Litz wire for AM ferrite Rod Antenna?

On 23 Mar 2007 21:50:54 -0700, "Bill Bowden" Gave us:

You are only talking microvolts/femtowatts here.

Loop antennas are what most stereo receivers use these days. Ferrite rod versions are for handheld portables.

The increase is enough that those portable radio makers use it.

Hell, just buy a cheap one or grab on at a yard sale and disassemble it. Better, just go to a military surplus store or industrial liquidator in your town.

If you would like to see some comparative experimental data, Ben Tongue has performed some experiments and posted the data to his web site.

Thanks John,

Yes, it looks like Litz wire has a significant advantage. If I read the results right, the unloaded Q factor at 943Khz is 141 using solid copper wire verses a Q factor of 1030 using Litz wire. That's quite a significant difference. Am I reading the results right?

Quote from website:

" Solid wire instead of litz?: Keep in mind that the work described here used close-wound 125/46 litz wire. If one duplicates 'Coil and Former B' in Table 2, except using 22 ga. solid copper wire (having the same diameter) as 125/46 litz, the Q values drop to about 1/6 of the values achieved with the litz wire. The cause is the large proximity effect resistive losses in the solid wire. The proximity effect, but not the skin effect loss may be much reduced if the wires are space-wound. New trade-offs now must be considered: Same wire diameter, and therefore a longer solenoid, or a smaller wire diameter and the same overall length? If one wishes to use solid wire, it should probably be wound directly on the ferrite, not on a former. The overall Q will still be much less than when using litz, but the loss from the high (tan =CE=B4) dielectric of the ferrite will be pretty well swamped out because of the now higher losses from the skin and proximity effect losses. The Q values, using a close-wound solenoid of 22 ga. solid copper wire on a polyethylene former, as in 'Coil and Former' B in Table 2 are: 520 kHz: 130, 943 kHz: 141 and 1710 kHz: 150 when using the "best core". The Q drops only 3, 3, and 5 points respectively if the "worst core" is used. "

-Bill

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I think you are interpreting this correctly. Keep in mind=20 that this is the Q of the coil, unloaded by any receiver=20 circuit. If the circuit adds a significant load, the Q=20 differences for a tuned antenna would be a smaller ratio=20 different. 22 AWG is also pretty heavy wire for a typical=20 antenna coil. With smaller wire, the Qs would be smaller=20 but closer.

y

That's the way I read it. Quite a large difference. And according to Table 7 using a smaller wire diameter, even with the higher DC resistance gives better Q. I found that interesting, I new I could get higher Q's when I spaced turns about one wire diameter, but it seem there's a little more to it. Oh, and that contrawound thing is neat to. I wonder if there is any advantage to three or four contrawound windings. Great article. Thanks, Mike

Quote from website:

Table 7: Simulation of inductor BB in FEMM at 1 MHz, with various conductor diameters (type 61 core material) Wire dia. in inches Inductance in uH Resistive losses in ohms Hysteresis losses in ohms Total losses in ohms DC resistance Q 0.02530 258.5 11.16 1.32 12.48 0.16 130.1 0.02320 259.6 8.04 1.33 9.36 0.18 174.2 0.02127 260.5 6.26 1.33 7.59 0.22 215.7 0.01951 261.1 5.13 1.34 6.47 0.28 253.7 0.01789 261.6 4.37 1.34 5.71 0.36 288.0 0.01265 263.4 2.91 1.35 4.26 0.64 388.1 0.008995 264.0 2.48 1.36 3.84 1.25 431.9 0.006300 264.4 3.02 1.36 4.38 2.62 379.7 0.008995* 264.5 2.57 1.40 3.97 1.00 418.6

Table 7 shows the benefits of space winding when using solid wire. All the inductors in Table 7 use centered have solenoids of 58 turns and a length of

1.624". The only variable is the diameter of the conductor, which controls the spacing of the turns (the winding pitch is held constant). The lesson here is that, when using solid copper wire, there can be a great Q benefit by space winding the solenoid, using an optimum size wire, in this case a Q of 431.9 vs 130.1 at 1 MHz.

On 24 Mar 2007 18:53:56 -0700, "Bill Bowden" Gave us:

Use insulated wire, and another good sub for litz is SPC (silver plated copper), as you get a slightly better skin, and the insulation gives the space winding. A twisted group of smaller SPC wires can give a slight Litz effect as well, like 7 32 ga SPC wires in teflon, or other sheathing twisted together evenly. Not true litz, but better than a single conductor. Particularly if the space winding effect are the main desire.

"MassiveProng" wrote in message news: snipped-for-privacy@4ax.com...

