How to attach a "random wire".....

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KJ7OTM, Jun 19, 2021.

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  1. KJ7OTM

    KJ7OTM Ham Member QRZ Page

    We see folks talk about using EFHW/random wire setups for antennas....how exactly does one go about attatching a random piece of wire (of appropriate length of course) to the back of ones radio, if say, it's PL-259 or SO-239, etc.?
     
  2. KC3PBI

    KC3PBI Ham Member QRZ Page

    You need a matching device to interface between the high impedance properties of that wire antenna and the low impedance expectations of the transmitter.

    The same device will provide the physical adaptation required to go from coaxial cable to single-conductor wire.

    There are a zillion ways to skin the cat, but a common solution would be a coax line with PL-259 connectors on both ends. One end to the radio, the other to a 9:1 un-un impedance matching device. The un-un is outdoors located at the near end of the random wire run.

    The matching device has two lugs for attaching single-conductor wires- typically one to ground and/or counterpoise, and the other for the actual random wire.

    I built my first un-un from a packaged kit, but they are available in finished ready-to-use form as well.

    A few web searches for "un-un" will probably teach you a lot. A 9:1 is a common ratio for the specific mismatch presented by random wires in the lengths most hams use, but there will be situations where a different ratio is required.

    Good luck!
     
    Last edited: Jun 19, 2021
  3. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    Not to be confused with "true" EFHW antennas, which use resonant lengths. The ratio for that matching device is typically 49:1 or 64:1.

    The matching devices are not interchangeable.
     
  4. W9WQA

    W9WQA Ham Member QRZ Page

    radio is looking for a 50 ohm load. the wire is ???. random.
    you need a tuner to match it. and decent ground,
    the built in tuner wont match a wire.
     
  5. KC3PBI

    KC3PBI Ham Member QRZ Page

    ^ good reminder! The radio will need a good tuner to use a random-wire antenna.

    Although some radios have built-in tuners, many cannot handle the range needed for a random wire; you'd have to add in an external tuner.

    The built-in tuner on my G90 has worked properly with two different random wires I've set up. I happened to try an FT-991a and it could not tune my wires until we put an LDG tuner in the path.
     
    N5HXR likes this.
  6. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    The term "random" is confusing. The length must be selected from a list of "non-resonant" lengths. It is, therefore, not random.

    The Elecraft manual for the KX2 shows an example of a wire and a counterpoise connected to a binding post - BNC adapter connected directly to the radio.

    However, the Elecraft tuners can handle larger than 3:1 SWR. Many radios can't do this with the internal tuner.
     
    KB2SMS likes this.
  7. K8JD

    K8JD Ham Member QRZ Page

    Random wires are just that, Random, and will have random, different, Z amd X on each band.
    End fed designs will have high RF fields at the feed end (RFI trouble in house) on some freqs !!
    Better Idea is a random DIPOLE fed in center with high Z feedline (twin lead, ladder line) and a balanced line tuner in the radio room and coax to the radio from that.
    I had great luck with a 90 Ft dipole with 450 Ohm line to a MFJ974 balanced line tuner in the shack Worked well from 5 to 18 MHz.
     
    Last edited: Jun 19, 2021
  8. KI4IO

    KI4IO Ham Member QRZ Page

    Well there might be a "list of non-resonant lengths", but with various antenna installations, WARC bands, etc., an end-fed
    antenna will present high impedances (and the possibility of RF-in-the-Shack) and low impedances which put a premium on
    a low-loss path for antenna return currents, i.e., a good ground system. Oh yeah, and everything in between. A modest
    L-network, preferably one that is "reversible" should suffice for most situations. Build an antenna coupler - only two parts
    in an L-network. Hard to get that wrong!

    KI4IO
    Jerry W
    Warrenton, VA
     
  9. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    Banana plug works fine and will plug right into the center conductor of the SO-239 on the rig ( or better yet to the plug on the external tuner that will handle the large mismatch). The wire just by itself without a counterpoise is not ideal but it will work.
     
    KB2SMS likes this.
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    There are two requirements for feeding a "wire" directly from the SO-239 on a typical ham transceiver: You are creating essentially a Marconi vertical antenna.

    1. That the length of the wire be very close to 95% of 1/4wl, 3/4wl or 5/4wl..., at the operating frequency (on a single band). For the 40m band, that would be 32, 101, or 170 ft, respectively. You can scale the required wire lengths to other bands.

    2. That the chassis of the transceiver be connected to a "RF ground system". To be an effective RF ground, the "ground system" would have to be an eight foot ground rod placed such that the wire from the chassis of the transceiver to the ground rod is no more than 4ft long (on 40m), or that there are at least two "radial wires", running parallel to the earth, each about 33ft long (so 66ft tip to tip for 40m). Just having the transceiver powered from the AC-line in your house will not cut it as a "RF ground system"!

    Even if you place a "tuner" between the transceiver and the "wire", the chassis of the tuner has to be connected to the same "RF ground system" described in 2 above.

    If you place a 50:2450 Ohm transformer between the transceiver 50 Ohm port and the wire (short coax jumper), then the wire would have to be ~0.5wl, ~1wl, or ~1.5wl (67.4ft, 135.9ft, or 204.6ft..., respectively). This also requires an RF ground connected to the transceiver chassis, just as in 2, above, however, the two radials could be a lot shorter, 3.5ft each. The transformer is the same kind as used with EFHW antennas.
     
    Last edited: Jun 19, 2021

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