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Thread: 300 Ohm "Twin Lead" Antenna

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  1. #1

    Default 300 Ohm "Twin Lead" Antenna

    Several years ago, while reading through some old ARRL manuals I came upon a statement that the center connection of a folded equal leg dipole has an impedance of 300 ohms. Well, I enjoy building HF antennas and decided to build a 40 meter folded dipole using 300 ohm TV-twin-lead (total length slightly less than 66 feet). The feed-line is also 300 ohm twin-lead, about 40 feet long, connected to a balun (MFJ) which in turn feeds about 30 feet of RG8X into my basement to my station. The antenna is hung in a grove of oak trees in a Vee configuration. The apex is about 30 feet high. I am a QRP aficionado and have experienced excellent results with this cheap and simple antenna. It produces an SWR of between 1.2:1 to 1.5:1 on 40 meters and 2:1 on 15 meters. I do not like to use tuners; even the built-in tuner on my Icom 703 if I can avoid it; opting instead to build resonant antennas (even if I am limited to only one or two bands). If anyone wants or needs to build a simple, cheap antenna for 40 meters and 15 meters, I'd recommend this set-up. You can even avoid using the external balun if you have an existing tuner capable of accepting a balanced line (with most tuners you can select "bypass" for direct connection from the tranceiver to the antenna) and can keep the feed-line away from any metal object. I have used Radio Shack's twin-lead product which sells for $20 for 100 ft. Therefore, you can make an excellent, home-brew antenna resonant on 40 and close to resonant on 15 meters for $20 plus some solder, rope and a little labor. If you need an external balun, then add another $25. I remember the old adage that simply states, in part: any antenna is better than no antenna; an outside antenna is better than an inside antenna and (this is the most important part) a home-brew antenna is more fun to use than any "store bought" one! During the SKCC's Weekend Sprint on March 10th I made contact with a ham in Alaska on 15 meters with an RST of 449. I was using my Icom 703 pushing 10 watts connected to my home-brew 300 ohm dipole. Ain't ham radio great! 73, Ken - KG4LLQ

  2. #2

    Default

    Even better back in 1965 when I was first licensed as a Novice and 300 Ohm twinlead of very high quality was readily available for five cents a foot or less, since "everyone" used it for TV antennas back then.

    My first antenna when I was a 13 year-old Novice on 40 CW was a folded dipole made of twin lead. Worked the world with it, using 60W DC input power (about 30W output) and crystal control. I think the antenna cost about $3 to build because the twin lead was really cheap back then. My "T-R" switch (to switch the antenna from transmitter to receiver) was a hardware store knife switch that cost $0.50 and worked very well for that.

    Yes, when you build your own stuff you get a 3 dB bonus.
    A government which robs Peter to pay Paul can always depend on the support of Paul.

    -- George Bernard Shaw

  3. #3
    Join Date
    Jan 2004
    Location
    Beautiful Downtown Colorado. (Montrose, SW corner)
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    Default

    A folded dipole has a greater bandwidth than a single wire type, dunno why.

    TOM K8ERV Montrose Colo

  4. #4

    Default

    Quote Originally Posted by K8ERV View Post
    A folded dipole has a greater bandwidth than a single wire type, dunno why.
    Ask Maxwell. :-)

  5. #5

    Default

    Quote Originally Posted by KG4LLQ View Post
    ... the center connection of a folded equal leg dipole has an impedance of 300 ohms.
    It's called a folded dipole. If one doesn't like tuners, here's a way to transform the 300 ohm impedance of a folded dipole to 50 ohms.

    XMTR---1:1 choke===0.06WL 300 ohm twinlead===-j147 shunt cap=====any length 300 ohm twinlead===folded dipole

    0.06WL from the 1:1 choke, install a -j147 ohm shunt capacitor across the twinlead. The cap causes an SWR on the twinlead of 6:1. Then 0.06WL more of twinlead transforms the impedance to 50 ohms.
    73, Cecil, www.w5dxp.com
    Can CO2 emissions save us from the coming ice age?

