ballpark value. (However, not now). Most WWVB antennas are ferrite loops. Good luck. My real circuit had some other components at the output that complicate the real circuit. The "receiver" is an FPGA with a very high input impedance. To bias the input to the threshold of the input there is an output of the quantized value which is filtered by an RC circuit and used to bias the other side of the CT secondary rather than grounding it. I haven't decided on the exact circuit for the digital side. High impedance means high voltages. If you use a realistic value for the input voltage instead of 1, it will show if you're going to overload your FPGA A/D converter or whatever you're using for input. Uh, I seriously doubt I can overload the input with just an antenna no matter how well it is constructed. Look at the E field for the WWVB transmitter and you will see it is very marginal receiving it at all on the east coast. I just want to fix a couple of typos in the formulas I posted earlier for my own benefit. These help me see what is going on. L ∝ r * ln(r) * N² R ∝ r * N Q ∝ N * ln(r) V ∝ r² * N * Q * ln(r) V ∝ r² * N² * ln(r) V ∝ (r * N)² * ln(r) l ∝ r * N V ∝ l² * ln(r) V = voltage l = wire total length L = inductance R = resistance r = radius of loop N = number of turns Q = quality factor -- Rick |
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