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Loop-On-Ground Antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AI5DH, Aug 1, 2018.

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  1. K4SAV

    K4SAV Ham Member QRZ Page

    The power lines are loaded with all kinds of noise. That means it's on your radio chassis and the shields of all your cables. That's the reason people with end fed antennas have so many problems with noise. If you send that noise up the coax to the antenna and it reaches the antenna, it can be converted from common mode noise to differential noise and you will hear it. That's the reason you need to choke off all that noise before it reaches the antenna. Noise on coax shields at the rig end usually isn't a problem because it can be prevented from entering the signal path by good connections and shielding.

    If you have ever traced powerline noise on 160 you will probably find that long straight HV powerlines serve as a long wire antenna and the signal radiates down that line very well. A big noise source on the line can be heard for miles in the direction the line runs.

    As an experiment, just connect a long wire, maybe 50 ft or so, to the center conductor of your radio antenna connector. The other half of your antenna now will be that noisy powerline. Tune around and see what you find. I can hear many S9 plus signals in places where they shouldn't be (in ham bands), and also everything in the house, including any light switch that is flipped. That powerline contains nasty stuff. I don't hear any of that on my normal antennas.

    It doesn't require a big noise signal to be a problem if you have a direct connection between that noise source and your receiver antenna input.

    Since the main subject is receiving antennas, most of them are very low gain and that means you need even more common mode rejection.

    Jerry, K4SAV
     
    Last edited: Aug 11, 2021
    N0TZU, W1PEP, N5YPJ and 1 other person like this.
  2. SWL37632

    SWL37632 QRZ Member

    All of the above.

    Chasing powerline based RFI a few years ago with the local power co "Interference Specialist" showed that an arcing insulator a mile away can set-up 'standing waves' that are powerline length dependant and show up periodically....in this case 50 feet away from my QTH and periodically there of on the powerline.

    My neighbor's stuff is conducted on the common powerline drop that feeds our common electrical branch into my panel....as well, the stuff is radiated to my antennas.
     
    Last edited: Aug 11, 2021
    N5YPJ likes this.
  3. SWL37632

    SWL37632 QRZ Member

    Ditto....what Jerry wrote..
     
  4. ULTRAVISTA

    ULTRAVISTA QRZ Member

    What is a recommended LOG length for LF/VLF? My 60 (15x15 square) foot LOG is not receiving anything < BC AM band. A Pixel loop and PA0RDT mini whip are capable of receiving NDBs and WWVB < BC AM but the loop does not.
     
  5. K4SAV

    K4SAV Ham Member QRZ Page

    A LOG is a very low gain antenna. You need a good low noise preamp to hear any weak signals, especially on the low bands. If you are only using your receiver built in preamp, likely you will only be able to hear the strongest broadcast stations.

    On 1.85 MHz the gain is -38.3 dBi at 45 degrees elevation and -40.9 dBi at 20 degrees elevation.
    On 1 MHz the gain is -48.6 dBi at 45 degrees elevation and -51.6 dBi at 20 degrees elevation.
    On 500 kHz the gain is -80.9 dBi at 45 degrees elevation and -64.3 dBi at 20 degrees elevation.

    If you are trying to receive WWVB on 60 kHz the antenna gain is less than -100 dBi.

    You could scale the antenna to a larger size to get more gain, but the best choice would be to use a different antenna.

    Jerry, K4SAV
     
    W9IQ likes this.
  6. ULTRAVISTA

    ULTRAVISTA QRZ Member

    I did scale the LOG to about 150 feet and irregularly shaped. Just what I had of a wire bundle, made to fit without cutting.

    With a direct connect to 75 ohm coax, I am pickup a local NDB, 415 khz. It is faint, but visible in the waterfall. When connected with a 1:9 balun, the signal disappears. Moving back to my smaller loop, it too was able to pickup 415 khz, but much weaker than the larger loop.

    The larger loop certainly helped, but it is strung around the backyard in an odd way, not sure I would leave this as-is since it doesn't appear to have much of an advantage over the smaller 60 foot loop.

    The direct connect did make a difference. Not sure why; I don't comprehend antenna theory.
     
  7. K4SAV

    K4SAV Ham Member QRZ Page

    The gain of a square LOG on 415 kHz is about -53.8 dBi at 45 degrees elevation. So you still need a very good low noise high gain preamp to hear anything with significant signal levels.

    A voltage mode balun shouldn't make much difference since coax loss at this frequency should be insignificant. A current mode choke or current mode balun should be useful to reduce common mode pickup. SWR of a 150 ft loop at 415 kHz is going to be over 100 to 1, no matter what impedance ratio the balun has. Resistive part of the antenna impedance is very low and the reactance is very high. You would need to correct that reactance and then have the right ratio transformer to achieve a low SWR . Likely the balun you have is very lossy on 415 kHz especially when operating at SWR levels above 100 to 1.

    Jerry, K4SAV
     
  8. N5YPJ

    N5YPJ QRZ Moderator QRZ Page

    A serious noise problem I can't seem to get resolved and a 75 X 130 ft city lot with a house in the middle have me using a pair of magnetic loops, then I built a 15X15 LoG. The mag loops usually outdo the LoG 80 - 30, on 20 meters it is kind of a toss up and on 17 meters the LoG wins. I have worked some pretty nice DX on 17 using it - propagation and a good radio don't hurt :):). My 17 / 20 meter antenna is a Mosley Pro 57B at 33 ft up but noise level is S-9/+10. 15 thru 10 mtrs usable.

    I'm going to try a couple of 102" whips spaced 16.5 ft apart connected to the NCC-2 phasing unit to see if it does better than mag loops and LoG. Yeah if I could just get the noise problem located and fixed :mad:
     
    Last edited: Sep 19, 2021

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