10643	2014-12-19 19:17:22	Chris Howard w0ep	high resistance inductor
Message	Date	From	Subject
I figured I would stop theorizing and measure something. I painted a piece of #30 type A nichrome wire with automotive clear coat and wrapped 31 turns on a 50-6 toroid. My cheap dvm says 11 Ohms resistance for the whole coil. I combined it in an L-network with a 470 pF capacitor. No resonance was found between .5-11 MHz. Frequency response was a steady decline over that range. I made a blog post about the measurements I made. http://w0ep.us/OM/?p=4635
10644	2014-12-19 20:28:13	kerrypwr	Re: high resistance inductor
A crude estimate of Q; reactance at 7 MHz about 150 ohms, resistance about 11 ohms. Q = about 14. No wonder it won't resonate. Kerry VK2TIL.
10645	2014-12-20 05:23:49	Nick Kennedy	Re: high resistance inductor
A couple of things will make the Q lower. The skin effect will make the resistance of the wire quite a bit larger than its DC resistance. Also, winding the coil on a toroid introduces some additional loss due to the toroid. And the reactance at the target resonance of 4.25 MHz is about 80 ohms. Trying to measure Q in a 50 ohm system (generator and detector both assumed 50 ohms) is difficult because the loaded Q of the system is even less than the combined L & C. But you can generally spot resonance, but I wouldn't connect it the way you've shown. It's connected like an L-network. Try putting L and C in series between source and detector and see if you can see a peak. 73- Nick, WA5BDU
10646	2014-12-20 05:39:48	Hector Pascal	Re: high resistance inductor
I am confused even as to why a "resonance" is expected. An "L" network will give either a low pass, or a high pass frequency response, depending on whether it is series L / shunt C, or series C / shunt L. In either case the -3dB point of the frequency response will be the same. By simulation, using an inductance value of 3.9uH (by mini Ring Core Calculator from the details given) and 470pF, and implementing a 50 ohm measurement system like that shown in the blog, the low pass insertion loss is about 7dB. and the -3dB frequency is about 4.4MHz. Did I miss something about this "problem"? David VK6JT
10647	2014-12-20 08:26:58	Chris Howard w0ep	Re: high resistance inductor - doing it wrong
You are so right! So, I went back to the book. There's a good explanation on how to measure the Q of an LC resonator on page 7.36 (section 7.9) of my EMRFD. I should be doing that!
10648	2014-12-20 13:58:19	Ephemeral	Re: high resistance inductor
If you are looking to increase the loss in an inductor a far simpler method is to connect a resistor in series or parallel with it. Then you have independent control of reactance and loss. Generally though we don't deliberately reduce the Q of our reactors; instead we use the input and output impedances to load them for us. With an L network you only have 2 component valuess so you cannot independently control impedances and Q. Usually Q is rather low so you won't see a peak in the frequency response. If you are looking for a response with a peak (resonance) then you can use a P or T network. Then you can choose 3 component values to have independent control of impedances and Q. Usually capacitors are more practical than inductors, so you would use 2 capacitors and one inductor - a P network for low pass or a T network for high pass. I'd recommend playing with a simulator until you get the idea. CTRL + Click to follow link" target="_blank" href="mailto:emrfd@yahoogroups.com">emrfd@yahoogroups.com Subject: [emrfd] high resistance inductor I figured I would stop theorizing and measure something. I painted a piece of #30 type A nichrome wire with automotive clear coat and wrapped 31 turns on a 50-6 toroid. My cheap dvm says 11 Ohms resistance for the whole coil. I combined it in an L-network with a 470 pF capacitor. No resonance was found between .5-11 MHz. Frequency response was a steady decline over that range. I made a blog post about the measurements I made. http://w0ep.us/OM/?p=4635 \|
10649	2014-12-20 14:41:56	Chris Howard w0ep	Re: high resistance inductor -got it right
I revisited the project today. First I read EMRFD section 7.9 about measuring the Q of LC resonators. I changed my circuit to the "Series TC" as given in Fig. 7.66. I also built two similar coils, one with NiCr wire and one with Cu wire. I was able to measure a resonant point on these two new coils and on the coil from yesterday's attempt and calculate the Q using the 3 dB method. Yesterday's coil (31 turns on FT50-6 with resistance of 11 Ohms) had a Q of 9 according to my calculations. The NiCr and Cu coils today were both 4 turns on FT37-61 ferrite. NiCr yielded a Q of 30.4, Cu Q 254 difference by a factor of 8. But both were resonant at the same frequency as we all expected. I made another blog post with pictures, etc. http://w0ep.us/OM/?p=4644
10656	2014-12-20 22:53:45	kerrypwr	Re: high resistance inductor
Now do some Q measurements on the 61 material inductor over a wide range of frequencies; say 1 MHz to 100 MHz. You will find that interesting. :) Q is a very difficult measurement to make with any degree of accuracy mainly because it is the ratio of a (usually) large value to a (usually) small value but, as our mentor Wes says, any measurement is better than no measurement. I would add ... provided that we understand the limitations of that measurement. Kerry VK2TIL.