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Tuning of magnetic loop antennas

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by I0PNM, Oct 1, 2013.

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  1. I0PNM

    I0PNM Ham Member QRZ Page

    I found some problems in the antenna tuning, mostly at higher frequency bands such as 21 or 28 MHz where the needed tuning capacitor has a small values ( from 30 to 10 pF ) i.e.quite close to that of the loop antenna distribuited capacity. I guess the effectiveness of the tuning capacitor is erased by the distribute capacity.
    Please some suggestion to overcome this problem.
    Thanks a lot
    Best 73 Max I0PNM
     
  2. IZ3NYT

    IZ3NYT Ham Member QRZ Page

    HI Max,
    can you tell me the diameter of the loop and Also the diameter of the tube, so I can tell you more on the various values​​.
    73
     
  3. KO6WB

    KO6WB Premium Subscriber QRZ Page

    I run a magnetic loop that tunes from below 40 meters to above 10 meters.
    Never have run into a problem tuning it to any band it'll cover.
    What problems do you believe the distributed capacitance is causing?
    Have fun
    73
    Gary
     
  4. KL7AJ

    KL7AJ Ham Member QRZ Page

    Hi Gary:

    The result of this is that it just won't tune above its self resonant frequency. One of the paradoxes is that a small tuned loop has to STAY small at the frequency of operation. On 28 MHz, it may be acting like a full sized loop. :)
     
  5. I0PNM

    I0PNM Ham Member QRZ Page

    Hi Andrew !I'm using 18 mm copper pipe in an octagon 1 meter in diameter loop configuration. I can't deeply tune the loop when I change the top capacitance
    73 Max
     
  6. IZ3NYT

    IZ3NYT Ham Member QRZ Page

    Hi Max,
    with your size 1m in diameter, 18 mm, these values ​​have to 21MHz:
    radiation resistance of 0.5ohm.
    you need to tune 22pf.
    28MHZ to:
    1.5ohm
    13PF to tune.
    Your loop is too small, you have a resistance to radiation too small, you should do it at least 200cm in diameter.
    with 200cm diameter have these values.
    21 -> 8ohm, 10pf to tune it.
    28 -> 23ohm 5pf to tune it.
    73 de Andrea
     
  7. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Hi Max,
    What kind of capacitor are you using to tune the loop?
    The Q of the capacitor is very important in a magnetic loop. A low Q capacitor will result in difficulty tuning the loop.
    The efficiency of the loop also decreases with a lower Q capacitor.
    A 1 meter loop should offer good efficiency when properly built.
    What shape is your loop (square, octagon, circle)?
    I am running a 750mm diameter square loop made with 28.5mm copper pipe. Total circumference is 3 meters.
    My tuning capacitor is a 3-250pfd vacuum variable. Tuning is easy on all bands from below 40 meters to above 10 meters.
    You can see the loop on my QRZ look-up page.
    The only difference is the larger vacuum variable fell and broke so a smaller unit was installed in it's place. That made it capable of tuning 10 meters.
    Hope this helps
    73
    Gary
     
  8. IZ3NYT

    IZ3NYT Ham Member QRZ Page

    Hi Gary,
    I did some calculations on your antenna, I would make my measurements.
    3.5MHZ->R=0.0008->YIELD 1.1%->C 965PF->2KV
    7.0MHZ->R=0.003->YIELD 11.2->C 241PF->4KV
    10MHZ->R=0.012->YIELD 30%->C118PF->4KV
    14MHZ->R=0.047->YIELD58%->C60PF->4KV
    18MHZ->R=0.128->YIELD77%->Q734->C36PF->4KV
    21MHZ->R=0.237->YIELD85.5%->Q510->C26PF->3KV
    24MHZ->R=0.469->YIELD91.4%->Q327->C19PF->3KV
    28MHZ->R=0.749->YIELD94.19%->Q237->C15PF->2KV
    30MHZ->R=0.987->YIELD95.38%->Q195->C13PF->2KV

    100w power

    I think the problem of MAX, both in isolation capacitor.Ad Anyway I recommend a larger loop, to have a greater resistance to irradiation.
    73
     
  9. I0PNM

    I0PNM Ham Member QRZ Page

    Thanks Gary,
    I'm using a good 300 pF Vacuum Capacitor, the antenna is 1 mt in diameter with Octagon shaping and made with 18 mm copper pipe.
    Probably I should have some improvements by using larger copper pipe diameters as well larger antenna loop diameters.
    Gary are you using Gamma match or coupled loop to feed the antenna ?
    Thanks for the help
    73 Max
     
  10. I0PNM

    I0PNM Ham Member QRZ Page

    Thanks Andrew,
    in principle I agree with you to increase the antenna diameter to have higher radiation resistance then better performances,however the Gary loop works fine even with 1 mt loop diameter and consequent low radiation resistance.
    Andrew, what copper pipe diameter are you using on your loop ? Do you have Gamma match or coupled loop to feed the antenna '
    Thanks a lot Andrew
    73 Max
     
  11. N0SYA

    N0SYA Ham Member QRZ Page

  12. IZ3NYT

    IZ3NYT Ham Member QRZ Page

    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
     
  13. KO6WB

    KO6WB Premium Subscriber QRZ Page

    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: Oct 9, 2013
  14. AA8C

    AA8C Ham Member QRZ Page

    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
     
  15. DL4TNR

    DL4TNR Ham Member QRZ Page

    Here is an interesting, Loop tuner.
    [video=youtube;vfjdwFCUAro]https://www.youtube.com/watch?v=vfjdwFCUAro[/video]
     
  16. DL4TNR

    DL4TNR Ham Member QRZ Page

    [video=youtube;j5nVVGccud8]https://www.youtube.com/watch?v=j5nVVGccud8[/video]
     
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