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Any special consideration for long run of twin-lead?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K0MB, Jan 29, 2015.

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  1. K0MB

    K0MB Ham Member QRZ Page

    My tower is some 300' from the shack. For my yagi I used LMR-600 coax. Now I am planning to add a non-resonant dipole fed with "window line" type twin lead. I plan to use this antenna for 160-40m, and do system matching via a quality manual tuner in the shack.

    Other than regular precautions like keeping it off the ground and away from metal, are there any other precautions I should be mindful of? I have a series of "T" posts made of pressure treated 4X4 running from the shack to the tower, and plan to run dacron rope thru the "windows" of the feedline to support it between the posts. The top cross-arms of these posts will allow 2' between the feedline and the bolts used to make the T-posts, and I have PCV pipe standoffs to keep the feedline positioned away from the tower legs.

    K0MB
    I can't spel
     
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    Think carefully about the type of "twin-lead" you are going to use - some of the commercially available stuff can be quite lossy in that application. For example, a 260ft dipole fed with 300ft of Wireman 551 will have about 4.5dB of feedline loss on 80m.

    Steve G3TXQ
     
  3. AC9KH

    AC9KH Ham Member QRZ Page

    I got a 335 foot run of homemade 400 ohm ladder line made with 14ga THHN on wood posts to my 160 dipole. It will work fine.

    I come right thru the walls of a steel building with the connectors to my ladder line and not a single problem with it

    edbcdbaf.jpg

    Like G3TXQ hinted at - you can build your own ladder line that will have less loss and handle higher voltages than what you can buy commercially. 160 will be the worst - even though it's resonant on 160 your dipole will have the lowest feedpoint impedance against a high-impedance feedline, which means high standing waves on the feedline. That commercial windowed stuff gets wet and your loss will quadruple, and don't be surprised if you get arc-over with a pleasant sound like bacon frying on 160 with an afterburner when it's wet. I don't run no rope that gets water logged thru my line - I just stretch it on the T-posts to the tower and use plastic ties to hold the line to glass fence post insulators.
     
  4. KD4MOJ

    KD4MOJ Premium Subscriber QRZ Page

    Nice setup there KH! Pretty cool how you got it through a steel building!

    ...DOUG
    KD4MOJ
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    When the rope gets wet, it will change the Z0, VF, and loss factor of the feedline. If you use something like The Wireman #554, you won't need a support rope.
     
  6. KM1H

    KM1H Ham Member QRZ Page

    With home made OWL you can drill a hole thru the center of the spacers to run the Dacron. I did that some years ago for 700-1000' 2 wire reversible Beverages using #14 stranded Copperweld. Ive no idea of losses but it certainly performed well from 160-30M and even used on 20M at times.

    My supports were steel garden fence stakes with 3/4" PVC pipe hose clamped to the posts, a T and some more 3/4" horizontal. A slight notch allowed the OWL to have free travel in the horizontals and the Dacron kept the stakes vertical for the 6 years it was up before moving. The Copperweld didnt do more than a slight extra grooving and it all survived lots of tree branches and ice storms.

    As with any OWL, twisting it slightly along its run helps to maintain RF balance.

    Carl
     
  7. WE6C

    WE6C Ham Member QRZ Page

    How large of mis-match did you figure in your calculations? When I tried this with a perfect match, there was virtually no loss, but with a 30:1 SWR, it pretty much matches your calculation above.
    Bob
     
  8. AC9KH

    AC9KH Ham Member QRZ Page

    On 160, 80 and 40 you're not going to get a perfect match with ladder line on a dipole with no tuner in the circuit. But it doesn't make any difference. The SWR will be the highest on 160m, but that's where the efficiency is the best (according to my analyzer) because there is no reactive load to tune out. You'll probably notice little difference between coax and ladder line on 160 because 50 ohm coax is a better impedance match to the dipole's feedpoint at its resonant freq. But tune the antenna on 80 or 40 with a tuner back at the shack, and then you may as well give up on coax because it's not going to work.

    The losses will be when the ladder line gets wet and then they can be quite high, especially with the commercially made "window line" that has a lot of surface area to hold water on it. So basically, just don't do anything (like running a rope thru it) that will hold the conductors near a wet surface and it will work fine. I've never liked the commercially made "window line" because it doesn't last and it has small conductors, so I make my own. But it's rather time consuming to make 300+ feet of ladder line too, so to each his own.
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    EZNEC says the feedpoint impedance of a 260ft centre-fed, 50ft over average ground, is 7670-j760 at 3.6MHz. That's an SWR of 19.3:1 on the Wireman 551. TLD predicts a loss of 4.3dB for 300ft.

    The SWR is a lot higher on 80m than on 160m.

    Steve G3TXQ
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    The Wireman #554 ladder line is #14 19-strand copperweld. I've had some in service for 20 years in AZ and TX and it's still doing a good job for me. I'll bet Tarzan could use it to swing through the trees.:)
     
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