Wire Size For Homebrew Tank Coils

Discussion in 'Homebrew and Kit Projects' started by WA2FXM, Feb 6, 2017.

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  1. WA2FXM

    WA2FXM Ham Member QRZ Page

    Is there a rule of thumb for minimum wire size when winding transmitter tank coils? I'm assuming legal limit circuits require large diameter wire, and pipsqueak QRP rigs can use smaller. How small? I've got some rolls of 28, and 26 magnet wire but to me they look too small for a low power (15 to 50 watts out) rig. What gauge wire should I be looking for?

  2. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page

    There was a time when it was thought that like 1/2-inch diameter copper tubing was needed for 5-watt transmitters!

    I have the pi-network coil from an old WRL Globe Chief 90 transmitter that used a pair of 807 tubes in the final amplifier. That coil is wound with either 24 gauge or 26 gauge wire. That transmitter was rated at 90-watts input and I
    have run at least 120-watts input in the "goode olde dayes" back in the early 1960s with my first Globe Chief 90A that used the same pi-network coil.

    Therefore, I don't think you will have any problem with a 50-watt output transmitter.

    Glen, K9STH
  3. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    At legal limit, I would use copper tubing, like the Henry 2K. Probably NOT necessary to silver plate the stuff though.
  4. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page


    A lot of the commercially built "legal limit" amplifiers use like 14 gauge or 12 gauge wire for the Pi-network coil up to either the 15-meter or 10-meter section. Then they go to either copper tubing or flat copper for the last section, or 2.

    For the complete coil to be made from copper tubing, the coil is going to be pretty large and, especially when 160-meters is included in the amplifier, will not fit within the confines of a 19-inch rack panel.

    Glen, K9STH
  5. VK4FFAB

    VK4FFAB Subscriber QRZ Page

    number 26 is what 0.4mm that should be more than good enough for qrp to 50w. 50w is 1amp @ 50R impedance if my math is right, and 0.4mm solid copper wire will take 3amps.
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Don't forget circulating currents in a tank circuit at resonance.

    As a rule of thumb, circulating currents are approximately equal to DC device current (e.g. plate current, collector current) multiplied by the loaded Q of the tank circuit which is usually in the 10 to 20 range. IOW, circulating currents can be 10 to 20 times higher than the DC device current. And of course DC device current must be calculated at the impedance seen by the device which may not be 50 ohms depending on the circuit and is a function of input power which is quite a bit higher than rated output power assuming 60% class AB amp efficiency.

    I'd use something larger than 26 gauge wire for winding the tank coil in a 50 watt transmitter both from current handling and coil Q perspectives. A lot of commercial 100 watt transmitters use 14 gauge wire for tank coils, I'd probably use that or perhaps 16 to 18 gauge wire if there was some compelling reason to make the coil smaller or lighter.
  7. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page

    Collins used like 20 gauge wire for all but the 10-meter section of the Pi-networks in the KWM-2- series and 32S- series transmitters. The wire is tinned but not silver plated. The 10-meter coil is 14 gauge.

    Glen, K9STH
  8. N1OOQ

    N1OOQ Ham Member QRZ Page

    Well I homebrewed an 18W amp for my QRP rig a few years ago... I think all the coils were 24 ga magnet wire. It worked fine. :D
  9. WA2FXM

    WA2FXM Ham Member QRZ Page

    Ok, 36 turns #22 for 30uH. Going to make it work on 60 meters with a 2E26.


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