10 meter inverted V

Discussion in 'General Technical Questions and Answers' started by KC2UGV, Apr 25, 2009.

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  1. KC2UGV

    KC2UGV Ham Member QRZ Page

    Ok, so I'm building an inverted V antenna right now.

    Basically, I have a chassis SO-239 with 24 GA wire soldered to the lead tip and to the shell ground.

    I've got 50 Ohm impedance on the thing, but the SWR is reading infinity (It's an MFJ-259 analyzer) on the meter.

    Is this ok, or is something wrong here (Bad solder joint, or something). Here's an ugly diagram of what I have:
    Code:
    ........................*.......................
    ......................./..\......................
    ....................../....\.....................
    ..................../........\...................
    
    * is the feedpoint
    / & \ is the antenna lines.

    I'm feeding it from a second story window to about 5 feet off the ground.

    Each line is 16.5" long, and the feedline is 20ft RG-58U pre-made cable.

    What am I missing, or is this good to go, and ignore the SWR reading since I have a perfect match?

    And if it's way wrong, any pointers about what is wrong?

    Thanks in advance :)
     
  2. KC8VWM

    KC8VWM Premium Subscriber QRZ Page

    RG 58/U is good for short runs (such as in a mobile radio installation environment) but it may become lossy and you may start experiencing performance issues when using too much of it. 20 feet doesn't seem excessive and it should *work* OK.

    The design frequency for your antenna is wrong for typical 10m CW/SSB work. I would probobly revisit that measurement a little and redesign it for 28.300 mhz or cut each leg for 8ft 3 - 1/4"

    A *high* VSWR reading may be a function of a nearby metal object or other unintended reflection. It seems indicative of *something* in proximity to the antenna causing a problem. That is if the antenna is designed and cut to specifications. I would try to get the ends up a little higher than 5 feet off the ground. Try for higher if you can but in no instance should the inverted V legs be closer than 45 degrees or it will start changing the antenna's feedpoint impedance characteristics.

    [​IMG]

    Also be sure to keep the feedline at 90 degrees to the feedpoint. For example, it's not a good idea to run the feedline in the same direction as one of the legs.

    A VSWR reading of *infinity* is typically indicative of an open or a short circuit somewhere in the connections. Perhaps one of the connectors have developed an internal short and it may require re installation and re-soldering.

    A quick check using a VOM meter can usually reveal an open or short circuit in the feedline connections.
     
    Last edited: Apr 25, 2009
  3. WB3BEL

    WB3BEL Ham Member QRZ Page

    Thats your problem. Each wire should not be this long. For ten meters it should only be a bit over 8 foot long each wire.
     
  4. KC2UGV

    KC2UGV Ham Member QRZ Page

    AHA! Serves me for following the directions on a website blindly lol

    Nevermind, I know what I did. Skipped the part where you "Take half of what the antenna formula spits out for each leg"

    Silly me :) And I was wondering why this was twice the size of a dipole.

    Thanks 'BEL
     
    Last edited: Apr 25, 2009
  5. KC2UGV

    KC2UGV Ham Member QRZ Page

    Thanks everyone for your help. After getting it down to 8.5" each leg, and clipping a combined 6", I got the impedance between 20-60 Ohms, and 1.7 or less across the 10 meter band :)
     
  6. W9GB

    W9GB Ham Member QRZ Page

    Why you have the Analyzer out, spin up to 14 to 18 MHz range -- should find a resonant point in that segment -- hey! 20 or 17 meters!

    w9gb
     
  7. KC2UGV

    KC2UGV Ham Member QRZ Page

    Yep yep. Resonant on 20 as well :) I found that out while "playing". Now, gotta get 20M going :) I think I got a 20M pixie around here somwhere...

    /me scratches head trying to remember where I put it...
     
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    I'm probably missing something obvious, but why would a 10m half-wave centre-fed antenna show a resonance on 20m or 17m ?

    Steve
     
  9. KD8GFC

    KD8GFC Ham Member QRZ Page

    I think and I could be wrong but a 10 meter 1/2 wave will act as a 1/4 wave on 20 meter. 14 MHZ x 2 = 28 MHZ Good luck with your dipole 10 M should be open with some E very soon.. Jim !!
     
  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    A 1/4 wave centre fed wire is not resonant - it has a highly reactive feedpoint.

    A half-wave dipole cut for 10m produces a 50 ohm VSWR of over 800:1 if used on 20m. That would cause 20dB of loss in 100ft of RG213.

    I just wish I was wrong - then I could halve the size of all my wire antennas ;)

    Steve
     
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