Loop Antenna

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

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  1. KI4NGN

    KI4NGN Premium Subscriber QRZ Page

    If tuning is off, does the 756 "assume" a 50 ohm load?
     
  2. WF7A

    WF7A Subscriber QRZ Page


    I, too, Fred, am a little bewildered; what happens when you load more than 15W into it? Does your SWR go sub-orbital or...?

    I've used motorized loops (AEA's and MFJ's) and didn't have any problems as long as:

    A) As suggested earlier, you don't use a radio's autotune function or antenna tuner;

    B) All the internal and external contacts are clean (i.e., not corroded);

    C) There's no backlash in the internal gearing; even the minutest backlash (and often rapid temperature change) would cause the SWR to fluctuate or go into orbit between the time you "nailed it" and when you transmit.

    As an aside, I used to use a Tuner-Tuner with the loops--that made it easy to tune to a minimum SWR (assuming 1 through 3 above were followed.) However, any way you look at it you need to have patience when tuning high-Q loops; if you're a contester or like to hop frequencies often then you'll have your patience sorely tested.
     
  3. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    You may want to check your math.
    That calculates to a Q of about 10.5 (f sub 0 / delta f @ 3db) with 14.210 being f sub 0 (center frequency).

    Since it is a loop you could treat it as an unloaded tuned circuit with a Q of 1000 since we don't know what the actual input resistance is. Then it's simply 1000/20khz or a Q of 50. Factoring in the circuit resistance (whatever it is) under load will modify the Q and it will again be somewhere near 10.
     
  4. KA5S

    KA5S Ham Member QRZ Page

    From Lionel Loudet's page on SID monitoring comes a very good Magnetic Loop Antenna Theory page:

    K0BG says, writing about antenna efficiency,
    And FWIW, the -3dB and 2:1 SWR formulas includes the effect of losses already, as losses lower Q and widen the bandwidth.

    I had one of these MFJ's and it worked at 100 watts. It had other problems, but that could be chalked up to "economizering" on the design.

    Cortland
    KA5S
     
  5. K8JD

    K8JD Ham Member QRZ Page

    I think the post was about the tiny 3 ft dia magnetic loop MFJ sells with a matching tuning box. Not anything like a fullwave loop.
    BTW, How can you use a 40M loop on 20M if it is matched with the 75 ohm quarterwave section ? the SWR must be off the scale ! My loops only work one band with the coax feed system ! They work well that way.
     
  6. K1AKI

    K1AKI Ham Member QRZ Page

    Now I know what to expect.

    Thanks, for the reply. The problem is the loop turner not working. MFJ seems to be not responding to my problem. This is my first experience with the company. And so far, it's not a pleasant one. I think I'll build my own loop.
    Thanks, Fred K1AKI /9
     
  7. K1AKI

    K1AKI Ham Member QRZ Page

    Thanks for the reply - The turner for the loop isn't working and MFJ isn't responding. ANy specs for the loop you use?

    Thanks, Fred K1AKI
     
  8. W7LPN

    W7LPN Ham Member QRZ Page

    FT-897 & 857

    These radios will immediately power down if they don't "Like" the swr or sense heat in the amplifier circuit. As stated before the resonant point will be very narrow and if you are tuning too quickly you'll miss it. It will dip and pop right back up. This is typical of any high Q antennas. IMO Hope you don't let the smoke out! :cool:
     
  9. W7LPN

    W7LPN Ham Member QRZ Page

    Skywire loop

    I wish I had room for one of these. Scroll down to "Skywire".
     
  10. WF7A

    WF7A Subscriber QRZ Page

    That's nothing new, unfortunately--MFJ has an international reputation...and I don't mean a positive one!

    If you do build your own motorized loop, Fred, the most critical part will be the tuning capacitor--it must have low-loss internal connections (and external ones to the loop itself) because there's a HUGE amount of current circulating through a loop so any resistive loss between parts/components/wipers/contacts also becomes huge as well. A well-made vacuum variable capacitor fits the bill nicely, but they're pricey. Weld, instead of nut-and-bolt, as much of the loop as you can to minimize the resistive losses.

    As for turning the capacitor, I'd go with a worm-drive configuration instead of a spur-gear arrangement--you'll have much finer control of turning the capacitor that way, which--as you know--is critical when tuning small loops.
     
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