Loop-On-Ground Antenna

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

ad: L-HROutlet
ad: l-rl
ad: HRDLLC-2
ad: abrind-2
ad: Left-3
ad: L-MFJ
ad: L-Geochron
ad: MessiPaoloni-1
ad: Left-2
  1. WN6F

    WN6F Ham Member QRZ Page

    OT: MLA 30+ loop test.

    On a lark, picked up one of these disposable loops for the occasional visit to the higher bands which my log is too large for. Seen all the reviews, which vary in content. Not interested in modifying because if I do that, I'll just get a W6LVP.

    Too much gain! Initially attenuated the output, but that didn't change the crappy s/n, just overall level. Got in the ballpark of usability by making sure my rig's preamp and attenuators weren't on. It took about a quarter-turn clockwise of the mla30+ preamp's gain pot to just tickle the s-meter at S1. NOW it is under control. Guess my overall noise level is pretty high, but no need to make it higher than about S1 with no activity. Otherwise it was just a noisy smoke-show which I almost tossed in anger.

    USB cable: I do use a portable battery with it, but the supplied usb cable presents what looks like an initial short when connecting. The Anker portable batts just turn off immediately. Tossed the oem cable, and used a Samsung phone cable. Tada, batt stays energized. The Anker batts seem ok with the little current drawn - whereas some other batts will time out quickly if not drawing more than 100ma current. Looks like the Anker's operate at 50ma minimum.

    Switching noise? From 160 to 10m, I'm not hearing the switching. Maybe down in BCB to VLF, but I'm not listening there.

    The funky frequency-vs-gain thing is there, but not a showstopper. Adjusting /reducing the internal gain pot to S1 on 30 meters seems to be ballpark. And now rotation actually seems effective.

    No common-mode seems to be evident, but as a precation, I did wrap the feedline with a few turns inside a ferrite a few feet away from the loop.

    Now if only 15 - 10m would open up more!
     
  2. N5YPJ

    N5YPJ QRZ Moderator QRZ Page

    Nice "spring like" SW TX day only 80F at noon so I got out to reorient the LoG feed point. The northeast corner was fed previously, today changed the feed point to the west side.
    The good: the 2 S units of noise that were present on 30 meters are now gone. The spikes every 18 KHZ on 20 mtrs (looks like a switching supply somewhere) are now gone.
    The bad: I now have about 2 S units of noise on 40 meters where there was none before.
    I have a phasing unit but do not have room to space the loops - best case scenario would be about 6 ft between loops.
    So I guess it is time to see if there isn't a way to do feed point switching or throw out another loop feed in the corner quietest on 40. Anyone have an idea how close an additional loop could be the one I have now?
     
    Last edited: Aug 5, 2021
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    I can probably explain the anti-noise directivity of a LOG with the results of a simulation:

    NEC2 is purported to be deficient in modeling very low wires.... However, Lewallen says (in the EzNec help file) that it is usable down to where a wire is ~0.001wl agl or a bit lower. 0.001wl at 7MHz is 0.13ft.

    Here is a 40m, square, horizontal, resonant, fed in the center of a side, over average dirt, wire loop that I move closer to the dirt. L is the total wire length required for resonance, and H is the height (ft), stepping down from 30ft to 0.1ft:
    upload_2021-8-8_17-22-32.png
    Row 25 is now officially the LOG! Notice what happens to the peak gain as the height H is reduced... That is a 12dB (2-S-unit) reduction between H=30ft (Skyloop) and H=0.1ft (LOG). Notice how the length of the loop is decreased to preserve resonance as H is reduced.

    What is interesting is what happens to the patterns as the loop is lowered close to earth. I separate vertical and horizontal polarization gains, and plot them separately at H=0.1ft agl (0.001wl at 7MHz is 0.13ft):
    upload_2021-8-8_17-24-47.png
    If vertically polarized RFI is arriving from the 80-100deg (or 260-280deg) azimuth sectors, then a LOG would appear to be a very good "noise cancelling" antenna, because the RFI is aligned with blue nulls. The null width for the LOG is wider in azimuth than if the loop was used broadside to RFI source with the loop perpendicular to the ground surface.

    This is borne out by looking at the elevation plot at an azimuth of 90 deg:
    upload_2021-8-8_17-28-0.png
    The red and green traces are perfectly superimposed, and there is virtually zero gain for vertical polarization (blue dot at graph origin, i.e very deep null for vertically polarized signals).

    At an azimuth range of 90deg+-15deg, (also 270deg+-15deg) there is a 2 S-unit reduction of vertically polarized radiation:

    upload_2021-8-8_17-29-29.png

    Perhaps this is why some folks report dramatic noise reduction with LOGs, while others do not. They unknowingly oriented their loop to maximize the RFI reduction.

    Knowing this, you might want to deploy your LOG such that the loop is square and the loop feedpoint is in the center of a side broadside to where you think your RFI is arriving from.
     
    Last edited: Aug 9, 2021
    WN6F, KK9W, M0AGP and 2 others like this.
  4. N5YPJ

    N5YPJ QRZ Moderator QRZ Page

    Thank you!
     
  5. M0AGP

    M0AGP Ham Member QRZ Page

    This is a really nice piece of analysis - it is consistent with the observations that a very low antenna has the most gain (least loss) for vertically polarized radiation, most man made RFI is vertically polarized and that numerous (but not all) hams find a strong improvement in SNR using the LOG.
     
  6. N5YPJ

    N5YPJ QRZ Moderator QRZ Page

    Have you found which is the better performer on 15 & 10 the shortened LoG or MLA 30+?
     
