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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. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    This is good advice for ordinary antennas like dipoles, verticals etc. However this is NOT necessarily true for negative gain antennas like the LOG being discussed or Beverages, SAL's, Pennants etc.

    The listener may find it helpful to use a preamp since the overall Signal + Noise level is quite low by nature of the antenna.
     
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Doug DeMaw discusses Pre-amplifiers for low noise antennas in this QST article about "On Ground Low Noise Receiving Antennas" April 1988. (free download).
    http://tm1o.free.fr/Beverage/On-Ground.pdf
     
  3. K0OKS

    K0OKS Ham Member QRZ Page

    Thank you both for the info. I will start without an amp, but may try one if I am not please with the results.
     
  4. AI5DH

    AI5DH Ham Member

    That is what I did and would do again if I had to.

    KOOKS this is not directed at you or anyone. Allow me to make this crystal clear in the event I did not make it clear from the start. IMO the LoG Antenna is a Special Purpose with niche applications for those of us with limited space in areas with high noise levels. Additionally although there is some info out there stating the LoG is good to 15 MHz might be a bit liberal, but I do not share that opinion. I only have one location to test against, my current location where I have a OCF 80/40/20/10/6 up about 25 feet with a Tuner, and now the LoG.

    Hands down the LoG outperforms the OCF from 400 KHz to 10 MHz RX. It is no contest. At 15 MHz and up, the OCF RX better. That is across 3 different radios. Kenwood TS590SG, AirSpy HF+, and SDRPlay-RSp2. The antenna realy excels in LF from 200 to 2000 KHz where it gets really noisy. I can see and RX signals with the LoG the OCF cannot even see because the noise is so high. So if your qth is noisy, limited on space, and want to at least listen to Commercial AM. 160, and 80 meters this would make an excellent RX only antenna. It is so inexpensive and easy to deploy for a test, give it a try sometime. All you need is 60 feet of wire wired directly into a balanced RX port on your radio to see how it works. Yes you will loose 10 to 20 dbm signal level, and along with it 30 to 40 dbm noise signal. Those two added together equal 10 to 20 db higher SNR
     
  5. AI5DH

    AI5DH Ham Member

    Thought I addressed that with a GO and NO GO test. Monitor Noise Floor with the Antenna disconnected and terminated. Connect Antenna and monitor Noise Floor Level. If it comes up more than 3 dbm, NO GO pre-amp. If Noise Floor is 3db or less, you might benefit with a filtered Pre-Amp. If I were to use one at a minimum use a 5 MHz low pass filter and would prefer to use a Band Pass Filters along with RF Gain Control up to no more than +13 dbm.

    If the Noise Floor comes up means the antenna gain is neutral to gain. All a pre-amp would do is raise the Noise Floor what you are trying to get rid of with a LoG antenna. The key is keep everything Balanced Lines all the way to the radio. Do not use a Balun or Unun at the Loop, only use isolation transformers. Keep earth (ground) off the antenna circuit.
     
  6. K0OKS

    K0OKS Ham Member QRZ Page

    I am in a suburban lot... older suburbs... 1 acre lot, but still close to the city, and thus noise. Due to the lot layout I am limited in some ways. The goal for me is an alternate receive antenna for 40-160m, especially for 80m. Since my main antenna is an old school Windom variant it is already fairly high noise.

    I just finished making up a transformer with 3 turns on the receiver side and 9 turns on the antenna side with a mix 31 2.4” torroid (what I had). Trying to get close to 50 Ohms as I am using 50 ohm LMR-400 (had a run made already that I am not using) and PL-259/SO-239. (Comments welcomed.)

    The plan is to pipe that into my preselector, which also had a variable preamp I typically keep off and then into my MFJ 1026 as a “sense” antenna. I can then phase it against my main antenna, or just use the 1026 as a TR switch, where I can listen only to the LOG, or to the LOG phased with my main antenna.

    Sometimes when I phase my main antenna with my 20m attic dipole I can get some pretty decent signal enhancement. Of course those are both horizontal antennae oriented in the same direction.
     
    K1VW, KK1N and KA0HCP like this.
  7. K0OKS

    K0OKS Ham Member QRZ Page

    If this works well I may try a mix 75 transformer, as mix 31 is not,recommended for other than 1:1 applications. I do wonder how much that would matter at these low power levels, though.
     
  8. AI5DH

    AI5DH Ham Member

    Yeah Mix 31 is not much good for what you want. Lower Frequency response is just not there and you take a lot of loss at 2 MHz and less. Like I said I used mix 75, and even with Mix 75 still takes 4 turns to 12 turns to pick up frequencies below 2 MHz. I have not ran the numbers on Mix 31 and the core you used, don't have to know because I know it will take a lot more than 3 turns on the low side. Would not surprise me if it took 20 or more turns on the primary. Get you some 75 mix if you can find them, otherwise 73 mix will work, but takes more turns on the primary.

    I have Agilent Analyzers and when I sweep my transformer insertion loss is .68 db from 400 KHz to 5 MHz. Does not get much better than that. FWI the 3db loss occurs at 170 KHZ and 12 MHz. Mix 73 works good from 2 MHz up to 30 MHz, but not so good below 2 MHz and falls off pretty quickly.

    Give it a try and post back here with results. I would like to hear how it works out for you. Just do not panic when you first hook up the antenna. Initially it will look like a Dumb Load because there is no noise ,and signal levels will be down. But SNR levels will more than make up for signal loss. A signal level of -100 dbm is a HOT signal when the noise level is down at -140 dbm, that leaves you 40 db SNR, a crystal clear signal.
     
  9. AI5DH

    AI5DH Ham Member

    OK for RX 75 Ohms is better than 50 Ohms, 300 Ohms is better than 75, and 450 is better than 300 Ohms. So maybe consider using 75 Ohm coax and F connectors like I did.
     
    KK9W likes this.
  10. K0OKS

    K0OKS Ham Member QRZ Page

    Thanks for the input, KF5LJW. Good info! I will review the relevant parts of the handbook, but could you point me to any other useful info on transformer core theory? I would love learn more about it.

    I had a closer look at the data sheet as well. I do see the impedance is at much higher frequencies, as you say. It would take a lot of turns.

    I could do a run of 75Ohm cable with F connectors. I just already had the 50Ohm cable, and my local shop was out of F connectors for RG-11. Can you elaborate on why the higher impedance cables better for receive? Isn't simply that the transmission line loads the receive, thus less load, more signal?

    I will go ahead and try it with the Mix 31 and the LMR-400, since I almost have it set up.

    Your insertion loss numbers are impressive. Since I have to order the cores from Mouser what size core do I need? At receive levels I assume the size is not very important. Any insights are appreciated.

    Thanks.
     

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