Minimizing Line Radiation

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W9ZD, Sep 16, 2017.

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  1. W9ZD

    W9ZD Subscriber QRZ Page

    I have a city row house and am anxious to not upset my neighbors by turning their electronically controlled devices on and off as I transmit. I also have an extensive junk box with a variety of loading coils, baluns and auto tuners. I'm planning to put up an 18' MFJ vertical up from a top floor balcony. to transmit on 40, 20 and 17 meters. My 100 watt transmitter will be located in the kitchen 30' below. I will be running 35' of new coax, either 52 ohm or 75 ohm, horizontally from the transmitter to a 30' vertical section of abandoned Direct TV 75 ohm coax running up the side of the house and then a 15' coax, either 52 or 75, from the top end of the cable line to my vertical. My question is where should I locate the auto tuner, balun, and adjustable loading coil to minimize losses and line radiation? I don't feel like replacing the 30' section of Direct TV coax with 52 ohm. Thanks. '73 Rick
     
  2. AI3V

    AI3V Ham Member QRZ Page

    Somewhere other than in a row of houses.

    Like out in the middle of a cornfield if you prefer Marconi style antennas, or maybe the woods if you like trees for hanging Hertz type antennas.

    Rege
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    What if your neighbor's RF sensitive electronic devices are on the second floor in a direct line with your antenna? What then?

    What are you using for a counterpoise? What if it radiates?
     
  4. N8AFT

    N8AFT Subscriber QRZ Page

    better stick with using the 75 ohm RG6 for the entire distance. it is near enough impedance that it should not give you a problem. vy likely it is braided shield with a foil 2nd shield as well so it's less likely to radiate your RF.
    there are coaxial adaptors available to convert from PL259 to F both in male and female fittings. place the tuner at the rig, the balun at the antenna. see what happens... unless you overdrive your ssb signal via excess mic gain and processor use I don't think you'll have a problem.
    you'll have to try it someday, right?
     
  5. W9ZD

    W9ZD Subscriber QRZ Page

    Counterpoise? Four 20' or so insulated wires routed away from human contact. If the neighbor bitches then I will be a good ham and check it out and eliminate the problem one way or the other. But not til it happens.
     
  6. W9ZD

    W9ZD Subscriber QRZ Page

    Thanks. All of that makes sense. The coax is quad shield. Fell off somebody's truck before I bought it at Dayton. I have the adapters. The antenna is an MFJ Hy-Gain 18VS (another proud hamfest acquisition.) It has an adjustable loading coil at its base. Should the loading coil come before or after the balun?
     
  7. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    It shouldn't make any difference if the coax is quad-shielded or not. Inconvenient radiation from the line in amateur installations is almost always caused by common-mode currents on the outside of the coax shield. Several shields are not an effective remedy for this; in fact, for this purpose they are no better than a coax with a simple braid shield, since the current will appear on the outside of the outer shield no matter how many shields are in use.

    The type of antenna and the details of the installation are a much, much bigger factor in whether or not there is RFI-producing radiation from the coax run.

    This is why Cecil asked about your counterpoise. An effective return path for antenna currents will do a very great deal to prevent common-mode problems with coax, as well as raise efficiency (and thus usually reduce the SWR50 2:1 bandwidth!).

    As you describe, you can start with four 20' radials and see what happens; but some installing this antenna would start with four elevated, tuned radials for each band of interest. Note the instructions imply (but do not overtly state) that elevated tuned radials are a good idea when they suggest four 33' radials and then state, "The 33 foot dimension is measured from the base assembly to the strain insulator, and is optimum for 40 meter operation only."

    So to get the most convenient installation, start with your four non-resonant counterpoises. But to get the best possible performance with this antenna from the get-go, consider installing four tuned radials for each band of interest.
     
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    The installation of a short vertical on a balcony is one of the worst available from an EMC and EMI standpoint.
    As the outer shield of the feedline will be part of the antenna structure and carry currents from the feedpoint, it is difficult to prevent them
    showing up in your neighbour's electronics. Also, there is an intense "near field bubble" around the feedpoint that may influence electronics directly.

    This antenna is not intended for multiband use, without manually adjusting the loading coil at each band and larger frequency change.
    Multiband operation may be possible using a wide-range autotuner and removing the loading coil entirely.
    The autotuner should then be placed at the feedpoint.

    73/
    Karl-Arne
    SM0AOM
     
    AI3V and WB5YUZ like this.
  9. W9ZD

    W9ZD Subscriber QRZ Page

     
  10. K0OKS

    K0OKS Ham Member QRZ Page

    As others have pointed out, this is not a great situation, but you can likely make something work.

    If you can somehow get a balanced antenna up like a dipole that would be ideal, but you are still going to be putting yourself and your neighbors in the near field. This is likely not a safety issue, at 100W, but it is likely to cause RFI issues. I destroyed a (poorly shielded) CyberPower UPS with 100W and an attic dipole.

    An MFJ Artificial Ground with some counterpoise wires hung around can help a lot with this. But you must remember to tune the Artificial Ground at low power before transmitting on a new frequency any considerable distance from the last tuned frequency (just like a tube amp).

    Once your neighbors know you have a transmitter they will likely blame you for any issues with anything at all, even the ubiquitous but usually unrelated, "My computer keeps crashing."

    Perhaps you could get the antenna farther away somehow? String something over to a tree?
     

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