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shielded loop

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  • dannibou
    Hi! On the topic of shielded loop, can anyone tell me if the metal for build a loop is important electrically speaking. I ve read on this group that aluminum
    Message 1 of 11 , Feb 2, 2010
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      Hi! On the topic of shielded loop, can anyone tell me if the metal for build a loop is important electrically speaking. I've read on this group that aluminum or copper are preferred. Is this because they are non magnetic metal? I have a round piece of metal that I could use to build a shielded loop but it is magnetized metal. Any idea? Thanks
      Danny
    • Jim Dunstan
      ... Hi, As with any antenna element the material should have the best conductivity possible. The most common of the best material is copper. If the antenna
      Message 2 of 11 , Feb 2, 2010
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        At 11:55 PM 2/2/2010 +0000, you wrote:
        >
        >
        >Hi! On the topic of shielded loop, can anyone tell me if the metal for
        >build a loop is important electrically speaking. I've read on this group
        >that aluminum or copper are preferred. Is this because they are non
        >magnetic metal? I have a round piece of metal that I could use to build a
        >shielded loop but it is magnetized metal. Any idea? Thanks
        >Danny

        Hi,

        As with any antenna element the material should have the best conductivity
        possible. The most common of the "best" material is copper. If the
        antenna element needs to be as light as possible then aluminium is the
        common choice. There may be better material ... of which I am not aware
        .... but silver is the premium plus material. However it is quite
        expensive to silver plate antenna elements .... although I have heard of
        it. If the material is magnetized it must be some Ferris material ....
        containing iron .... such as steel.

        I am no expert on the conductivity of steel vs copper ... although I have
        heard anything from 30 times to 6 times worse than copper. Whatever that
        means .... steel is very useful for its weight to strength ratio and
        antenna wire is often constructed by copper or silver coating steel wire.

        For small loops ... e.g. 1/10 of a wavelength or less the absolute amount
        of current carrying ability required for efficient operation pretty much
        precludes the use of steel. This of course also applies to a loop shield
        which of course is part of the loop element. If you would like to make a
        shielded loop then it will be small ... and I would stay away from using
        ferrous metals .... not because of the their magnetic properties ... but
        their RF conductivity properties.

        Jim
      • Rick Karlquist
        ... Any metal that is magnetic will be very lossy at RF. Forget about it. Rick N6RK
        Message 3 of 11 , Feb 2, 2010
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          dannibou wrote:
          > Hi! On the topic of shielded loop, can anyone tell me if the metal for
          > build a loop is important electrically speaking. I've read on this group
          > that aluminum or copper are preferred. Is this because they are non
          > magnetic metal? I have a round piece of metal that I could use to build a
          > shielded loop but it is magnetized metal. Any idea? Thanks
          > Danny

          Any metal that is magnetic will be very lossy at RF. Forget about it.

          Rick N6RK
        • Missouri Guy
          ... build a loop is important electrically speaking. I ve read on this group that aluminum or copper are preferred. Is this because they are non magnetic
          Message 4 of 11 , Feb 2, 2010
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            >Hi! On the topic of shielded loop, can anyone tell me if the metal for
            build a loop is important electrically speaking. I've read on this group
            that aluminum or copper are preferred. Is this because they are non
            magnetic metal? I have a round piece of metal that I could use to build a
            shielded loop but it is magnetized metal. Any idea? Thanks
            Danny
            >

            Danny,

            Copper or AL is used because it has less RF resistance than
            iron. Also, those two metals won't have a tendancy to rust
            away outdoors.

            The metal "shield" is actually part of the antenna. You can
            prove that to yourself by touching the metal loop and noticing
            that the loop detunes when you touch it. So, really, if your
            loop is mounted "in the clear", it doesn't make a bit of
            difference in sensistivity or directivity whether it's shielded
            or not (read more below). If the loop is very near some
            metal objects, there might be some benefit of having the shield.

            A loop that is UN-shielded may have a higher 'Q' because of
            less distributed capacitance....depends on how it is wound.
            Having a higher 'Q' usually results in a more sensitive
            loop, but with narrower (sharper) tuning. The crystal
            radio boys know this and some have constructed
            large loops of large litz wire (the right type of litz) for
            that very purpose.

