ARRL Offering an EFHW Kit

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W9IQ, Jun 10, 2021.

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  1. W9IQ

    W9IQ Ham Member QRZ Page

    To help new hams prepare for the exciting world of HF opening up during Cycle 25, ARRL has partnered with HF Kits to bring you this easy-to-build four-band antenna kit: an end-fed half-wave (EFHW) antenna. Unlike the dipole antenna, which is comprised of two quarter-wavelength wires and fed at its center, the EFHW is a half-wavelength antenna with the coaxial cable for your transceiver attached at one end. It has become popular with portable operators because it’s very simple in its construction and deployment. This antenna, which works on 10, 15, 20, and 40 meters, has a very high impedance of around 2,500 Ohms. The kit includes the parts needed to construct a 49:1 impedance matching network, which will transform the impedance to 50 Ohms, which will suit most transceivers.

    More details here.

    - Glenn W9IQ
     
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  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    I ordered one. Will build an EzNec model of it as I built it...
     
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  3. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    I hope the instructions discuss a short stub and/or a choke at an an appropriate point on the feed line, and feed line lengths to avoid.
     
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  4. WA7ARK

    WA7ARK Ham Member QRZ Page

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  5. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

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  6. W9IQ

    W9IQ Ham Member QRZ Page

    A reasonable common mode choke for this antenna is 12 turns of RG400 on an FT240-31 core. One 12 feet or so away from the feedpoint (is that right, Mike?) and one before the coax enters the shack.

    I am not aware of any coax lengths to avoid. What is the concern?

    - Glenn W9IQ
     
  7. W4HWD

    W4HWD Premium Subscriber QRZ Page

    The concern is most people do it wrong and then come on here to whine about RF in the shack. Unfortunately there is no fix for that.
     
  8. W9IQ

    W9IQ Ham Member QRZ Page

    The fix is proper common mode chokes. They are easily added even after the fact. Coax length is inconsequential. What am I missing?

    - Glenn W9IQ
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    If an EFHW antenna is installed without a choke on the coax, there is a possibility that at certain coax lengths, on certain bands, the coax radiates and/or causes "RF-in-the-shack" problems. Even if there is no problem on the antenna's lowest band, it is likely that a problem might happen on one of the harmonic bands. Choking the coax cures this.

    There is a lot of confusion about how to use an EFHW multiband antenna such as the one offered by the ARRL. It comes down to this:

    1. Am I going to use the antenna as an off-center fed Hertz dipole (radiating part of the antenna (including the transformer) elevated by tens of feet, isolated from earth, and more or less horizontal)?

    This requires that the wire be stretched between two masts, the higher the better. One of the masts holds up the transformer, and if the mast is metal, the coax and transformer have to be isolated from the mast. No connection to the grounding screw on the transformer. The coax drops straight down below the transformer. The part of the coax between the transformer and choke cannot be taped to the mast.

    The choke is necessary to separate the short piece of coax that radiates (and completes the "dipole") from the part of the coax that is just the feedline to reach the station. The optimum distance from the transformer to the choke is about 12% of the EF wire length.

    Since the antenna acts as a Hertz dipole, the higher it is, the better. Folks that place this antenna so that its average height is a small fraction of a wavelength shouldn't be surprised that the radiation goes straight up.​

    2. Am I going to use the antenna as an earth-referenced end-fed Marconi (the radiating part of the antenna mostly or partially vertical, with the transformer mounted within feet of the earth)?

    This configuration is where the EF wire goes mostly up, such as with a ground mounted half-wave vertical, an inverted-L, a sloper, and an inverted-V which starts like the sloper to the mid point of the wire, but then bends back down so that far end is close to earth.

    If the transformer is within a couple of feet of earth, then it is possible to use a single ground rod connected to the grounding terminal on the transformer to complete the antenna. Alternatively, very short radials, or just the coax laid parallel to, but isolated from the earth acting as a single radial with the choke placed as described above will do. This is the topic of this recent QRZ forum posting: https://forums.qrz.com/index.php?threads/feeding-efhw-vertical-antenna.758486/page-4#post-5882163

    If you start with a Marconi, lay it on its side so that the average height of the entire antenna is a small fraction of a wavelength, you shouldn't be surprised that the radiation goes straight up.​
     
    Last edited: Jun 10, 2021

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