Loop-On-Ground Antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF5LJW, Aug 1, 2018.

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  1. N3PM

    N3PM Ham Member QRZ Page

    Try the LOG into the regular co-ax connector on the radio.
    Check the connectors and adapters. Some SMA hardware reverses the pin and shell (has an R in the prefix or suffix) Some imports are just plain bad.
    Try the LOG into another receiver.
    Mike N3PM
     
  2. K4SAV

    K4SAV Ham Member QRZ Page

    Sounds like you have a problem of some kind. The gain of the LOG is very low but you should still be hearing more signals than you are describing. The gain is approximately 160 = -44 dBi, 80 = -35 dBi, 40 = -19 dBi (all measured and calculated numbers). If you are listening on 40 meters unless you have a 19 dB preamp, the signals are going to not be as strong as your 6BTV. For 80 and 160 meters you will need a high gain preamp to hear anything but the strongest signals. Your rig's preamp may not have enough gain to make up for the -19 dBi gain of the antenna on 40 meters. My rig's preamp is 15 dB but most do not have that much gain.

    Check your RDX-7300 modification to make sure that is working OK. You can do that by temporarily moving the 6BTV to that input and see if the signals sound the same. Also you can move the LOG temporarily to the main input for the radio and see how it sounds. DO NOT transmit into this antenna.

    It is difficult to use SWR to determine if the antenna is working right, or not, because the SWR is very high on the low bands. I measured on 1.8 MHz SWR = 110 to 1, 3.6 MHz SWR = 59 to 1, and on 7.1 MHz SWR = 24 to 1. Those numbers are relative to 75 ohms, which was the feedline impedance. The SWR was lowest on 12.6 MHz. If measured at the end of a piece of coax, the numbers will be a lot different.

    You should actually have a signal to noise ratio increase for stations arriving at high angles. The antenna has good high angle response but very poor low angle response. That means it will be poor for DX stations. In general most of the FT8 stations will be close, so overall you should see a signal to noise increase. Location of the antenna may be important if there is a noise source very close.

    Jerry, K4SAV
     
    Last edited: Jul 7, 2020
  3. NQ8J

    NQ8J Ham Member QRZ Page

    Very good thing to remember about SMA connectors! I've had this cause me problems.
     
  4. KK5JY

    KK5JY Ham Member QRZ Page

    I would be interested to hear what kind of ground constants you have at your location. My experience was very different at low angles.
     
  5. K4SAV

    K4SAV Ham Member QRZ Page

    I have typical red clay ground. This type dirt is common in nearly all the states in the southeastern US (except for Florida, which is just sand).

    Jerry, K4SAV
     
  6. KK5JY

    KK5JY Ham Member QRZ Page

    No problem, I'll look up the constants at FCC. :)

    ... clicking ...

    Looks like the ground conductivity where you are is on the order of 2 mS/m. At my location, it's closer to 30.

    The ground conductivity strongly influences the low-angle pattern response of vertically-polarized antennas (*) in the far-field, and since this antenna is very close to the soil, the conductivity affects its efficiency in the near-field, as well.

    Since we have such very different soils between our two locations, it's no surprise that the pseudo-Brewster angle is much lower here than there. That would readily and easily explain why I get much better DX reception on the LoG than you do there, assuming that the antennas are otherwise equivalent in their design, installation, etc.

    (*) ...and this antenna's response is vertically-polarized, since the proximity to the ground cancels the horizontal component of its response...much like a Beverage antenna.
     
  7. K4SAV

    K4SAV Ham Member QRZ Page

    A performance comparison between antennas is required to determine how an antenna performs. You can't access that using only a single antenna.

    Here is a quick summary of my test data on signal to noise. A low noise preamp was used to make up for the low gain of all the test antennas.

    Measurements of low angle signals on 160 was difficult because most of those signals were below the noise floor of the LoG. I was able to copy some big gun European stations on 80.

    S/N performance was very poor. 80 was the worst band and 160 was the best. On 80, my 80 meter inverted L (transmit antenna) beats it by 0 to 10 dB S/N. A BOG beats it by about 6 to 10 dB S/N.

    On 160 my 160 inverted L beats it by about 3 dB. A BOG is about 12 dB better than the LoG on 160.

    The LoG S/N is significantly worse than my transmit antennas and much worse than a BOG.

    I live in a low noise location. If you have local noise sources, a receiving antenna of any kind is likely to be an improvement if it is further away from the noise sources.

    I did notice a signal to noise improvement on high angle signals but I have no need for that.

    Jerry. K4SAV
     
  8. KK5JY

    KK5JY Ham Member QRZ Page

    Given the ground type you have there, I'm not surprised.
     
  9. KK5JY

    KK5JY Ham Member QRZ Page

    As an interesting footnote, I re-ran the EZNEC models I use here for the 15' square loop-on-ground, on 80m. I ran the far-field plot once with good soil (30 mS/m) and once with very poor soil (2 mS/m).

    The -3dB elevation angle for the good soil was about 8 degrees above the horizon. For the very poor soil, the -3dB elevation angle was roughly 20 degrees. For chasing DX, that's a huge difference. The peak gain was only changed by about 1.6dB between them, so the high-angle response was essentially unchanged.

    For vertically-polarized antennas, low-angle performance really is dominated by ground quality. Unfortunately, the only way to improve the ground quality under one's station is to move. :(

    Then again, that's why there are so many different antenna designs to choose from. :)
     
  10. K4SAV

    K4SAV Ham Member QRZ Page

    I ran all the EZNEC calculations for all different grounds too but actual measured data is more conclusive. Listing the stations you have heard doesn't say anything about signal to noise ratio. Comparison to other receiving antennas is required. I also have a phased EWE array I use for receiving. It is a little better than the BOG, so the LOG would look worse when compared to that. I just gave the comparison between common simple receiving antennas.

    I haven't measured my ground constants. It can't be too bad since I have 239 countries on 160 and 297 on 80 in the last 13 years.

    Jerry, K4SAV
     

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