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
     
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