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Sirio Tornado 27 modified to 6m band.

Discussion in 'CB Antennas' started by nav2010, Dec 30, 2020.

  1. nav2010

    nav2010 Active Member

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    My son ordered a Sirio Tornado 50-60 as a Xmas present for me. On opening the box on Xmas day I was presented with the correct paperwork for a 50-60 but the wrong antenna, they sent the Tornado 27 by mistake.
    Bob85 and myself discussed the options, it was either send it back with $25 shipping costs between myself and the vendor or go for a conversion.
    I slept on it and after a little research decided to go for the conversion. Initially we were unsure if there was a parallel LC network between the mounting pole and the driven element but after watching a video of a broken Tornado I decided that the two poles didn't overlap and that any network was probably between the tuning coil and the bottom of the driven element.
    Tuning coil.
    First thing to modify is the tuning coil. This is not a dc grounded coil on either the 27 or the 50-60, its an open circuit design with probably some parallel capacitance between the coil and the element. The Tornado 27 is about 6.5 turns and from pictures we concluded that the 50-60 is just 3 turns. It was easy to modify, just cut the coil at the top to the desired length then move the retaining ring and bolt the coil back on.
    Torcoil.jpg
    Another photo of where to cut. The red marked part of the coil is the part you keep.



    tor277.jpg
    Radials.
    This was easy. The Tornado 27 comes with a 2 section set of 4 radials approx quarter wave at 27.5mhz. The 50-60 comes with 4 quarter wave radials approx 1170mm. As it happens Sirio use the same length radials for both antennas but the 27 has an addition inner section. The Sirio 27 radial that bolts to the hub is exactly 1170mm without its inner section. So all you need to do is use the first half of your 27 radials at 1170mm. No cutting required. Another instance of Sirio using interchangeable parts.
    Driven element.
    I kept the same number of sections (6) and cut them so I finished with a total length from the radials to the tip of 3855mm. Make sure the adjustable tip has at least 12 inches of adjustment in either direction so you can make it resonant anywhere from 48mhz to 54mhz. In my case I was aiming at 50mhz because that's where I'm transmitting. 3855mm is exactly 50mhz.
    If you notice I cut more from the bottom two sections than the other 4. This made the antenna lighter in weight and from pictures I looked at of a 50-60 was more proportional to the amount of lager diameter tube. It isn't massively important but keeps the same kind of inductance.
    Tuning.
    It tuned at less than 1.1:1 right from the get go at 50mhz. It has 3mhz of bandwidth at less than 1.3:1 and 4mhz of bandwidth at less than 1.5:1. It will tune anywhere between 48 and 54mhz at 1.1:1 with the adjustable tip.
    We now know that Sirio use the same parts in its range of Tornado antennas but with different tuning coils and element lengths. Thanks to Bob85 for his advice in this project.
    tornado.jpg
     
    Slowmover, bob85, sp5it and 1 other person like this.

  2. nav2010

    nav2010 Active Member

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    An important note: On Sirio tuning charts for this antenna, the tuning length is measured from the radials to the tip and not the length of the driven element.
    tuning.jpg
     
    Marconi likes this.
  3. Slowmover

    Slowmover Elmer

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    Nicely done, thanks for the time & effort!
     
    nav2010 likes this.
  4. ForestRunner98

    ForestRunner98 917 SoCal

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    Great job on making it work!!

    BTW...was the antenna purchased at DNJ? I ask only because they messed up my antenna order as well(wrong antenna)
     
  5. bob85

    bob85 Supporting Member

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    "Return shipping & more waiting, fck that where's my hacksaw" (y)
     
  6. nav2010

    nav2010 Active Member

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    No this was in the United Kingdom
     

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