Inverted V antenna doesn't tune at 40M

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK4JJ, Nov 21, 2021.

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  1. VK4JJ

    VK4JJ Ham Member QRZ Page

    I made an linked inverted v for 40/20/15 (before I realised I could use the 3rd harmonic of 40 on 15!) out of 19AWG PVC coated wire.

    15 and 20 work fine - I did my first parks activation on those bands and it was a blast. My Anderson powerpole crimp failed so I couldn't try it on 40.

    This weekend, I took it to the park to tune it. I hung the centre connector with a 1:1 balun through a bit of rope through a tree (probably 12 feet high), fed it with a handy 12M length of RG58 coax and swept it with my nanoVNA. It seemed resonant at 6.5MHz and around 20MHz. I trimmed about six inches off, equal on both sides. No change.
    Trimmed some more. Another six inches. No change.

    At this point, I got a little worried. I didn't want to keep trimming if there was obviously nothing happening as the dipole was getting shorter and shorter.

    I've measured it again now:

    Banana plug to first link - 5.35 metres
    First link to second link - 1.570 M
    Second link to insulator - 3.2M

    Can anyone help me understand why this inverted v antenna appears 'stuck' and won't tune?
     
  2. N8PKN

    N8PKN Ham Member QRZ Page

    My suspicion is you have a bad connection through your balun, and are only measuring the feedline. Cutting six inches should have made a change in resonance if you were actually sweeping the antenna.
    Check all connections with an ohmeter.

    Good luck.

    N8PKN
     
    K7TRF likes this.
  3. K1VW

    K1VW Premium Subscriber QRZ Page

    Hi,

    Having a real 15m antenna in your linked dipole arrangement is a good thing. It will have a more proper dipole radiation pattern vs using a 40m antenna on 15 will have a cauliflower pattern. Most of us who use our 40m antennas on 15 tolerate it because we are too lazy to do linking and unlinking on band changes on a permanent setup.

    Did your trimming really yield zero change? Or just a lot less than you expected? If you are moving from 6.5mhz to 7.2 mhz, that may require trimming something like 7 feet (total), so trimming a foot from each side should move it to 6.7mhz. If that is what you saw, then you are headed in the right direction, you just have a ways to go. If it really yielded zero change, then yes you may have a connection problem.

    Good luck,
    Joe K1VW
     
  4. WN1MB

    WN1MB Ham Member QRZ Page

    I'd be very wary of SWR/fres readings of a 40 meter inverted vee with an apex at only 12' high. How high are the ends? I'm envisioning more of a low dipole than an inverted vee.
     
    AK5B likes this.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    How long is the coax?
    Is there a coax CM choke where the coax connects to the wires?
     
    AK5B likes this.
  6. W1VT

    W1VT Ham Member QRZ Page

    I'd test the balun by changing the feedline length, ideally with another feedline.
     
  7. AK5B

    AK5B Ham Member QRZ Page

    Besides that, summer is beginning in your part of the world now, and your earthworms underneath your antenna do not need any extra warming at this time.
     
    WN1MB likes this.
  8. WN1MB

    WN1MB Ham Member QRZ Page

    Reread the original post:
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    ... raises a new question:

    If the OP is operating in the park, presumably everything is on batteries and there is no conductive path from the battery/rig/coax to earth ground? Everything is isolated, no car near the antenna/coax?

    Please explain this in more detail:"I hung the centre connector with a 1:1 balun through a bit of rope through a tree (probably 12 feet high),"

    I need to get a detailed description of the coax CM choke. Type? Where placed?

    Finally, you didn't say where the ends of the dipole where while you are attempting to tune it. Is is stretched out such that the ends are also 12m agl, or did you let them sag. How close were the dipole ends to the earth?
     
    Last edited: Nov 21, 2021
    WN1MB likes this.
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    My preliminary calculations say that these values of J, K, and L (in m) yield resonance at 7.15, 14.175, and 21.225 MHz:
    upload_2021-11-21_15-27-50.png

    Formulas:
    upload_2021-11-21_15-35-17.png

    Model:
    upload_2021-11-21_15-37-35.png
     
    Last edited: Nov 21, 2021

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