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5/8 Wave Base Antenna, no luck

Discussion in 'General Technical Questions and Answers' started by CUCTU, Nov 24, 2008.

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  1. CUCTU

    CUCTU QRZ Member

    Hi,

    I've had no luck with this antenna. I've made it basically as illustrated in another post here:

    [​IMG]

    I started with a 1/2" brass coupling and soldered #14 copper wire to the rim. Then I screwed in a 1/2" x 48" PVC pipe threaded on both ends to wrap the 10.5 turns and continued the copper wire straight up the pipe. Then I soldered on some brass rods through the coupling for radials.

    I did a 100 MHz to 200 MHz sweep to measure the vswr over this range and note the valley:

    r=20 x=6.5 ohms @ 135.70 MHz (vswr = 2.55, lowest)

    I tried shortening the "whip" and moving the tap, but not much really changed.

    Ironically, the vswr looks more promising up around 70cm:

    r=40.5 x=11 @ 434.10 MHz (vswr = 1.39)

    Smith chart (red is VHF, black is UHF, green are the markers above):

    [​IMG]
     
  2. K7JEM

    K7JEM Ham Member QRZ Page

    Try a series coil instead of a tapped coil. The coil will be about 3 turns of wire, about 1" in diameter, turns spaced with about one wire diameter.

    That should be about 9-10 inches of wire, overall. Adjust overall whip length to match SWR, but 48" will be a close start.

    The PVC pipe may affect the length, but if you sweep it you should see the match is good close to 2M.

    Make sure there is a good ground plane or radials in place, about 20" radials, 3 or 4 should work.

    Joe
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Please describe your radial/ground-plane system.
     
  4. AC0FP

    AC0FP Ham Member QRZ Page

    There are some good suggestions here. The 5/8 wave antenna requires a good ground plane to work properly! Try measuring the antenna by itself, it should be capacitive and when the capacitance is tuned out with a small coil at the base of the antenna the impedance on the smith chart should rotate around to somewhere around 50 ohms +/- j 0.

    73,

    Frank:)
     
  5. CUCTU

    CUCTU QRZ Member

    2 rods, ~40" long (4 ground radials) are inserted perpendicular to each other through the brass coupling, about 1/4" and 3/8" down from the upper lip of the coupling so they cross in the center of the coupling.
     
  6. K8JD

    K8JD Ham Member QRZ Page

    On the comercial antennas I tore down , the coil feed tap is quite close to the ground end since it is operating a bit like an autotransformer where the larger end is toward the higher Z of the whip (2K?)and smaller turns toward the low Z (50 ohms) feedline.
     
  7. CUCTU

    CUCTU QRZ Member

    Moving the tap to 2 turns down from the whip made a big improvement.

    r=35 x=12.5 (vswr = 1.6) @ 155 MHz
     
  8. CUCTU

    CUCTU QRZ Member

    forgot the plot:

    [​IMG]
     
  9. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    That's definetly inductive reactance. Add a small piston capacitor, around 2 to 10 pf between the inductor and ground and see what you get. Better yet, make the antenna about 3 inches shorter.
     
  10. KA1MDA

    KA1MDA Ham Member QRZ Page

    According to my ARRL antenna handbook, coild form should be 3/4" diameter and 3 1/2" long. Coil consists of 10 1/2 turns of #10 or 12 wire. Whip is 47" long. Coax sheild connects to ground side of coil, coax center conductor is tapped approx 4 turns down from whip end of coil.

    I built one of these 2 meter 5/8 wave antennas about 20 years ago, and it worked fine just as described in the ARRL antenna handbook.

    Tom, KA1MDA
     
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