Type of wire used for shunt coil; bare/insulated? Alan, white courtesy phone please

Discussion in 'Mobile Radio Systems' started by AK5B, Nov 8, 2014.

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  1. AK5B

    AK5B Ham Member QRZ Page

    I'm going to be making up a shunt coil for my Hi-Q 6/80 antenna soon. While I know I could order some Thermalese enameled wire from Amidon I already have a lot of 14 ga. THHN insulated solid copper wire on hand. I could easily make up some 7-9 turn coils with this (either stripped or with the insulation left on) which would save me ordering something I might not really need to buy.

    I will be measuring the reactance as outlined in Alan's website tutorial using a RigExpert analyzer and a short piece of coax (with a ferrite choke in place) out in the clear so my readings should be accurate enough---but I was wondering if the coil's type of wire used will be of any particular consequence?

    Would bare be better and is the Amidon wire easier to work with than the THHN?

    73, Jeff
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    The matching coil is Far less critical than the loading coil...the currents are far less, as is the Q. 14 ga is fine.
  3. K0BG

    K0BG Ham Member QRZ Page

    I use #14 hard drawn copper, and just about any motor rewind shop will have enough scrape around to wind a bunch of them. Amidon, HRO, DXE, and others also sell the wire. You can use regular old building wire, but the fact it has insulation on it will make the band width a bit less.

    As Eric points out, the Q isn't too important, but should be about as much as the loading coil Q, if you want the match to be linear over several octaves.
  4. AK5B

    AK5B Ham Member QRZ Page

    OK, thanks guys; I'll try using bare 14 ga. for starters and mount the coil off to the side of the hefty Breedlove insulator at the base (feedpoint).

    73, Jeff

    PS: Alan, I also forgot to ask you earlier about the need or reason for using only stranded wire when winding a choke; is it because it's a bit easier to conform or does it have to do with inductance or skin effect? I did find the correct 18-2 wire you specify at Home Depot but am still curious why solid won't do---thanks!
    Last edited: Nov 8, 2014
  5. K0BG

    K0BG Ham Member QRZ Page

    Mainly because the application is in a mobile scenario—vibration is other words. The reason for the insulation is thickness. If you use ordinary automobile wire, like that found at AutoZone, the insulation is too thick, and you can't wind enough turns.

    If you do all of the calculations, the impedance of the choke is perhaps a bit of overkill. The problem is, I can't judge how folks install their antennas, and in too many cases the method used ends up having excessive ground loss. The more ground loss there is, the more RF imposed on the control leads and the coax cable. Hence, I can easily justify the overkill. And, after all, the difference is effort is small, and it sure beats rewinding the choke when you discover it wasn't adequate in the first place.
  6. AK5B

    AK5B Ham Member QRZ Page

    Roger that, thanks!
  7. KK4YDR

    KK4YDR XML Subscriber QRZ Page

    Good info and Alan thanks for your help yesterday. Ill be sending some sound clips to you once my visitors leave.
  8. KC7YRA

    KC7YRA Ham Member QRZ Page

    It's already been well answered, but here is my experience.

    I typically use 12ga solid purely for survival from ice and other environmental factors.

    I once made a shunt coil from 4ga solid copper in use on a heavy off road vehicle. It was designed to survive 6+ inch radial ice, or mud caked onto it.

    I worked perfectly (trying to find a pic), until it got so covered in mud the entire base of the antenna looked like a bowling ball. It changed the tuning so drastically that it was useless.

    Smaller gauge coil mounted in a shielded location worked much better.

  9. KH6AQ

    KH6AQ Ham Member QRZ Page

    Minimum VSWR and zero reactance coincide at the antenna but not along a length of transmission line (coax) that is not of zero length or an integer multiple of 90 degrees. Your VSWR readings will not change with line length and therefore VSWR is the thing to tune the antenna with, not zero reactance. To that end I would install the radio and coax - the final installation - and adjust the shunt coil and antenna for minimum VSWR as shown on the transceiver VSWR meter.

    Using a short length of coax and a balun (that is not included in the final installation) to tune the antenna adds two installation steps and cost while resulting in less than optimum tuning.
    Last edited: Nov 9, 2014
  10. K0BG

    K0BG Ham Member QRZ Page

    Well, David, think about this. The letter V in VSWR stands for voltage. If you measure the voltage along a transmission line, is changes between its peak, and zero volts. It is, after all, a sine wave.

    And, it isn't the voltage, or the current for that matter, that you have to be concerned with. Rather, it is the reactance—in this case hopefully zero—and the resistive value we need to know in order to adjust the matching coil correctly.
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