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Litz wire for AM ferrite Rod Antenna?

Discussion in 'Electronic Basics' started by Bill Bowden, Mar 24, 2007.

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  1. Bill Bowden

    Bill Bowden Guest

    How much improvement can I expect using Litz wire to wind a AM ferrite
    Rod antenna as opposed to using solid copper wire?

    Is it worth the trouble to obtain Litz wire, or can I expect almost
    the same response at say 1 MHz using regular solid enamaled copper

  2. MassiveProng

    MassiveProng Guest

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

    Bill Bowden Guest

    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 δ) 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. "

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

    amdx Guest

    That's the way I read it. Quite a large difference.
    And according to Table 7 using a smaller wire diameter, even with the
    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
    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.
  7. MassiveProng

    MassiveProng Guest

    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.
  8. amdx

    amdx Guest

    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.

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

    John Larkin Guest

    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.

  10. 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.
  11. So what would be a reasonable Q for the tuned antenna?
    Something close to 100, I suspect.
  12. John  Larkin

    John Larkin Guest

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

  13. John  Larkin

    John Larkin Guest

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

  14. Bill Bowden

    Bill Bowden Guest

    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.

  15. MassiveProng

    MassiveProng Guest

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

    amdx Guest

    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
    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.
  17. 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. ;-)
  18. Rich Grise

    Rich Grise Guest

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

    Have Fun!
  19. John Larkin

    John Larkin Guest

    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.

  20. Bill Bowden

    Bill Bowden Guest

    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.
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