  6. #6
    Join Date
    Jan 2000
    Location
    Clearwater, FL
    Posts
    4,888

    Default

    I just use a 6:1 balun outside my shack entrance on my 80 meter folded dipole fed with 300 ohm twinlead and hook 50 ohm coax to it for the short run inside. Works great!
    ex-W4DFW Ham since 1970. ARRL Life Member and Volunteer Counsel

  7. #7
    Join Date
    Jan 2000
    Location
    Estherville, Iowa USA
    Posts
    267

    Default 40m FOLDED dipole ?????B

    Somewhere back in archives of my ole brain,
    I remember reading (I think I do anyway) about the FOLDED DIPOLE, in one of the old Handbooks,
    that the Folded dipole was indeed a wide banded antenna, compared to a regular Dipole.

    BUT, in those clouded brain cells, I seem to recall that the author of that article said it was
    also a SINGLE BANDED ANTENNA. Meaning that it wouldn't do such a good job working on 15 meters.
    Meaning that it wouldn't work very well on an odd multiple of the basic frequency. (7-21 mcs)

    Just to get the actual construction correctly, the FOLDED DIPOLE is two (or three) runs of spaced wire where the upper
    run of wire IS NOT broken in two pieces like the lower portion of the antenna is.
    The two runs of wire are both shorted together on either ends.
    l------------------------------------------------------------------------------------------l
    short l insulator l short
    l--------------------------------------------l0000l---------------------------------------l
    l l Center is the 300 Ohn feed point on the Folded Dipole
    l l
    l l
    300 ohn feedline

    Where as a regular DIPOLE is constructed like this:
    insulator ------------------------------------oooooo------------------------------------------- insulator
    50 Ohm feed point

    (I hope these kb drawings come out right)

    To properly feed (balance feed system) for each antenna can be accomplished by the following:
    So--------
    Matching between the feedline and the antenna is easily covered by feeding the FOLDED DIPOLE with 300 ohm line,
    and use a 300/50 ohm balun outside the shack and 50 ohm coax on into the shack.

    The REGULAR DIPOLE is fed by a 50 ohm COAXIAL line or a 50/50 ohm BALUN, and 50 ohm coax on into the shack or
    directly with 50 ohm coax and straight back to the shack.

    If you use twin lead to build your FOLDED DIPOLE antenna, you'd better off feeding both wires in the horizontal portion of the antenna as a REGULAR DIPOLE, one cut for 40m and the other cut for 15m.
    No shorts at the ends and a single insulator at the center and coax on back into the shack.
    de
    Tex WAĜKZL - FISTS # 8092 - SKCC # 1368C
    Talk is Cheap - REAL HAMS BEEP !

  8. #8

    Default

    Quote Originally Posted by K8ERV View Post
    A folded dipole has a greater bandwidth than a single wire type, dunno why.
    Because the elements are "fatter"!

    If you build a conventional dipole using the same twin-lead for the elements, you get pretty much the same bandwidth:
    http://www.karinya.net/g3txq/folded_dipole/

    73,
    Steve G3TXQ

  9. #9
    Join Date
    Jan 2000
    Location
    Commerce MI (Detroit area)
    Posts
    8,024

    Default Folded Dipoles

    I still use twinlead for outdoor FM folded dipole recv. ant and feedline for DXing the 3M band ! One benefit is the stuff has lower loss on VHf and UHF bands. Interesting Tropo conditions sometimes bring in FMBC from 300+ miles away.
    When I first got on 30M I made a dipole out of twinlead and feedline fed into the bal input of a MFJ948 versa tuner. the internal 4:1 balun was a great match for this antenna.
    My twinlead dipole became a camper antenna since it was light weight and could be rolled up and packed up with the QRP rig.
    73.....JD, FISTS #3853,cc 455,SKCC # 1395,tribune #12,
    Official US Taxpayer

  10. #10
    Join Date
    Mar 2003
    Location
    3763 Lyle Avenue, North Pole, AK 99705
    Posts
    22,422

    Default

    I've used folded dipoles with good success for many years. They tend to be a little more broadbanded than standard dipoles, and you can feed them directly with low loss twin-lead (Which USED to be as common as table salt, but alas, no longer). You can then use a simple 4:1 balun at the shack end for a pretty doggone good match across most upper ham bands.

    Twin lead is great for making ZL special beams too...probably my favorite kind of beam.
    "The more you know, the less you don't know."

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