17/12 inverted vee

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KP4SX, Nov 21, 2021.

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  1. KP4SX

    KP4SX Premium Subscriber QRZ Page

    I just finished 'rebuilding' my fanned vees for 17 and 12 meters.
    The 17m segment works fb but the 12 is nowhere near resonance. On both the VNA and the MFJ 259B various resonances are showing but not where I cut it for!
    The vee is on a 20 ft mast on a concrete roof edge and its additionally 15 ft above ground on one side.
    Do 17 and 12 just not play well? The previous antenna behaved the same way.
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I've built fan dipoles that included both the 17m and 12m bands and though all parallel fed antennas have some interactions I didn't have particularly tough problems with those antennas or interactions between those two bands. Does your design include a fair amount of separation between the individual wire elements at the feed point and at the ends or are they fairly close together?

    I'd probably just start doing fold back tuning of the 12m element assuming it tunes low in frequency making sure I periodically re-checked the 17m tuning and keep iterating towards a good balanced tuning solution.
     
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    My assumptions:

    bare #14awg copper wire
    apex height 35ft
    apex down angle 35degrees
    average earth
    Coax well choked at feedpoint (Gotta have a CM choke or all bets are off)​

    Suggest changes if I guessed wrong

    Model:
    upload_2021-11-21_7-34-19.png

    Variables of the model:
    upload_2021-11-21_7-35-35.png

    The required wire lengths are X and Y. This geometry produces this:
    upload_2021-11-21_7-39-12.png

    These results depend on having a CM coax choke right below the feedpoint. If one side of the antenna is shorted to the mast or to the coax sheild, that completely changes everthing!
     
  4. W1VT

    W1VT Ham Member QRZ Page

    I have a 20 ft flagpole that I lowered to resonate on 17 instead of 30M. Then I added a 12M element in parallel. SWR is 2:1 on 17 and 3:1 on 12. Low enough on 12 to just use the autotuner in the shack.
    I have five other antennas designed to work on 12 meters, so I'm not inclined to mess with it by adjusting element lengths.

    It did show that my front yard used to be a lot noisier than the back yard before they put up a new power pole in my front yard.
     
    Last edited: Nov 21, 2021
  5. KP4SX

    KP4SX Premium Subscriber QRZ Page

    1. Only a coil of several turns of RG213 at the feed point.
    2. The two vees are not exactly in the same plane. I don't really know how to quantify that. Let's just say the 17m runs exactly E-W and the 12m is ENE-WSW.
    3. Mast isn't grounded and nothing is touching it.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Likely worse than no choke at all. Ugly (coiled coax) chokes are totally unpredictable (worthless) unless you actually measure their Self-Resonant Frequency using a grid-dip oscillator or set them up for a Vna measurement to determine their SRF. If the coils are not tightly held together (on a ridgid coil form), their SRF can change over time.

    Even if you carefully tune the SRF to say the 17m band, they do not act as a choke on the 12m band or vise versa (only work as a choke on a very narrow range of frequencies "high Q"). Forget Ugly chokes, and get a real ferrite torioid choke made by winding Teflon coax on a FT24- type31 ferrite donut (they work as a choke over at least three frequency octaves).

    If you use a proper ferrite choke at the feedpoint, it is not necessary to splay apart the two sets of dipoles; they can be parallel and as close as a few inches with little interaction. It is the lack of coax Choking that makes hams believe that it is impossible to make a fan dipole without there being interaction between the dipoles. Without choking, energy is stored as a standing wave along the coax shield, and that causes the interaction (coupling) during tuning the individual dipoles to the various bands. If you block the coax-shield Common Mode standing wave by putting a choke just below the feedpoint, the interaction is almost totally eliminated.

    Sounds like if you look down on the two sets of dipoles from above, then they are splayed about 45 degrees? How was my guess on the down angle? When I get some time, I will see what splaying the two dipoles does to the tuning of the two dipoles.
     
    Last edited: Nov 21, 2021
    AK5B and W9IQ like this.
  7. AK5B

    AK5B Ham Member QRZ Page

    Bolded and enlarged for the benefit of future generations of hams that will see this most salient point about TUNING A FAN DIPOLE / FAN VERTICAL as they search fruitlessly for how to tune multiple elements far more easily than previously thought...;) :)

    All thumbs up for this one!

    73,

    Jeff

    Longtime fan of fans
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    I rotated the shorter set of wires

    Top view:
    upload_2021-11-21_13-28-9.png

    Side view:
    upload_2021-11-21_13-29-24.png

    Splaying the short legs by 45 degrees has virtually no effect on feedpoint R, jX, with the same wire lengths as before.

    This result is predicated on having the proper CM choke at the feedpoint. If that choke wasn't there, I would have to include the coax shield as a conductor in the antenna model, and I would have to know how the rig is connected to earth via the house's in-wall wiring or explicitly grounded for lightning protection if you are doing anything about that.
     
    Last edited: Nov 21, 2021
  9. KP4SX

    KP4SX Premium Subscriber QRZ Page

    That looks pretty close, maybe more than actual splay but if it doesn't matter...
    BTW, they are splayed because of available tie-offpoints.
    I'll look into adding a ferrite choke
     
  10. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Will I be committing an antenna crime if I place the CM choke about a foot down the coax from the feedpoint?
     

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