            Charlie, N0TT

            [Non-text portions of this message have been removed]
          • Pronto. (Military)
            Because the RF currents run on the skin or surface of the metal, You can use copper plated steel for lightness and strength. Remember many of our military
            Message 5 of 11 , Feb 3, 2010
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              Because the RF currents run on the "skin" or surface of the metal, You can use copper plated steel for lightness and strength. Remember many of our military whips in the past had that construction. Today they tend to use fibreglass.
              At VHF and UHF the plating is frequently silver plated for conductivity but at HF frequencies copper plating is sufficient and even unnecessary.
              You could use thinwall copper tubing if you can source some.
              Copper becomes magnetic at high current but because , in our case it is oscillating at high frequency, it doesn't show.
              I used to work in a copper mine where 1200 amps DC would run through bus bars and you couldn't walk nearby without having your wrist watch destroyed. Carrying a tool box nearby was downright frightening. Fortunately the voltage was low.
              I am currently considering building a loop antenna in a square configuration using copper plated steel sections or at least plated where the junctions are. My objectives are lihtness , portability and simple takedown/assembly.
              Jim VE3DDY/W4



              --- In loopantennas@yahoogroups.com, "dannibou" <dannibou@...> wrote:
              >
              > Hi! On the topic of shielded loop, can anyone tell me if the metal for build a loop is important electrically speaking. I've read on this group that aluminum or copper are preferred. Is this because they are non magnetic metal? I have a round piece of metal that I could use to build a shielded loop but it is magnetized metal. Any idea? Thanks
              > Danny
              >
            • Dave
              ... In general, ferromagnetic materials are a VERY poor choice for antennas. The problem is that, at RF frequencies, the currents flow on the very outer edge
              Message 6 of 11 , Feb 3, 2010
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                --- In loopantennas@yahoogroups.com, "dannibou" <dannibou@...> wrote:
                >
                > Hi! On the topic of shielded loop, can anyone tell me if the metal for build a
                > loop is important electrically speaking. I've read on this group that aluminum
                > or copper are preferred. Is this because they are non magnetic metal? I have
                > a round piece of metal that I could use to build a shielded loop but it is
                > magnetized metal. Any idea? Thanks

                In general, ferromagnetic materials are a VERY poor choice for antennas.
                The problem is that, at RF frequencies, the currents flow on the very
                outer edge of the conductor (e.g., skin effect). For ferromagnetic materials,
                the ferromagnetic properties reduce the skin depth even more, and this
                tends to make ferromagnetic materials even more lossy than you'd
                normally expect.

                There are, of course, ways around this. The common approach is to
                plate the ferromagnetic material (and, we're usually talking about steel or
                some other Iron compound here) with a higher conductivity,
                non-ferromagnetic material. The prototypical plating material is Silver,
                due to it's high conductivity, but it's also possible to plate Copper onto
                steel (and, this is commercially available as CopperWeld (TM) antenna
                wire). Some television lead-in wire, and some telephone wire was also
                Copper plated steel.

                Failing that, the best wire is Silver, although price usually prevents
                the use of that except in very special circumstances (e.g.,
                military/government applications, very small microwave antennas, etc.).
                Following that, Copper wire is the next best material, quickly followed by
                Aluminum wire.

                > Danny

                Dave
              • dannibou
                ... Hi again. Thanks to those who replied to my question. But I did a mistake in that question. Replace electrically by RF. In fact, I want to build a
                Message 7 of 11 , Feb 4, 2010
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                  --- In loopantennas@yahoogroups.com, "dannibou" <dannibou@...> wrote:
                  >
                  > Hi! On the topic of shielded loop, can anyone tell me if the metal for build a loop is important electrically speaking.

                  Hi again. Thanks to those who replied to my question. But I did a mistake in that question. Replace "electrically" by RF. In fact, I want to build a shielded loop because the electric noise level here is strong. My question should have been: if the metal I use for a loop is the kind that attract a magnet, will it lower the RF signal or is it unimportant? I'll upload a photo of the loop in the evening.

                  The good side of my wrong question is that I learned a lot of things from your answer that I didn't know. Thanks for your help.

