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Loop feed question: should shorten both lines of the transmission line?

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  • alejovarona
    Hi all, Yet another apartment owner looking for a compact antenna. I ve done some reading and I ve gone through most of the files in the Files section of the
    Message 1 of 2 , Feb 2, 2011
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      Hi all,
      Yet another apartment owner looking for a compact antenna.
      I've done some reading and I've gone through most of the files in the "Files" section of the group and still one question bothers me. It is concerned with feeding the loop:
      I can imagine a dipole folded into a big circumference cut by half, connecting a tuning capacitor on one of the cuts and feeding the circumference on the other cut as if it were a normal dipole...

      But in most of the pictures I see that is not the case. You seem to use an "inductive feed loop" acting kind of as a transformer. In essence you are connecting both the core and the braid of a coax line with a coil between them. Which is OK with me (a million loop antenna builders cannot go wrong). But then I cannot stop wondering about the consequences to my rig.

      You see I've only built dipoles and yagis and so the excited element as always some dielectric separating both end points of the transmission line. So the question is: will I damage my rig if I transmit with some tens of watts to the inductive feeding loop?

      Thanks in advance
    • pappy_hiram
      Many people look at that and say, Hey, that s a short circuit! It s a short circuit at DC, but not at RF. With a few tens of watts, you need a big honking
      Message 2 of 2 , Feb 3, 2011
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        Many people look at that and say, "Hey, that's a short circuit!"
        It's a short circuit at DC, but not at RF. With a few tens of watts, you need a big honking variable capacitor capable of several kilovolts else it will flash over. A large transmitter type variable or a vacuum variable is necessary for this part. High RF voltages will be present even at a few watts of RF power.

        Here's what you do, tune up the rig into a dummy load, switch over to the antenna, tune the variable on the antenna for a dip in final current, indicating resonance. Do this at the lowest power your rig will operate at. You need some sort of SWR indicator or wattmeter to do this. You also need a "chicken stick," some sort of insulated shaft to turn the variable on the loop, because it is a high voltage point on the antenna. After you know you are tuned properly, you can increase power.

        The "inductive feed loop" is typically 1/5 the size of the main loop.

        Let us know what you build and how it works out.

        Pappy Hiram


        --- In loopantennas@yahoogroups.com, "alejovarona" <alejovarona@...> wrote:
        >
        > Hi all,
        > Yet another apartment owner looking for a compact antenna.
        > I've done some reading and I've gone through most of the files in the "Files" section of the group and still one question bothers me. It is concerned with feeding the loop:
        > I can imagine a dipole folded into a big circumference cut by half, connecting a tuning capacitor on one of the cuts and feeding the circumference on the other cut as if it were a normal dipole...
        >
        > But in most of the pictures I see that is not the case. You seem to use an "inductive feed loop" acting kind of as a transformer. In essence you are connecting both the core and the braid of a coax line with a coil between them. Which is OK with me (a million loop antenna builders cannot go wrong). But then I cannot stop wondering about the consequences to my rig.
        >
        > You see I've only built dipoles and yagis and so the excited element as always some dielectric separating both end points of the transmission line. So the question is: will I damage my rig if I transmit with some tens of watts to the inductive feeding loop?
        >
        > Thanks in advance
        >
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