  7. WN6F

    WN6F Ham Member QRZ Page

    Wow - great analysis!

    To be sure, try different feedpoint locations with the log. The simplest is changing from one corner to the other, or trying putting the feedpoint in the middle of a side.

    One problem that I face is that I can't "relocate" my log due to space considerations. And depending on what the neighbors are doing, sometimes the wire is closer to a qrm source and can't be easily moved.

    So as not to limit myself to just one tool in my bag, an SVL (amplified or not) can be an assett. KK5JY also has numerous articles about those for both rx and tx.

    In the case of the MLA30+ I did modify it without going nutzo:

    1) Reduced the gain output at the outside loop preamp pot to be just under S1 on my receiver with band noise.

    2) Stacked two loops on top of each other vertically in the same plane to cover more area in a cross-connected fashion for the bottom loop.

    3) Discarded the usb cable that came with the kit. Something wrong, so I used a quality usb cable. They aren't all wired the same!

    Basically just took 84 inches of #14 thhn wire I had lying around, and at the mla30+ connectors, one wire crosses over the front of the amp, and the other crosses over at the back side. In essence, this is a "cross-connected co-planar" version. Less inductance. Lower noise floor. Maintains pattern as the original. Tastes great. See LZ1AQ and PA0SIM, W6STI stuff. Easy first mod! Could easily be applied to Wellbrooks, W6LVP and others without having to find super large conductors.

    Anyway, didn't want to turn this into an SVL thread, just a note on what is *also* in my bag. The LOG is still the best s/n, but that means nothing if a neighbor fires up something stupid next to those wires. A smaller SVL of your favorite flavor should be familiar in pattern.
     
    Last edited: Aug 10, 2021
    N5YPJ likes this.
  8. N6YWU

    N6YWU Ham Member QRZ Page

    That’s a useless question, not a real question. A more realistic question would involve a set of constraints. None of your comparison receive antennas fit a small (say 1000 sq ft) yard and are below fence height. Nor did you specify whether they were surrounded on 2, 3, or all 4 side by neighbors solar panels, plasma TV sets, cheap LED lighting, or broken power line transformers. So none of your comparisons are applicable to possibly a majority of QTHs.

    A more realistic comparison for a lot of people for a receive only antenna might be against an indoor small loop, or a ferrite loopstick. Try that.
     
  9. K4SAV

    K4SAV Ham Member QRZ Page

    There is absolutely nothing wrong with that statement. You will never find an antenna that works better than an antenna that works better. That is totally illogical. If you don't know how well an antenna works how do you know that another antenna might not be better?

    The LOG antenna is included in that list, but only data for 160 meters.

    I did explain what those number represent, and also the basics of how a receiving antenna works. None of that data is for performance against local noise sources from multiple directions at random distances. For people that are surrounded with close noise sources, there is no antenna that receives the signal and rejects noise form all directions and polarities. I gave some options for rejecting a single noise source from one direction. Noise arriving from more than one direction will require a lot of compromises. Although it's possible to design an antenna with multiple steerable nulls, it will be very complex and not something most people would consider practical.

    Any antenna that is pointed at a noise source is very likely to receive that noise source very well. If you don't know where the noise sources are located, then it's pretty much a try it and see what happens. Analysis won't help you. The data I gave was for atmospheric noise arriving from all directions and on 160 meters. Your are correct if your dominant noise sources are local, then it's likely info about performance against atmospheric noise may be useless to you. Trying to find an antenna that rejects all those local noise sources is also likely impossible.

    Performance data for a ferrite loop or indoor small loop emersed in local noise from all directions will yield pretty much random data. Experimental data from others indicate the problems associated with antennas that are very close to noise sources. The suggestions are always, eliminate the local sources if you can, move the antenna away from them, or add other equipment that attempts to null the worst noise source.

    If there is a common characteristic of most of the local noise sources (like low arrival angle) then an antenna that doesn't respond well to low angle signals may be useful (as long as you are don't care about DX). On the low bands a lot of vertically polarized signals arrive from a city. An antenna that is horizontally polarized could reject some of that but to get a horizontally polarized antenna to respond to DX on the low bands requires that the antenna be placed at a great height.

    Jerry, K4SAV
     
    Last edited: Aug 10, 2021
    N5YPJ likes this.
  10. WN6F

    WN6F Ham Member QRZ Page

    Jerry brings up a good point re point-sources and loops. That's great, but in the modern world, much of the qrm isn't a point source as crap is fed into powerlines and other cabling.

    For instance, my loops became much less effective when all the neighbors started pumping BPL into their powerlines to hook all their computers together. Totally killed SWL, but fortunately in the US, the model's they bought notched out +/- 100 khz away from all the amateur bands. I felt sorry for the UK hams, where apparently stuff like this didn't notch-out the amateur bands. Just one example of many..

    Used to test my loops balance and common mode by using a battery-powered drill motor (sparky!) where the trigger was duck-taped on, and placed on a chair at the other end of the back-yard. Nulling was easy. With a helper, had him run the drill motor close to my transmission lines, with me in the shack looking for hot-spots which would just peg the meter and improved my grounding / choking there - making sure I just didn't move the problem. :)

    Ah, the good old days of point-sources. Now I just rotate (or use the log) casually to attenuate local qrm to just try and get the s/n the best I can.

    Thing is, unless you're going to just hang it all up in the closet, you gotta' do *something*.
     
    SWL37632 likes this.

Share This Page

ad: hrd-2