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Feeding a odd shape beverage or rombic

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N4AEQ, Nov 18, 2009.

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  1. N4AEQ

    N4AEQ Ham Member QRZ Page

    I have a 350' x 750' unused fence I have run a wire on top of the post. I hoped to use this as a quite RX
    antenna but not sure how to feed or terminate it. I
    will be using 50 ohm coax & will have to feed it about
    200' down one of the 750' sides.
    I am looking for options on how to feed and the term
    the opposite end of this odd shape loop/long wire.
     
  2. W0BTU

    W0BTU Ham Member QRZ Page

    Is the fence conductive (metal) or wood? Please describe it for us.

    FWIW, found this just now in a search:
    http://lists.contesting.com/archives//html/Topband/1997-04/msg00080.html

    The latest version of ON4UN's Low Band DXing is an excellent reference on Beverage antennas.
     
  3. N4AEQ

    N4AEQ Ham Member QRZ Page

    Fence is a unused all metal

    Welded, hog wire type fence with metal post and a few wooden ones, its about 3.5' tall. I installed 4" long plastic insulators at the very top of the post and strung a big loop around the area. I will be using it for RX so Im not sure
    if a closed loop, beverage type or use it as a long wire with one end grounded would be the best?
     
  4. K4SAV

    K4SAV Ham Member QRZ Page

    Assuming it is a wooden fence and you add a 750 ft wire and feed it unterminated at the 27% point, it won't make a very good Beverage on 160. It will be bi-directional and the take-off angle will be very high (about 36 degrees in the forward direction and 60 degrees in the reverse direction). However it may be of some minimal benefit on 80 and 40 meters. Its pattern does vary a lot across the 80 meter band.

    A better option is to implement a 550 ft terminated Beverage. That will significantly outperform a 750 ft off-center fed one on 160, 80, and 40 meters. You will need a ground rod and termination resistor at the far end and a matching transformer at the feedpoint. The exact termination value depends on the height of the fence, but that should be close to 300 ohms. The matching transformer also depends on antenna height but will probably be close to 500 ohms (50 ohms to 500 ohms). You will need to wind this transformer with good primary to secondary isolation, or you will have to add a very good common mode choke on the feedline at the feedpoint to keep feedline currents from messing up the pattern and inducing noise. If you don't do this, it's very likely the antenna will be useless.

    Another more useful option may be to convert it into a switchable bi-directional Beverage. ON4UNs Low Band DXing book gives detailed info on how to do that.

    If this happens to be a four sided WIRE fence then it is unlikely that that it will work very well. Coupling to the fence will destroy the pattern. If it's like most wire fences, the poor contact between sections (and the rusty connections) will cause a lot of noise, and rectification of the signal impressed by local AM stations creating lots of intermod.

    This antenna will only work well in the direction it is pointed. All other directions should have worse signal to noise ratios.

    Jerry, K4SAV

    edit: I see you answered the question as I was typing. With a wire fence I think you are on your own. Please report back on the performance.
     
    Last edited: Nov 18, 2009
  5. AD5ND

    AD5ND Ham Member QRZ Page

    I once used the electric fence wire for an antenna. It was 660' square, hog wire fence with metal T posts, the top strand was insulated from the rest of the fence. I opened up one corner and used it as a 2640' closed loop. It worked fairly good for receive but acted like a 5 ohm dummy load for transmitting. I'm only guessing at this value. Once the homebrew tuner was adjusted I didn't have to change it when changing bands. A dipole would always out perform it on transmit. During a thunderstorm I watched sparks fly on the spark plug I was using as a lightning arrester every time the cloud lit up. I terminated this antenna experiment the next day.
     
  6. N4AEQ

    N4AEQ Ham Member QRZ Page

    Too bad all that length isnt good for anything

    I had hoped you had better results with your electric fence wire, it sounds
    exactly like what I have. I guess I will stick with a rubber duck for VHF &
    a base loaded vert for all HF and leave the other 22 acers to the weeds.
     
  7. AD5ND

    AD5ND Ham Member QRZ Page

    Just because it didn't work well for me is no reason why you shouldn't give it a try.
     
  8. KB9MZ

    KB9MZ Ham Member QRZ Page

    I would not be to quick to discard it! What you have going for you is that the joints are welded. I would feed it at the top of one end and ground the same bottom end. You can feed it with coax but disconnect the braid at the radio and then ground the chassis at the radio end ie Let the coax braiding float
    Aussies do this all the time with the kangeroo fences.
     
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