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Gamma Feed Point...

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  • n6dmb
    Hello fellow loop enthusiasts, I have a question or two for you if I may. My loop 5 loop is made of 1.5 copper tubing which has been rolled into a
    Message 1 of 5 , Jan 26, 2010
      Hello fellow loop enthusiasts, I have a question or two for you if I
      may. My loop 5' loop is made of 1.5" copper tubing which has been
      rolled into a continuous loop by a local metal fabricator set up for
      such operations. The gap is about 3 inches. An internet source shows a
      plan for a loop which is very similar to mine except for the relatively
      narrow tubing. The link will take you to this plan, a plan I offer for
      illustration purposes only...
      http://nmwilliam.tripod.com/mla.jpg
      If you take a moment to look at the plan, you will see that the gamma
      feed location point is placed 300 mm to the right of center, with a
      spacing of 100 mm. My question to the group is how was that point
      determined to be the "sweet spot" for the gamma feed location and how do
      I determine where that point is on my loop antenna? If I understand
      correctly, shouldn't that feed point should allow for a minimum SWR of
      1:1? Using my antenna analyzer, the lowest SWR I can achieve is 1:3.
      These readings are "without" the capacitor installed. During 1 test, I
      touched the loop and watched the SWR drop to 1:1. That would bring me
      to my second question, what gives with that? Will my body act as a
      capacitor if I am ungrounded? Does it matter to the gamma feed point if
      the capacitor is installed or not?
      Thanks in advance for any advice you might have for me.
      Don N6DMB






      [Non-text portions of this message have been removed]
    • Jim Dunstan
      ... Hi Don, I assume the loop is 5 in diameter and you are planning to use it on 80m. I have built a number of transmitting loops ... several with the gamma
      Message 2 of 5 , Jan 26, 2010
        At 11:52 PM 1/26/2010 +0000, you wrote:
        >
        >
        >Hello fellow loop enthusiasts, I have a question or two for you if I
        >may. My loop 5' loop is made of 1.5" copper tubing which has been
        >rolled into a continuous loop by a local metal fabricator set up for
        >such operations. The gap is about 3 inches. An internet source shows a
        >plan for a loop which is very similar to mine except for the relatively
        >narrow tubing. The link will take you to this plan, a plan I offer for
        >illustration purposes only...
        ><http://nmwilliam.tripod.com/mla.jpg>http://nmwilliam.tripod.com/mla.jpg
        >If you take a moment to look at the plan, you will see that the gamma
        >feed location point is placed 300 mm to the right of center, with a
        >spacing of 100 mm. My question to the group is how was that point
        >determined to be the "sweet spot" for the gamma feed location and how do
        >I determine where that point is on my loop antenna? If I understand
        >correctly, shouldn't that feed point should allow for a minimum SWR of
        >1:1? Using my antenna analyzer, the lowest SWR I can achieve is 1:3.
        >These readings are "without" the capacitor installed. During 1 test, I
        >touched the loop and watched the SWR drop to 1:1. That would bring me
        >to my second question, what gives with that? Will my body act as a
        >capacitor if I am ungrounded? Does it matter to the gamma feed point if
        >the capacitor is installed or not?
        >Thanks in advance for any advice you might have for me.
        >Don

        Hi Don,

        I assume the loop is 5' in diameter and you are planning to use it on 80m.
        I have built a number of transmitting loops ... several with the gamma match.
        I have never used a formula to determine the structure. I simply did quick
        seat
        of the pants experiments to get approx dimensions that work. It is usually
        quite easy to get a good match near or at 1:1. There is some interaction
        between resonance and gamma match settings. (it is easier than getting the
        match with a coupling loop).

        The loop needs to be tuned to resonance for the experimentation to work ...
        without the capacitor the loop is like a dipole formed into a circle and would
        probably resonate somewhere higher than 10m. If you wanted to make the
        gamma match for 10m the structure would be very much smaller. However
        if you want to build the gamma match for 80m ... it will require the loop to
        be resonant at 80m in order to build and adjust it.

        The 5' diam. loop made of 1.5" copper tubing should work very well on 80m.

        Jim

        VE3CI



        [Non-text portions of this message have been removed]
      • capntripps2000
        First, as previously posted, hook up a capacitor. Heck, use a cheap 365pf broadcast variable since you are using an analyzer. I use a cheap var. cap. to get
        Message 3 of 5 , Jan 26, 2010
          First, as previously posted, hook up a capacitor. Heck, use a cheap 365pf broadcast variable since you are using an analyzer. I use a cheap var. cap. to get the ballpark resonance and then cut a length of RG8 coax and trim it for resonance. I have several monoband magnetic loops in the attic. Easy and cheap to build.

          Connect a short length of wire with a hose clamp and start adjusting for best match, 1:1 will definitely be obtainable.

          Using gamma matching you might not get 5 band coverage as is typical using a feed loop. I design my mag loops to cover 2 bands and gamma matching works great.

          Nearby objects will alter the impedance of the antenna and affect the gamma match size, also limiting how many bands you can achieve a low acceptable SWR.

          You can always compromise, as we do with feed loops, and adjust the gamma match for 1.5:1 SWR across all the bands you intend to cover.

          Trial and error, no substitute for that. I guarantee if you tune the loop outside or indoors and finally mount the loop in the attic, for example, you will need to touch up the matching and the tuning for resonance.

          The work will be well worth the effort as you will be amazed with the results on receive as well as signal reports.

