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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

    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 Ham Member

    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

    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

    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

    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

    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

    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

    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

    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

    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
     
  11. KD8GFC

    KD8GFC Ham Member

    I guess things just are not that simple. Thanks for the correction now I know. Jim !!
     
  12. KC2UGV

    KC2UGV Ham Member

    You know, I didn't look at the SWR, I was just sweeping through the freqs to see where else I got a 50 Ohm impedance.

    Figures, I borrow a new toy, and use it wrong lol
     
  13. G3TXQ

    G3TXQ Ham Member

    Corey,

    As you just found out, because the resistive part of the impedance ("R=" on the MFJ259) is 50 ohms that doesn't make the VSWR low; you also need the reactance to be close to zero - that's the "X=" bit on the MFJ259.

    For example, if R=50 and X=35 you have a VSWR of about 2:1; if R=50 and X=100 you have a VSWR of about 6:1

    Have fun learning,
    Steve
     
  14. K4UUG

    K4UUG Ham Member


    http://www.hamuniverse.com/10metertechniciandipole.html

    http://www.hamuniverse.com/balun.html
    :)
     
  15. KC2UGV

    KC2UGV Ham Member

  16. K7FE

    K7FE QRZ Lifetime Member #1

    Your antenna is a currently 20 meter inverted V, because it is 16.5 feet on each leg.

    If you want it to work on 10 meters, it would need half the length that you used.

    Roughly: 465/frequency in MHz = 465/28.4 = 16.37 ft = 1/2 wave length Then divide that number by 2 for a 1/4 wave = 8.19 ft per side.

    73,
    Terry, K7FE
     
  17. KC2UGV

    KC2UGV Ham Member

    Ok, so the inverted V isn't working out so great right now. Probably because it's plastered against the wall of my house on the outside.

    I'm considering moving it indoors, and ran across the ceiling. At least until I get my hands on the coax to get it on the roof.

    Would this be better or worse than the inverted V on the outside like I have it?
     
  18. K0RGR

    K0RGR Premium Subscriber

    How are you judging the performance of the inverted V, Corey?

    At this time of year, the dominant mode of propagation is Eskip, and almost anything should work for that when the band is open. When the band is not open, you would have only a bit of extended 'scatter' type propagation on 10, and if there aren't any locals, you aren't going to hear much.

    It really all depends on what the materials are in the end of the house. If the antenna is close to metal gutters or 'chicken wire' in the walls, it's not going to work well. Some shingles also contain serious amounts of metal, and have much the same effect.

    But a friend and I built a 'turnstile' for 10 many years ago - basically two dipoles at right angles to each other - and laid it on his roof, with one dipole running the length of the ridgeline and the other drooping down the sides at right angles to the ridgeline. This was not a great 10 meter DX antenna, but it worked very well for satellite reception and domestic contacts on 10. You can build a matching harness using 75 ohm coax and a 'tee' connector. You need 1/4 wavelength of the coax on one side of the tee and 1/2 wavelength on the other. Instead of the normal dipole formula, use the formula for radio waves in free space (wavelength in meters = 300/F in Mhz) and divide the result by 2 or 4. Then, multiply times the velocity factor of the coax and convert from meters to feet if needed. That should get you plenty close for 10 meters.
     
  19. KC2UGV

    KC2UGV Ham Member

    I guess the only judgment is that I'm not hearing anything but noise. I figured I'd hear at least a beacon or two :)

    After your advice though, I might just wait until I get up on the roof to change anything. As far as I know, the house is all wood, and the roof has a nice peak to it.

    I'll give your design some thought, and look at some other options if it's going on the roof :) Thanks.

     
  20. W0IS

    W0IS Ham Member

    There's probably not much to hear right now. I think most of the beacons between 28.2 and 28.3 are just a couple of watts. So unless there's one in your local area, you won't hear any unless the band is open to somewhere.

    The best way to test whether it's receiving is to tune to 27 MHz and see if you hear anything. Even then, it might sound pretty quiet for a couple of reasons. First, contrary to popular opinion, there's really not that much CB activity these days. And secondly, you'll be cross-polarized to most local signals. But in most areas, particularly if you're near a major highway, you should hear something.

    I would try to get it away from the side of the house. Even a couple of feet will probably result in some improvement.

    In addition to listening and calling CQ, the following web page is a good way to alert yourself to openings on 10:

    http://www.vhfdx.net/spots/map.php?Lan=E&Frec=28&Map=NA&mycall=&myloc=&freq=&prop=
     
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