I'd be interested in seeing the results of that experiment. Ben's best Q is 431 using a single #31 wire. The results shown in Table 7 suggest that, getting the wires close to each other reduces Q. Twisting 7-#32 wires (with teflon) together and winding with that bundle would probably end up with no space between turns. Mike

Let the name calling begin, but try to use something new.

In a superhet, high Q will make it that much harder to track the LO, so you may well lose signal with a q=1000 rod. Why do you want a high antenna Q? In the AM band, gain is cheap and s/n is dominated by ambient noise, so it won't matter much.

John

I think the main point to keep in mind is that it is easy to throw Q away, but hard to make it if the L and C don't have it, to start with. You might want to calculate the ideal Q, and then use an antenna coil construction technique that is pretty sure to exceed that requirement.

So what would be a reasonable Q for the tuned antenna? Something close to 100, I suspect.

At 800 KHz, q=1000, the resulting audio bandwidth will be 400 Hz!

John

Or maybe a bit less... 50? Again, for a superhet, the tracking problem isn't trivial.

John

It's just a little portable AM radio I've been wanting to build for years. I took a radio class in 7th grade 50 years ago and never got around to finishing the superhet design. But I got an A anyway. We used tubes in those days.

I'm using the Signetics NE602 balanced modulator IC that produces about 13dB gain. The antenna rod is buffered with a JFET so there is minimal load on the antenna rod yielding another 12 to 18 dB. The front end is pretty hot.

But as you say, the bandwidth is narrow with a high Q coil, so I'm using a switch to short a couple turns on the antenna rod to load the antenna for local strong stations. Local/DX select.

The biggest problem is separating a distant station 40KHz away from a strong local 50KW station 5 miles up the road.

-Bill

On Sun, 25 Mar 2007 17:42:10 -0500, "amdx" Gave us:

Of course there would be space between the turns. The insulation alone edicts it. Can you really be that thick? The solid wire nests right next to itself from turn to turn because it is mag wire, dipshit.

Like your 8 blank line dumbshit, followed by yet another retarded remark after your reply?

"MassiveProng" wrote in message news: snipped-for-privacy@4ax.com...

Oh, I thought you might have read the article. The length of the windings of all coils in Table 7 is 1.624". His High Q coil has about .019" between each turn. I doubt you could even duplicate his experiment with your suggested wire. But as I said, "I'd be interested in seeing the results of that experiment."

Come on Massive, dipshit, dumbshit and retarded, is that the best you can do? Entertain us with something original. You use the same old tired names over and over. They've lost their oomph, their punch, their pizzazz.

Space intentionally left blank for new improved original name calling. Mike

I take it this is connected to pins 1 and 2? How did you do your oscillator on 6 and 7?

What are you using as an IF filter and AM detector? I've been tinkering around trying to build a 10MHz WWV receiver and I have an NE602 that I'm injecting my "PIC locked" 9.545MHz LO into (pin 6) and using a 10MHz tank circuit ( 2.8uH and ~100pF) as a preselector with the antenna coupled to the coil. I only have that much done so far, but it's time to do some filtering and detection and I'm not sure what direction to take on that. Just playing around, nothing critical.

I always wanted to build a crystal set using an oatmeal box sized coil form with a sliding tap. Maybe when my daughter gets a little older she'll wind it for me. ;-)

I've read somewhere that the idea is to build the selectivity into the IF part.

Have Fun! Rich

Yes, but you have to be careful that nothing overloads in the front-end, or in the first IF stage, from that 50 KW monster. A decent antanna Q helps some. Fets are really good for avoiding nonlinearity.

So, low-gain jfets or mosfets in the front end and maybe the first IF, and pile up selectivity and gain in later IF stages. Manual stage gain pots, rather than AGC, would be fun.

John

The oscillator is the Hartly version with a tapped coil. Pin 7 goes to the tap through a 0.1uF cap. Pin 6 goes to the high side of the coil through another 0.1uF cap. Low side of coil goes to ground and tuning cap goes across the coil.

But, I had to rewind the oscillator coil because the tap on the regular (red slug) coils is too close to one end for good oscillation. I put the tap about 1/3 the way up and used a resistor (1K) in series to get it about right.

I have 2 IF stages planned, but just using one right now. It just uses the normal (black slug) coil with a 2K secondary to drive the detector. The detector is a Schottky diode that feeds the gate of a JFET. There is a 2 Meg resistor from gate to +V to bias the diode at a few microamps and a 1000pF cap from gate to ground to filter out the RF. Audio comes off the source of the JFET.

I'm not sure exactly how this works, but it does the best job of all the other configurations I tried. The audio is good from a weak station. I have an article about using a zero bias JFET so that no diode is needed, but I haven't tried that one.

-Bill