twin lead

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KK4YWN, Nov 28, 2014.

Thread Status:
Not open for further replies.
ad: L-HROutlet
ad: l-rl
ad: MessiPaoloni-1
ad: Subscribe
ad: Left-2
ad: L-MFJ
  1. KK4YWN

    KK4YWN Ham Member QRZ Page

    this stuff was commonly used for tv antenna so my assumption is it can be used on 2m. if this assumption is incorrect please let me know.

    rant:
    dear god how many websites do we need that explain how to make a crappy jpole out of ladder line? i mean really?
    rant off.

    i'm looking at a collinear broadside array similar to the cushcraft dx-120's. the feed impedance is 300 ohms and i was thinking that a 3x3 stack with equal length twin lead would give 50 ohms at the feed point.

    IF i can find a pair of v8 chevys to turn the thing, it should be a decent antenna for local rag chewing. i use the word local in the relative sense.
     
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    TV twin-lead works fine on 2-meters. Like any balanced line, you need to keep it away from metal.

    My first 2-meter yagi, made by HyGain, was 10-elements with a 300-ohm balanced feed. I used TV twin-lead to feed it with a coaxial 4:1 balun made from RG-59/U cable. Worked all over Indiana, Michigan, Illinois, Wisconsin, Ohio, and even into Ontario with less than 10-watts output from a home-brew transmitter with a 2E26 final amplifier tube. This was back in the 1960 - 1962 time period.

    Later, first in the Atlanta, Georgia, area, and then down here in Richardson, Texas, I "flipped" the polarization to vertical and used the antenna for 2-meter FM.

    Glen, K9STH
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The CC DX Array colinears didn't use any twin lead to interconnect and phase the bays: It used parallel widely spaced aluminum rods, and of course they had a 180 degree "twist" at every other bay to keep the phasing correct. This was a good, low-loss way to do it and of course it would handle a kilowatt or more easily since the conductors were large and there was no dielectric spacers between them other than air.

    If building a "copy" of this design, I'd do the same thing. Aluminum rod is cheap and can be nut-n-bolted together very tightly to assure long-lasting connections even in high winds, etc.
     
  4. KK4YWN

    KK4YWN Ham Member QRZ Page

    yeah no the twinlead is the phasing lines. some amateur astronomer built a copy of the 16 element version. he says its 300 ohms at the feedpoint. its a handy number. easy to feed and match to 50 ohm if "stupid-big array" happens to be on your agenda.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I wouldn't use twin lead on 2m unless I was pretty desperate.

    I can't find any on the market with sufficient conductor size to handle the power I routinely use. And all the stuff I've seen has unstable dielectric (PVC or Type I PE) which degrades with UV exposure and ultimately falls apart. And most of the "300 Ohm" stuff isn't...it measures about 220 Ohms. (Then, the "450 Ohm window line" usually measures about 380 Ohms, so this seems to be a common issue.)

    And of course solid dielectric twin lead increases in loss quite a lot when it's not operated in clean air. Salt, snow, ice or almost anything on the line degrades it quite a lot.

    On 80 meters, most of that makes little or no difference. On VHF, it does.

    This is the reason "TV" antennas, which all used twin lead from the 1940s through the 1960s, all switched to 75 Ohm coax because it simply worked better, reduced "ghosting" and lasted longer.
     
  6. KK4YWN

    KK4YWN Ham Member QRZ Page

    I guess I'll roll my own balanced feeders then. Thanks for the info
     
  7. K8JD

    K8JD Ham Member QRZ Page

    When I moved to the far suburbs, about 25 years ago, I needed a good TV antenna so I put up the vhf/uhf combo log periodic and thinking it "modern" I used RG6 feedline and a 4:1 balun. The TV had a 75 ohm input on the back.
    The results ; great VHF and snowy UHF channels. I went back to the twinlead with the 4:1 balun on the back of the TV. GREAT UHF AND VHF reception.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'm guessing your balun with the RG6 didn't work on UHF. Many of those old baluns didn't. I had buckets filled with ones that were very bad on UHF.

    Normally RG6 is very low loss on UHF, unless the line is terribly long. Twin lead can be a bit better when dry, but when "wet" or snow or ice covered, it can easily be lossier than RG6.

    Here, for OTA broadcasts I use a simple butterfly reflector antenna on UHF, fed with RG6 quad shielded cable, and get 44 channels from LA and San Diego, distances from 26 to 110 miles. If I switch to twin lead and baluns (tried that), half the channels disappear.
    :p
     
  9. N7EKU

    N7EKU Ham Member QRZ Page

    HiKK4YWN,

    If you are still considering 300ohm line, the "Rooftop" kind that Leviton sells is better than the average stuff. It's much heavier duty, uses 20AWG stranded wire (most I've seen use 22 or 24), and has foamed insulation around the wires. I haven't got my roll outside yet, but I think it would last a long time.

    Best of luck on your new antenna :)

    73,


    Mark.
     
  10. W2VW

    W2VW Ham Member QRZ Page

    Stay the heck away from insulated twin lead. Minimal surface area spreaders are in order.

    Balanced feeders work very well when all the rules are followed. That goes for flat or not flat.

    Pity some long time hams will never grasp that concept.
     
Thread Status:
Not open for further replies.

Share This Page