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Measure the impedance of a loop

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

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  1. K4SAV

    K4SAV Ham Member QRZ Page

    With the transformer, as described, and 150 ft of RG-6, the SWR looks like this. A different transformer won't improve the performance. Bands are marked with yellow stripes.

    Jerry, K4SAV

    Log Z coax plus trns.jpg
     
    W1BR likes this.
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Our old pal Owen Duffy, VK2OMD, (ex-VK1OD) from Australia rarely posts to the Zed anymore, but he clearly reads the topics and often comments about the technical matters in his blog. Here he suggests a useful transformer for matching the loop-on-ground to 50 Ohms:

    "Feasibility study – loop in ground for rx only on low HF – small broadband RF transformer using medium µ ferrite core for receiving use – 50:200Ω"
    https://owenduffy.net/blog/?p=13039
     
  3. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Thank you all for all the great ideas. I will give it a shot.

    I certainly get that an antenna analyzer is not making lab grade measurements. I am more interested in relative measurements and quantifiable performance comparisons. As you know, band changes make any sort of on-air testing pretty unscientific.

    I was thinking perhaps I could set up different matching networks or transformers for use with different bands (possibly switch with relays). Of course I would have to measure some marked improvement for this to make sense.
     
  4. AI5DH

    AI5DH Ham Member QRZ Page

    KOOKS well you know me and I am going down this road. I agree matching Impedance on a non resonant LoG is pretty much a waste of time. If you are like me my Loop is 60 -feet of wire laid out in a 15 x 15 square. If I sweep the thing with my Anritsu Site Master or EXFO Analyzer tells me the antenna is resonant around 15 MHz and impedance of around 400 Ohms. However I and I believe you have no intention of operating at that high of a frequency. As for me 5 MHz and below where impedance is all over the place from a few hundred ohms up to a few K-Ohm's.

    What I have found is 4:1 ratio works best for me and I tried 2:1 and then 3:1 before settling on 4:1. Might even try 5:1 because it is so easy to modify. So experiment until you find what works best and good luck.
     
  5. AI5DH

    AI5DH Ham Member QRZ Page

    I ran across that but not sure it applies directly to what KOOKS and I are doing. Example I want to listen to LW, MW, 160, and 80 meters so the Medium u core is no good for that frequency range as it takes a high u core or 5000 mix 75 to get frequencies that low. I am running 11:44 winding ratio and may go to 11:55. Thanks for sharing.
     
  6. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Check out the other related posts in his blog
     
  7. AA5MT

    AA5MT Ham Member QRZ Page

    I built a cubicle quad at a club picnic. I had only a new analyzer made by MFJ, with no digital readings, only swr and an output jack for a frequency counter. I made a 1/2 wave coax for 2m, so that I would have an accurate reading. I built the driven loop to the formula for loops. I connected directly with coax, just like in operation. I measured the exact frequency that I was aiming at. But, the swr was 2.5:1. It hung me up. I asked several others, who scratched their heads, with no answer. I piddled around awhile and even built another loop with the same results. A couple of hours later, I give up decide to pack it up and figure it out at home. So, I built the reflector and installed it. Miracle! The swr dropped to 1.1:1 on frequency. I did not know that the load would affect the driven this much. Much later, I read an article which explained that the impedance of a single loop is 120 ohms, just what I was seeing. Live and learn.

    Tom
     
  8. K4SAV

    K4SAV Ham Member QRZ Page

    Worrying about antenna matching is not what you should be looking at. What you need to know is antenna pattern, or ultimately the resulting signal to noise ratio for whatever purpose you desire. What you may desire could be for DX use or maybe just for NVIS signals.

    Any antenna located in a quiet area is likely to outperform any other antenna located in a noisy location. That should be obvious. To understand this antenna it should be compared to another antenna, both of which are located in a quiet area. There are several other small receiving antennas that it can be compared against. Some of those are a BOG, K9AY, flag, pennant, EWE, delta, diamond, DHDL, etc.

    Comparing it to your transmit antenna doesn't tell you much other than one is better than the other. I have several transmit antennas that I can select and the noise on the low bands is different on all of those. Just using that data, I cannot determine if any of those should be classified as a good receiving antenna, or not. However since I also have some good receiving antennas, I can say that all my transmit antennas are much poorer on receiving that even the simplest common receiving antenna.

    Do the tests, then determine where to classify this antenna relative to other simple antennas in terms of performance.

    Jerry, K4SAV
     
  9. K0OKS

    K0OKS Premium Subscriber QRZ Page

    The Owen Duffy links are especially useful. I had seen one or two of them before, but had not found all his info on various loops on ground.

    I am tempted to try his 3m square buried 20mm below the surface. I would also put it in a different location and perhaps use 75 Ohm cable since I have long run of brand new direct bury rg-11 the cable guy mis-measured and left for me.
     
  10. K0OKS

    K0OKS Premium Subscriber QRZ Page

    This is wise advice. It seems the best antenna is the one with which you can hear the other party best. There are so many variables in the real world that it is hard to make direct comparisons anyway. Sometimes a poorly designed antenna will have a nice peak lobe directly towards the other part, and that point, for that QSO, that is one of the best designs.

    Part of this for me is a learning experience as well. I want to know as much as I can about a topic and explore the various sub topics in detail. Sometimes this means the equivalent of chasing a dB of S/N here or there. The fruit of these efforts may not be helpful in this instance, but I can use the knowledge learned to avoid the build up of small “mistakes” in future endeavors.

    As a programmer I like to break things down in to small, efficient, elegant functions. These functions then become part of my personal library I apply to future projects. Sometimes I spend too much time make a function called one time as efficient as possible because I know in a future project it may be called a trillion or more times on a huge dataset.

    So I may be chasing local optima for insignificant things, but they may come in handy later. And most of all... I am HAVING FUN.
     

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