                  Danny
                • gmcjetpilot
                  Danny It seems you are determined to have a shielded loop. I would suggest you look at a magnetic loop. Do some googling, the available materials, your ability
                  Message 8 of 11 , Feb 5, 2010
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                    Danny It seems you are determined to have a shielded loop. I would suggest you look at a magnetic loop. Do some googling, the available materials, your ability to fabricate come into the picture. I am sure gold might be best but not practical. Silver may be better than copper and so on. Better than what? Litz wire was mentioned. That is what I have in a hula hoop of PVC for a magnetic loop. I could have used copper pipe but this is lighter and works as well. Some make Shielded Loops from coaxial cable!


                    I think you got some good answers. The answer is best found in antenna books (ARRL books are good) and looking at other successful designs. The topic of antennas I maintain is a science mixed with a little art. Antenna design can go very far into theoretical and mathematics and very fancy spectrum analysis gear to test. At our level and the HF band we are interested it..... there is not much new. Not to say someone might no strike upon a new configuration or design or material no one ever tried before. So research the subject in some good ARRL books and steal.... look at other antennas.

                    I tend want to assure the results before putting huge effort into an antenna project. I have no desire to invent something new in the antenna World. Clearly there is some creativity in how you make it, which I enjoy. The real joy is making an antenna that works and gives you more gain but best of all is better SNR - Signal to Noise Ratio. It took me a long time to realize that is really the name of the game, SNR. Every antenna has limitations. I still say (sorry know this is a loop group but) the a random wire or variations of it of sufficient length is the "standard" I use. Clearly loops can do better in many areas because it is usually a tuned and directional device, which is also it's drawback.

                    I don't know what you have tried before but a well installed random wire usually gives good results. The problem is few people install a random wire properly (grounds, baluns). If you have the best random wire installation and it's noisy, than you need to look into getting rid of the noise.

                    Noise is a big problem for everyone: atmospheric conditions, sun spots and so on, area interference from other stations or bad power transformer and the last area, usually the worst is your home. It is a RFI factory. That is why an outdoor antenna (a reasonable distance from the home) is best (but not always possible). It does not do a lot of good to have the best antenna ever made inside or bolted to your house. Antennas are not magic but amplified magnetic loops (like the welbrook) is the way to go in my opinion for SWL.

                    Any loop in general has good noise rejection off the sides. I started off with simple unamplified passive resonate loops. They do wonders. You can make two and have one to pick up a signal and another to block interference in an direction, like a phased array antennas. I do this "phased array" with my two MW passive loops. Two loops are better than one.

                    AM and HF stations use phased arrays to transmit very spacific directional signals in one or more directions while being NULL in other directions. So good luck. George


                    --- In loopantennas@yahoogroups.com, "dannibou" <dannibou@...> wrote:
                    >
                    >
                    >
                    > --- In loopantennas@yahoogroups.com, "dannibou" <dannibou@> wrote:
                    > >
                    > > Hi! On the topic of shielded loop, can anyone tell me if the metal for build a loop is important electrically speaking.
                    >
                    > Hi again. Thanks to those who replied to my question. But I did a mistake in that question. Replace "electrically" by RF. In fact, I want to build a shielded loop because the electric noise level here is strong. My question should have been: if the metal I use for a loop is the kind that attract a magnet, will it lower the RF signal or is it unimportant? I'll upload a photo of the loop in the evening.
                    >
                    > The good side of my wrong question is that I learned a lot of things from your answer that I didn't know. Thanks for your help.
                    >
                    > Danny
                    >
                  • Mike
                    Hi. Think I will come in on this, 30 turns from a slinky {child s toys} metal type LOL, over a hula hoop, taped via a 150pF cap and fly lead worked for me as I
                    Message 9 of 11 , Feb 6, 2010
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                      Hi. Think I will come in on this, 30 turns from a slinky {child's toys} metal type LOL, over a hula hoop, taped via a 150pF cap and fly lead worked for me as I taped around the turns re band, good from RX and TX I used 10 watts as that is what my licence permits. see back pages of Practical Wireless, the guy also had 3 x Slinky's making a long wire and a broom stick vertical, but the loop worked great. Mike M3EMB.
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