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Loop Antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K1AKI, Mar 17, 2010.

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  1. K1AKI

    K1AKI Ham Member QRZ Page

    Gentleman:
    I have an MFJ – 1788 loop antenna (I have read the manual)
    ICOM IC-756PROIII (the rig is working)
    50 feet of rg-213.
    The antenna is on a balcony. I’m hearing stations worldwide on 20m.
    Problem is that I can’t load more than 15 watts onto the loop. I have had all types of antennas but this one is strange – any ideas or suggestions would be most appreciated.
    Thanks, Fred, K1AKI /9
    :mad:
     
  2. N0SYA

    N0SYA Ham Member QRZ Page

    Hi
    Any chance you can place some ferrite cores around your coax?
     
  3. KB0EO

    KB0EO Ham Member QRZ Page

    I run a home brew 40 meter loop on 20 meters. It uses a 1/4 wave 75 ohm coax matching stub, and then directly fed with 50 ohm coax. It works like a champ...

    Is there a matching section on your loop? Have you measured the SWR? Without the matching section, my loop wouldn't load up either...
     
  4. KI4NGN

    KI4NGN Premium Subscriber QRZ Page

    What's the SWR displaying on your ProIII?

    That antenna is supposed to auto-tune and then beep when finished. Does it?

    When you press the Tune button on your ProIII...does it tune, or does it give up?
     
  5. KE5FRF

    KE5FRF Ham Member QRZ Page

    I don't think this is a full wave loop, but rather a miniature mag loop. Great for what they are, but at only 36 inches across, I'm not sure what I'd expect from one.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I don't understand what you mean.

    First of all, the automatic tuner in your PROIII should be off.

    Then, transmit a carrier from the PROIII and tune the antenna using its control box and dip for minimum SWR (which in my experience, should be about 1.0:1 at any frequency between 14 and 30 MHz, once you get the hang of this).

    Then, increase power and you should certainly be able to run 100W.

    If you have done exactly this and your rig simply won't "turn up" to more than 15W, that's a mystery.

    With this antenna, you always have to tune the antenna using its remote control box, and not use the tuner built into the Icom. The SWR dip is very sharp and the antenna needs re-peaking if you change frequency even 10-20 kHz.
     
  7. KE5FRF

    KE5FRF Ham Member QRZ Page

    Steve, I looked up the price for one of these thingies. WOW.

    What is the Q factor for such a small loop? Does having a high Q noticeably increase the performance at the point of resonance?
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It's very high, in the hundreds.

    Actually based on one that I have:

    fo = 14.200
    f2 = 14.210
    f1 = 14.190

    (20 kHz BW at -3 dB points, approximately)

    That calculates as a Q of 710.

    It's amazingly good at resonance, and amazingly bad anywhere else!
     
  9. K1AKI

    K1AKI Ham Member QRZ Page

    Thanks for the reply - the antenna tuner is off and the remote is working properly. Everything seems to be working. I have an FT-897 which also won't load ??
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    What are the symptoms of "won't load?" You crank up the Power control and it just stops at 15 Watts?

    What kind of power supply are you using?

    Possibly RF from the loop is getting into the power supply and causing a problem -- or something like that.

    I have one of the MFJ Loops and have only used it portable (generally with battery power) but have never had a problem like this.
     
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