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SGC SG-239 Questions (40m multiband center-fed doublet)

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  • Jeff Hall
    Hi everyone, I am designing a new antenna for portable use. I have an SG-239 tuner, and plan to feed it with 300 ohm ladder line (from DX Engineering). The
    Message 1 of 3 , Jul 30, 2010
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      Hi everyone,

      I am designing a new antenna for portable use.  I have an SG-239 tuner, and plan to feed it with 300 ohm ladder line (from DX Engineering).  The antenna will be configured as an inverted V, with the apex at 40 feet (at the top of a Spiderbeam telescoping fiberglass pole).  I will be weather proofing the tuner as shown on the SGC website (http://www.sgcworld.com/homemadetechnote.html).

      Question #1: I do not plan to use a balun.  From a quick perusal of the Smartuners group, it appears that is is acceptable, so long as the length of the ladder line is adjusted correctly to tune on all desired bands.

      I plan to follow DX Engineering's formula for calculating the antenna and feed line lengths.  The 300 ohm ladder line has a velocity factor of 0.88.  The lowest band of operation will be 40 meters.  DX Engineering states the length of the transmission line must be an odd multiple of 1/8th of the lowest frequency to be used:

      Length = (123 / 7) x 0.88 = ~15.4 feet

      (where 123 = 1/8th wavelength factor and 7 = lowest planned operating freq in MHz)

      The apex will be at 40 feet, so we need to multiply by 3 to get an odd wavelength multiple of 46.2 feet.  The excess 6.2' of ladder line will slope down to the SG-239, which will be sitting on a small milk crate near the base of the antenna mast.  Out the other side of the tuner will be some length of coax, probably 25' of RG-8X, back to my operating position.

      DX Engineering recommends a shortened dipole wire length of 55 feet (27.5' each side).  The apex angle will be between 90 and 120 degrees.

      Question #2: For a balanced antenna, I don't need to ground the tuner itself, correct?

      I plan to power the SG-239 from a nearby 12V AGM battery (e.g., 12V 7Ah).  Or I could run a longer power cable back to a RigRunner where my station is.

      Question #3: Is it preferred to power the SG-239 from a battery right at the base, or is it ok to run a long power line back to the operating position where it ties into the same power source used by the radio (e.g., via RigRunner)?

      Thank for you any advice you can give me!

      73 de Jeff, W6UX


    • Bryan Johns
      Depending on how tight your budget is, another option would be to use a pair of those gizmos, I forget the proper name for them is at the moment, that lets you
      Message 2 of 3 , Jul 30, 2010
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        Depending on how tight your budget is, another option would be to use a pair of those gizmos, I forget the proper name for them is at the moment, that lets you inject a dc voltage into your coax at the station power supply and recover it at your tuner.  SGC sells them, or if you're feeling a little adventurous you might be able to build them yourself.

        --
        Bryan Johns
        K4GDW
        http://www.greendragonweb.com

        Do not meddle in the affairs of dragons, for you are crunchy and taste good with ketchup.


        On Fri, Jul 30, 2010 at 11:14 AM, Jeff Hall <w6ux@...> wrote:


        Hi everyone,

        I am designing a new antenna for portable use.  I have an SG-239 tuner, and plan to feed it with 300 ohm ladder line (from DX Engineering).  The antenna will be configured as an inverted V, with the apex at 40 feet (at the top of a Spiderbeam telescoping fiberglass pole).  I will be weather proofing the tuner as shown on the SGC website (http://www.sgcworld.com/homemadetechnote.html).

        Question #1: I do not plan to use a balun.  From a quick perusal of the Smartuners group, it appears that is is acceptable, so long as the length of the ladder line is adjusted correctly to tune on all desired bands.

        I plan to follow DX Engineering's formula for calculating the antenna and feed line lengths.  The 300 ohm ladder line has a velocity factor of 0.88.  The lowest band of operation will be 40 meters.  DX Engineering states the length of the transmission line must be an odd multiple of 1/8th of the lowest frequency to be used:

        Length = (123 / 7) x 0.88 = ~15.4 feet

        (where 123 = 1/8th wavelength factor and 7 = lowest planned operating freq in MHz)

        The apex will be at 40 feet, so we need to multiply by 3 to get an odd wavelength multiple of 46.2 feet.  The excess 6.2' of ladder line will slope down to the SG-239, which will be sitting on a small milk crate near the base of the antenna mast.  Out the other side of the tuner will be some length of coax, probably 25' of RG-8X, back to my operating position.

        DX Engineering recommends a shortened dipole wire length of 55 feet (27.5' each side).  The apex angle will be between 90 and 120 degrees.

        Question #2: For a balanced antenna, I don't need to ground the tuner itself, correct?

