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Thread: Tuning of magnetic loop antennas

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  1. #11
    Join Date
    Jan 2002
    Location
    harms way
    Posts
    13,615

  2. Default

    Hi Max,
    I calculated the antenna of the friend Gary,
    with its diameter, there is attention on variable high voltage.
    It's a little small but it works, greetings
    73 de andrew

  3. #13
    Join Date
    Feb 2004
    Location
    Bakersfield, CA
    Posts
    4,653

    Default

    Hello Max, you can see my loop that I'm presently using, with a different vacuum variable capacitor, on my look-up page here on QRZ.
    I use a Gamma match system with fairly good results. Got tried of the screwdriver motor drive and replaced it with a stepper motor drive.
    Cost about $20.00 to get the stepper electronics exciter and an additional H-Bridge for the final drive to the stepper motor.
    Wire was an interesting story, later on that.....
    Found through experience that certain things near a magnetic loop are undesirable.
    On the 10 foot 80-40 meter loop I wanted to raise the loop so I purchased two 20 foot fiberglass beams. These were proudly advertised as RF transparent.
    They weren't and the loops tuning went far to the low end of the spectrum missing 40 meters entirely.
    Just had the same experience with the drive belt on my stepper drive. It has a fiberglass core and it definitely affect the loop. Didn't think that tiny thing would but it did.
    Removed it and replaced it with a Kevlar core belt and the Q of the loop improved dramatically. To the point where there was zero effect from the drive or the belt.
    So, you have to be careful with just about anything in the loops field.
    A larger loop works better but how much better is up for questions. You may be okay sacrificing a few DB's to obtain the coverage you want.
    Plan on making a new loop soon.
    Have fun
    73
    Gary

    P.S. An octagon has multiple joints that if they are not done perfectly will introduce additional losses. A complete circle at 1 meter is easily done and there are no joints to have losses in.
    Of course I didn't do that either but its an idea you may want to consider.
    If you can connect a dropping resistor from the shack power supply. Enough to provide about an amp or two through a short circuit. Connect that in series with your loop and have a current meter
    in series with that. Turn on the power supply, measure the current flowing and the voltage across the loop. This will give you the E/I=R and that should equal the additional losses from the materials making up the loop.
    Mine measured in at 650 uOhms. There were more losses through the Q of the capacitor then the resistance of the loop.
    Last edited by KO6WB; 10-09-2013 at 01:35 AM.

  4. #14

    Default

    I think the original question was how to get the loop to tune up on 10m.

    Make the loop a little smaller. As it is the loop is a bit longer than 0.25 wavelength in circumference and is no longer working as a pure magnetic loop. It has begun to act like a dipole. Regardless, reduce the circumference of the loop, the inductance decreases and the capacitance required to resonate it at 10m will increase. Ideally for 10m you want a magnetic loop that is about half a meter in diameter. What you have will work, it is just not a "pristine" magnetic loop.


    good luck

    Glen
    AA8C

  5. #15

    Default

    Here is an interesting, Loop tuner.

  6. #16

    Default


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