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Simulation Fun: Rotary 5-Band Antennas - Let's Compare Them

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by SP3L, Jan 25, 2016.

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  1. SP3L

    SP3L Ham Member QRZ Page

    The antenna looks like that.
    All wires are Copper, 2mm in diameter.
    Transmission line characteristic impedance is 450 ohms. Both lines are 2.5 m long in the NEC model but when building this antenna one must multiply this value by the VF (Velocity Factor) of the transmission line used. VF usually equals 0.9-0.91 for a 450 ohm window line. So, in the real world implementation, each TL should be 2.25 +/-0.05 m long. The feed point will be a little closer to the driven element than shown in the picture above. The main feed point is on the boom, the split feed points are 1.87 m above and below the boom.

    The antenna impedance is approx. 450 ohm. So, you will need a 9:1 balun to feed it. A 3-core Guanella balun seems to be the best choice.

    The distance between the driven element and the reflector is 3.2 m.
    The table below presents dimensions of the loops.


    The SWR, gain and F/B ratio plots of the 5-band SPF Cubical Quad are shown below.




    To be continued.
  2. SP3L

    SP3L Ham Member QRZ Page


    I compared the SPF Quad with the Flat and Spider Quads. See the graphs below.



    SPF Quad gain is on par with the classic quads. F/B ratio seems to be somewhat worse on 12 and 10 m but I designed this quad to cover the whole 10 m band with low SWR what can not be said about the other two. Probably, if you brought more close together the driven element and the reflector, the F/B would improve but the usable bandwidth will decrease.

    All in all, the SPF Cubical Quad is quite close in performance with the other quads and does not require any relays for band switching. The goal of this design has been met.

    I am attaching the updated Comparison Table and the Model Pack.

    I also design the diamond version of the SPF Quad. I will share it soon when I find some time to prepare an article. Sometimes, I think that preparing an article takes longer than modeling a new antenna...


    Attached Files:

  3. SP3L

    SP3L Ham Member QRZ Page

    The SPF Diamond Quad

    The view
    The driven elements and reflectors are 2.5 meters apart. The split feed points are 2.225 m above and below the boom. This time, the ideal 450 ohm transmission lines are 2.5 m log in the model but when corrected by VF, they will be about 2.25 m long. It means that they will be almost exactly in the same plane as the driven elements. The dimensions of the loops are shown in the table below.


    Find below the SWR, gain and F/B ratio plots of the 5-band SPF Cubical Quad.


    And the comparison with the other quads:

    The SPF Diamond Quad has quite decent gain. Its F/B is not extremally high but quite stable (about 15 dB in free space) no matter what frequency. That means that also over the real ground its radiation patterns will not change dramatically throughout the band. See below an example from 20 m band: lowest, mid and highest frequencies. 11 m above the ground.

    The model pack and the comparison table will appear in the following post as I have just reached the limit of 10 attachments...
  4. SP3L

    SP3L Ham Member QRZ Page

    And the model pack and the comparison table attachments.

    Well, I think that the last posts demonstrated that the split feed point approach is an interesting option for an antenna designer.

    Yesterday, I tried using it for a 5-band fan dipole (a.k.a. multi-dipole). This trick allowed me to design the antenna with wires separated by only 15 cm (6”) from one another while limiting the SWR measured at the feed point below 2:1 for any amateur frequency in the 14.0-29.7 MHz range. Not bad. In the traditional design, the wires can not be closer than about 30 cm but even then, it is not possible to cover the whole 10 m band. If anybody is interested, I can share this design too.

    Attached Files:

    UA3TW likes this.
  5. W0JRN

    W0JRN Ham Member QRZ Page

    I am intriged and will build one, have you considered a reflector exactly the same just 5% larger?
  6. SP3L

    SP3L Ham Member QRZ Page

    The reflector is actually the same size as for a regular cubical quad - a couple percent lager than the classic driven element. But the SPF driven element needs to be slightly larger than the reflector. That's a small penalty for getting rid of the band switching relays.
    Keep us up to date about your progress in building the antenna. Good luck!
  7. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Gain without beams also works for amateur radio check out my Vee beams and rhombics, I can switch nine Direction's with 12 to 14 DB gain on 40 meter, no rotors or moving parts, I had multiple bands stacked beams before the ice out here on the Prairie of KS kept taking them down every winter

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