          Keep us posted.

          Andrew
          N3LCW



          --- In loopantennas@yahoogroups.com, "n6dmb" <N61W160@...> wrote:
          >
          > Hello fellow loop enthusiasts, I have a question or two for you if I
          > may. My loop 5' loop is made of 1.5" copper tubing which has been
          > rolled into a continuous loop by a local metal fabricator set up for
          > such operations. The gap is about 3 inches. An internet source shows a
          > plan for a loop which is very similar to mine except for the relatively
          > narrow tubing. The link will take you to this plan, a plan I offer for
          > illustration purposes only...
          > http://nmwilliam.tripod.com/mla.jpg
          > If you take a moment to look at the plan, you will see that the gamma
          > feed location point is placed 300 mm to the right of center, with a
          > spacing of 100 mm. My question to the group is how was that point
          > determined to be the "sweet spot" for the gamma feed location and how do
          > I determine where that point is on my loop antenna? If I understand
          > correctly, shouldn't that feed point should allow for a minimum SWR of
          > 1:1? Using my antenna analyzer, the lowest SWR I can achieve is 1:3.
          > These readings are "without" the capacitor installed. During 1 test, I
          > touched the loop and watched the SWR drop to 1:1. That would bring me
          > to my second question, what gives with that? Will my body act as a
          > capacitor if I am ungrounded? Does it matter to the gamma feed point if
          > the capacitor is installed or not?
          > Thanks in advance for any advice you might have for me.
          > Don N6DMB
          >
          >
          >
          >
          >
          >
          > [Non-text portions of this message have been removed]
          >
        • n6dmb
          Hello Jim and Andrew, Thanks for taking the time to reply to my post. Yes, the loop is 5 OD. The circumference is just under 15 feet. The intended band is
          Message 4 of 5 , Jan 27, 2010
            Hello Jim and Andrew,

            Thanks for taking the time to reply to my post. Yes, the loop is 5' OD. The circumference is just under 15 feet. The intended band is 15 meters. Using one of the loop design formulas in the files section, I came up with 1/4 wavelength divided by 3.14 or 15.66' circumference. Actually, I plugged information into ALL the loop design formulas listed and couldn't get any two to agree. Since 1.5" heavy copper tubing comes in 20' lengths, the best the metal fabricator could do for me is the loop I now have, 5 feet across with a circumference just under 15.66'. It works out because this should keep me away from self resonance issues within my desired band. We'll see.

            I brought this up a few months ago, but got frustrated with the project and rolled my loop out of sight, behind a shed. My "Buddipole" antenna system works fine. It's time for me to see this loop project through. I will keep everyone informed of my progress.

            73,
            Don N6DMB








            --- In loopantennas@yahoogroups.com, Jim Dunstan <jimdunstan@...> wrote:
            > Hi Don,
            >
            > I assume the loop is 5' in diameter and you are planning to use it on 80m.
            > I have built a number of transmitting loops ... several with the gamma match.
            > I have never used a formula to determine the structure. I simply did quick
            > seat
            > of the pants experiments to get approx dimensions that work. It is usually
            > quite easy to get a good match near or at 1:1. There is some interaction
            > between resonance and gamma match settings. (it is easier than getting the
            > match with a coupling loop).
            >
            > The loop needs to be tuned to resonance for the experimentation to work ...
            > without the capacitor the loop is like a dipole formed into a circle and would
            > probably resonate somewhere higher than 10m. If you wanted to make the
            > gamma match for 10m the structure would be very much smaller. However
            > if you want to build the gamma match for 80m ... it will require the loop to
            > be resonant at 80m in order to build and adjust it.
            >
            > The 5' diam. loop made of 1.5" copper tubing should work very well on 80m.
            >
            > Jim
            >
            > VE3CI
            >
            >
            >
            > [Non-text portions of this message have been removed]
            >
          • jacob_twotwo@q.com
            Message 5 of 5 , Feb 7, 2010
              On 1/26/2010 4:52 PM, n6dmb wrote:
              > Hello fellow loop enthusiasts, I have a question or two for you if I
              > may. My loop 5' loop is made of 1.5" copper tubing which has been
              > rolled into a continuous loop by a local metal fabricator set up for
              > such operations. The gap is about 3 inches. An internet source shows a
              > plan for a loop which is very similar to mine except for the relatively
              > narrow tubing. The link will take you to this plan, a plan I offer for
              > illustration purposes only...
              > http://nmwilliam.tripod.com/mla.jpg
              > If you take a moment to look at the plan, you will see that the gamma
              > feed location point is placed 300 mm to the right of center, with a
              > spacing of 100 mm. My question to the group is how was that point
              > determined to be the "sweet spot" for the gamma feed location and how do
              > I determine where that point is on my loop antenna? If I understand
              > correctly, shouldn't that feed point should allow for a minimum SWR of
              > 1:1? Using my antenna analyzer, the lowest SWR I can achieve is 1:3.
              > These readings are "without" the capacitor installed. During 1 test, I
              > touched the loop and watched the SWR drop to 1:1. That would bring me
              > to my second question, what gives with that? Will my body act as a
              > capacitor if I am ungrounded? Does it matter to the gamma feed point if
              > the capacitor is installed or not?
              > Thanks in advance for any advice you might have for me.
              > Don N6DMB
              >
              >
              >
              >
              >
              >
              > [Non-text portions of this message have been removed]
              >
              >
              >
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