        I plan to power the SG-239 from a nearby 12V AGM battery (e.g., 12V 7Ah).  Or I could run a longer power cable back to a RigRunner where my station is.

        Question #3: Is it preferred to power the SG-239 from a battery right at the base, or is it ok to run a long power line back to the operating position where it ties into the same power source used by the radio (e.g., via RigRunner)?

        Thank for you any advice you can give me!

        73 de Jeff, W6UX





      • Christopher E. Brown
        You don t need and with a SGC or similar generally should not use a balun on the coupler output. You should run a good choke (A.K.A Choke balun) on the
        Message 3 of 3 , Jul 30, 2010
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          You don't need and with a SGC or similar generally should not use a balun
          on the coupler output.



          You should run a good choke (A.K.A Choke balun) on the coax/control/power
          running into the coupler.



          For a unbalanced antenna it is good practice. Prevents the coax shield
          from being an extra radial, helps prevent RF from coming back in on the
          control/power and causing coupler issues, and helps prevent RF from riding
          the coax shield into the antenna.



          In the case of a SGC feeding a doublet, it prevents the coax shield from
          becoming a 3rd leg on your doublet.


          Take a look at K9YC paper on the subject or section in 2010 handbook for
          easy ferrite toroid choke info. Bundle the coax feed and any
          power/control and wrap through choke together.



          I run multiple SG237 and a SG235 and run power/control from the shack on a
          multi-conductor run with the coax.

          Feeding via power adapters down the coax is also viable, so is local
          power.



          As far as grounding. Standard lightning protection for the area applies,
          whatever that means. The SGC internal transformers bleeder resistor do
          provide a 230kohm or better path to the coax shield. Assuming the coax
          runs to the shack, and the radio (at least) is properly grounded (or the
          coax protector bonded at the entrance panel) this will provide static
          bleed for both legs of the antenna.



          If you were to drive a rod at the coupler and connect to the "gnd" lug on
          the coupler, this output is cap isolated from the coax shield (230k resist
          and feedthrough cap in parallel), but you would be tying one leg of the
          doublet to gnd.

          If you were to drive a rod and connect on the coax side you would be
          setting yourself up for current on the coax if there is a potential
          difference from shack ground to coupler ground.



          Generally you use rods + radials on the antenna side gnd for unbalanced
          (cap isolated from coax shield) and no earth tie for balanced use (in both
          cases one assumes the coax enters the shack through a properly bonded
          entrance panel.)


          On Fri, 30 Jul 2010, Jeff Hall wrote:

          > Hi everyone,
          >
          > I am designing a new antenna for portable use. I have an SG-239 tuner, and plan
          > to feed it with 300 ohm ladder line (from DX Engineering). The antenna will be
          > configured as an inverted V, with the apex at 40 feet (at the top of a
          > Spiderbeam telescoping fiberglass pole). I will be weather proofing the tuner
          > as shown on the SGC website (http://www.sgcworld.com/homemadetechnote.html).
          >
          > Question #1: I do not plan to use a balun. From a quick perusal of the
          > Smartuners group, it appears that is is acceptable, so long as the length of the
          > ladder line is adjusted correctly to tune on all desired bands.
          >
          > I plan to follow DX Engineering's formula for calculating the antenna and feed
          > line lengths. The 300 ohm ladder line has a velocity factor of 0.88. The
          > lowest band of operation will be 40 meters. DX Engineering states the length of
          > the transmission line must be an odd multiple of 1/8th of the lowest frequency
          > to be used:
          >
          > Length = (123 / 7) x 0.88 = ~15.4 feet
          >
          > (where 123 = 1/8th wavelength factor and 7 = lowest planned operating freq in
          > MHz)
          >
          > The apex will be at 40 feet, so we need to multiply by 3 to get an odd
          > wavelength multiple of 46.2 feet. The excess 6.2' of ladder line will slope
          > down to the SG-239, which will be sitting on a small milk crate near the base of
          > the antenna mast. Out the other side of the tuner will be some length of coax,
          > probably 25' of RG-8X, back to my operating position.
          >
          > DX Engineering recommends a shortened dipole wire length of 55 feet (27.5' each
          > side). The apex angle will be between 90 and 120 degrees.
          >
          > Question #2: For a balanced antenna, I don't need to ground the tuner itself,
          > correct?
          >
          > I plan to power the SG-239 from a nearby 12V AGM battery (e.g., 12V 7Ah). Or I
          > could run a longer power cable back to a RigRunner where my station is.
          >
          > Question #3: Is it preferred to power the SG-239 from a battery right at the
          > base, or is it ok to run a long power line back to the operating position where
          > it ties into the same power source used by the radio (e.g., via RigRunner)?
          >
          > Thank for you any advice you can give me!
          >
          > 73 de Jeff, W6UX
          >
          >
          >
          >
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