Connecting a random wire

Discussion in 'Ham Radio Discussions' started by KI7MWA, Jan 14, 2019.

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  1. KI7MWA

    KI7MWA Ham Member QRZ Page

    Antenna novice, here...

    I've got a 58 ft length of stranded household wire, length chosen on the basis of the analysis performed by Jack, VE3EED. I have one end thrown up into a tree (maybe 40 ft), and will connect to a Multi Z Tuner (from QRP Guys based on the circuit designed by Charlie Lofgren, w6jjz).

    My question is this: If I were to connect the household wire to the tuner by means of a 26 ft piece of coax that has its shield connected directly to earth ground, will it behave more like a 58 ft piece of wire or like a 84 ft piece of wire (which is also among the recommended lengths)?

    Or something else entirely?

    In other words, will the fact that the coax shield is grounded effectively prevent the center conductor of the coax from receiving and/or radiating?

  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'll take door 3....

    When you run a random wire straight out of a matching network along with some form of ground/radials/counterpoise, etc. you have a matching network right at the feed point and all of the random wire is part of your radiating antenna. If the matching unit is able to establish a decent match any rig or coax attached to the matching unit sees a good match to 50 ohms and losses are relatively low.

    When you add coax between the matching unit and the random wire feed point that stretch of coax almost certainly sees a very poor match and very high SWR on some if not all bands. So even if your matching unit can match into that coax the coax itself is generally presented with a very poor match which translates to high losses in the coax.

    If it's further complicated by having your ground system (whatever that is) back at the matching unit and then just the coax center conductor feeding the random wire you'll have several things going on including common mode on the coax shield as it acts as the missing ground return system for the antenna.

    A random wire out of a tuner with decent RF return system is one thing, making it a lot more random and a lot more lossy by adding coax between the tuner and the antenna is another. IOW, there's random that can more or less work and then there's random that's not really a great idea.
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    1. Because you have chosen a length for use on desired bands, it is no longer a Random Wire.
    2. The coax will tend to limit radiation from the shield, so your desired length will probably behave as you plan but may require trimming. No model can exactly predict antenna performance in the real world.
    2.a. If you have problems with RFI in the shack you may need to install chokes/isolators or upgrade to a better antenna system.
    3. The traditional name for your antenna system is called an "End Fed Wire Worked Against Ground". Countless numbers of hams and SWL'ers have started this way.

    -Earth/soil is a very poor conductor. Your performance will greatly improve by attaching any sort of wire radial(s) to the ground stake. Copper is over 1,000 times more conductive than average soil.

    -I started as an SWL'er and Novice with a Random Wire against Ground with ordinary wire. RFI on the radio controls (ouch!) led me to add a piece of coax out the window to the ground rod. RFI tamed.

    Making every mistake along the way ever since, I have climbed the ladder of antenna success wrong by wrong! ;)
    Last edited: Jan 14, 2019
    KL7KN and K4AGO like this.
  4. W1VT

    W1VT Ham Member QRZ Page

    A Smith chart is the simplest graphical method of seeing how the impedance of your end fed wire is transformed by that 26 ft piece of coax.
  5. KP4SX

    KP4SX XML Subscriber QRZ Page

    In a random untuned scenario doesn't the 26 ft piece of coax look like a capacitor to ground?
    KA4DPO likes this.
  6. KC3SWL

    KC3SWL XML Subscriber QRZ Page

    Feeding the wire with a coax run without a balun is going to be an issue. You are taking a wire with high impedance and trying to match it to a coax with low impedance. In my SWL part of the radio hobby I got around this with a 300ohm to 75 ohm TV type transformer and used one wire for antenna and grounded the other to the outside faucet. For transmitting you will need something for the 50 ohm to whatever probably at least 100ohm wire. Then you have to get the wire resonant on a frequency with that tuner as well. I'd just go with a random wire WITHOUT coax and ground the tuner really good.
  7. KP4SX

    KP4SX XML Subscriber QRZ Page

    Balun doesn't apply here. There's no 'bal' in the setup. Unun matching transformer does, though.
    KU4X, KC8VWM and KA0HCP like this.
  8. KM6CND

    KM6CND Ham Member QRZ Page

    I built about the same.

    - 33 foot MFJ telescoping mast
    - 25 feet of coax out to where it connects to the wire at the base of the mast. I have since forgotten how long that wire is:p

    I use an LDG AT-100 auto tuner and I have seen low SWR 1:15 with the analyzer and less than 3 watts of reflected power on 75 M phone with 100 watts. I can hear myself on the Utah SDR (I am in So Cal)

    Previously I had the wire come into the shack and connect directly to the home brew tuner I have. All new members of our hobby will eventually learn the hard way what RF burn is all about :eek:

    I currently have 3 random length elevated counterpoise.

    I also have a 10 pound spool of 22 ga solid copper (a mile or more). Been wondering about using it for as many counterpoise as I can lay down.
  9. KC8VWM

    KC8VWM Ham Member QRZ Page

    Random antenna = Random results.
    KU4X and KP4SX like this.
  10. KM6CND

    KM6CND Ham Member QRZ Page

    You got that right!

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