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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


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


    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):

  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.

  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.


  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:

  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
  11. K8ERV

    K8ERV Ham Member QRZ Page

    I've made a few 5/8 for 2M, needed a cap in series with the tap. Have to juggle the ant length, coil, tap, and cap. Eventually worked well.

    TOM K8ERV Montrose Colo
  12. K4EEZ

    K4EEZ Ham Member QRZ Page

    here u go
    making the antenna like this you can use it on 2 meters and 6 meters to.
    click on the attachment/pic below.

    ps QRZ messed the picture up and can't re edit the pic... but you can still see the comcept of the idea

    the whip is around 54 to 56 inchs long
    the coil is wound over 1 inch Diameter 3 turns with 1/4 inch spacing, and the top of the coil connects to the whip
    the bottom of the coil connects to the center conductor of your coax/feed line ONLY
    HINT: use a mirror mount or stud mount to accomplish this
    the shield MUST go to your ground plane this is a 5/8 wave on two meters and 1/4 wave on 6 meters.

    contact me if you need the picture in its true format for my email address
    again 7 3
    Last edited: Nov 28, 2008
  13. WA7KKP

    WA7KKP Ham Member QRZ Page

    good start . . . .

    Reading the books on antenna theory, the 5/8 wave has the maximum radiation at the hoirizon, but the feedpoint is nowhere near 50/75 ohms resistive, and there's a reactive component.

    Most mobile 5/8 wave antennas use a small series coil to electrically make it looke like a 3/4 wavelength antenna -- and as you should remember, odd multiples of a 1/4 wavelength have pretty much the same nominal impedance.

    Go one step further, and you'll discover that a 6 meter whip quarter wavelength will work rather well on 2 meters, and a 2 meter 1/4 wave whip will work pretty well on 440 MHz, because of the 3/4 wavelength thing.

    Little known fact -- most commercial high band 5/8 wave antennas will work on 6 meters, with an SWR of less than 2:1 in almost every case.

    Okay, you can figure out the rest.

    Gary WA7KKP
  14. W5DXP

    W5DXP Ham Member QRZ Page

    Gary, "work rather well" is a false statement. "Load rather well" might be true, but 3/4WL vertical monopole antennas send the majority of the radiation up at a high angle of radiation to be lost in space forever. If such configurations "worked rather well", initiated amateur radio operators would be using them but rarely is that the case. Most initiated hams use collinear phasing for their dual-banders.
  15. K4EEZ

    K4EEZ Ham Member QRZ Page

    DITTO... :D

  16. CUCTU

    CUCTU QRZ Member

    Well, I went the route of some of the suggestions and ended up with a 3/4 wave antenna. Not what I wanted, so I started over and finally got the 5/8 wave antenna to work:

    I found the tap point to be about 3-2/3 turns from the top of the coil. The "whip" part is 47.5 inches long. (I also discovered that I had some bad coax causing most of my problems before)



  17. CUCTU

    CUCTU QRZ Member

    BTW, the S)239 is screwed into the bottom of the brass coupling, so I need to figure out a way to mount it!
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