Measure the impedance of a loop

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K0OKS, Aug 9, 2018.

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  1. K0OKS

    K0OKS Premium Subscriber QRZ Page

    So We have this thread going on the Loop on Ground antenna here:

    https://forums.qrz.com/index.php?threads/loop-on-ground-antenna.622669

    It is not necessary to read that thread to answer this so I am starting a new thread.

    I am curious how to specifically test the impedance of my Loop on Ground. It has a transformer to convert from a theoretical 470 Ohm to 50 Ohm. But how can I use my coax based AA220Zoom antenna analyzer to measure the impedance of a loop antenna WITHOUT any transformer?

    I'd like to know the actual impedance of the loop so I can properly produce the transformer to convert it to 50 Ohm.

    Thoughts?

    Thanks.
     
  2. K0OKS

    K0OKS Premium Subscriber QRZ Page

    In case I did not make my intentions clear...

    I want to know how to physically connect my antenna analyzer which expects 50 Ohm input (N, but I have a SO-239 and a BNC adapter) to the loop in order to measure its impedance. Can I just connect a BNC to alligator clip cable, which is 50 Ohms up to the alligator clips?

    Thanks in advance .
     
  3. VK2TIL

    VK2TIL Ham Member QRZ Page

    I consulted the manual for your analyser and learned that it is capable of open-short-load (OSL) calibration.

    A short length of co-ax with an N connector on one end and two clips on the other would do; use adaptors if you must, this is certainly not a precision measurement so an adaptor or two will do no harm.

    In fact, the co-ax may be unnecessary if the loop terminals are accessible; use a couple of short wires with a N on one end and clips on the other. (I did say that this is not a precision measurement : ) ).

    Loops, like any high-impedance device, can be susceptible to the presence of test instruments or human bodies so a short co-ax line might give better results.

    You can work this out; it's part of the experiment.

    Calibrate the instrument at the clips.

    For Open calibration, separate them by an inch or so; for Short, clip them together; for Load, connect the best non-inductive resistor, with the shortest-possible leads, that you can find. A common metal-film resistor will do, although it usually has a little inductance but, as I said earlier, this is not a precise measurement, but it is adequate for the task.

    What calibration does is tell the instrument that it must compare whatever it "sees" at the calibration plane (the ends of the clips in this case) to its 50-ohm Zo (Zo = 50 +/-j0); if you do not calibrate, the line length, however short, between the instrument N-connector and the antenna or other test device (DUT) will transform the DUT impedance and the measured impedance will be in error, often substantially.

    The line would not transform if it were a Zo line terminated at each end by Zo but that is not the situation here.

    Calibration effectively "removes" the effect of the line from the measurement.

    Once you have calibrated the test setup, measure away. If the DUT impedance is a very long way from Zo, the results may not be very accurate; it is in the nature of these instruments, even high-end VNAs, that accuracy decreases with distance from Z0 but I believe that any measurement is better than total ignorance, as long as we understand the limitations of the measurement.

    .
     
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Frankly, I think you'll find it's a waste of time trying to measure the impedance . . .

    Aren't you using this Loop on several different bands?

    The impedance will vary drastically from one band to another . . . so you would need to use a different matching network for each band to make it look like 50 ohms.

    BUT most people simply don't bother, as it's just a receiving antenna. Signals will be way down already, so additional loss due to mis-match is irrelevant.

    Roger G3YRO
     
    AK5B and K5RCD like this.
  5. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Yes. As VK2TIL said, you can calibrate it at the clips if you like. I think the short test lead will make little practical difference at the HF frequencies you are probably interested in.

    I made a small test clip adapter for my antenna analyzer by soldering two short wires with alligator clips to a PL-259.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    If you put the analyzer on an insulating surface, with no usb cable connected, getting your hands and body away after commencing the scan, it will give you a reasonable measurement...

    I have an AA600 and it works well on the lower HF bands using a type-N male to clips with wires about 2" long even without re-calibration.
     
    AI3V likes this.
  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    I am with G3YRO in this.

    The impedance of the loop will vary wildly over its
    frequency range, starting with a very small real part in series with an increasing inductive imaginary value, the impedance will pass through a high value and then change sign and become capacitive.

    In the long run the impedance will converge around a value of a few hundred ohms, but then you are far outside the practical frequency range.

    A practical impedance-matching circuit for the low part of the frequency range could be some kind of feedback amplifier with a quite high impedance so the voltage division of the source EMF between the imaginary part of the loop impedance and the amplifier input becomes as small as possible.

    For the whole frequency range, a compromise must be made.

    73/
    Karl-Arne
    SM0AOM
     
  8. AI3V

    AI3V Ham Member QRZ Page

    There is also the field expedient method to hook up the log, tune in a nice steady signal, then add your impedance matching device and see if signals get stronger or weaker.

    Fiddle with the turns, both number and ratio.

    Keep a log of how your log tunes :)

    Rege
     
    AK5B likes this.
  9. W1VT

    W1VT Ham Member QRZ Page

    I would just experiment with different transformers via trial and error to find one which one gives the best match to 50 ohms on the SWR analyzer. When you have one that works you can then measure that transformer to calculate the feedpoint impedance.

    Zak W1VT
     
  10. K4SAV

    K4SAV Ham Member QRZ Page

    It should look similar to the figure below. Your ground may be different from mine so there may be some difference. Small changes in height above the dirt should make a significant difference. No transformer or coax was included for this plot.

    Connect the wires of the loop directly to the analyzer. Don't touch the analyzer while it is making the plot.

    Jerry, K4SAV

    Ant Z.jpg
     
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