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Re: [loopantennas] Re: Help making the most out of my new purchase

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  • Jim Dunstan
    ... Hi, almost any length of wire will work whether configured as a loop or as a straight piece with the feed point located in the middle (dipole or
    Message 1 of 3 , May 30, 2010
      At 06:35 PM 5/30/2010 +0000, you wrote:
      >
      >
      >Hello again,
      >
      >I now realize after doing more reading that my post is very misinformed.
      >
      >I was under the impression that I needed to make the wire in a loop a 1/4
      >wave in order for it to work if I were to do an all out loop (if that
      >makes sense). I did more research and have learned this is not the case.

      Hi,

      almost any length of wire will "work" whether configured as a loop or as a
      straight piece with the feed point located in the middle (dipole or
      doublet). Both configurations loop and dipole have a common characteristic
      in that they are considered "balanced". By that it is meant that the
      antenna element is complete and does not require a reference to ground,
      earth, counterpoise, or whatever you want to call it ... in order for RF
      currents to flow through the feed line to the radio.

      When you speak of 1/4, 1/2, or 1 wavelength etc.. you are talking about the
      electrical length of the element at a particular frequency and
      resonance. I just so happens that an element that is 1/2 wavelength at
      some particular frequency and is arranged as a dipole represents some
      desirable attributes .... e.g. presents approximately 50 ohms impedance
      (with no complicated inductive or capacitive reactance). If you increase
      the length to 1 full wavelength and configure it into a loop it presents
      approximately 100 ohms.

      The information above only makes sense for frequencies in the HF range of
      3.5 to 30 mhz. Frequencies lower than 2-3 mhz become too large to be
      practical in terms of resonant lengths.





      >I got on the radio last night at 0000 UTC and noticed that the setup I
      >have here is not much good on the SW bands. I did indeed pick up some
      >stations but the noise floor was very high (S6-8). On MW the wire wrapped
      >around my room fared much better and I was able to do some good MW DX. In
      >a sense what I have here is a big loop around the ceiling.
      >
      >I still have questions. A lot of the info I find about magnetic loops has
      >to do with transmitting, something I won't be doing. Do the same rules apply?

      Generally speaking NO. The most critical aspect in constructing a small
      loop (generally one that is considerably smaller than 1/4 wavelength in
      circumference) is to make the element, tuning, and coupling components
      robust enough to withstand high RF currents and voltages.



      >Also wondering about the size, I read that I can use 1/10 wavelength and I
      >am really lost as to how to calculate that. I don't understand the size.

      As mentioned in the first comments the smallest (naturally resonant) loop
      antenna is considered the 1 full wavelength. The rule of thumb formula for
      calculating this length is 1005/Frequency. So a naturally resonant loop
      for 14 mhz would be 1005/14 = 72' 8" It has certain characteristics in
      terms of input impedance and radiation pattern. A loop that is 1/10
      wavelength would be only a little more than 7ft in circumference and it
      needs to be "tuned" in order to become resonant (to make up for the
      shortened length) and has much different feed and radiation characteristics.

      When the loop is large ... it will work well even if it is not resonant ...
      as in the case of the loop around the room. If it is small (in the 1/10 WL
      range) it generally must be tuned in order to perform adequately. (there
      are exceptions .... eg. providing much additional RF amplification).



      >I also am wondering about the variable capacitor, does this have to be
      >attached to the loop or can I move it closer to my station? The way I have
      >the radio here the radio will be on one side of the room and the loop on
      >another. If I can attach the capacitor to the coax it will be much easier.

      Yes, the capacitor can be located at a distance from the loop



      >In front of the window I have about 3.5 feet in width I can play with, if
      >I were to move the loop to an inside wall I got about 5 food in width at
      >that point. I live in a vinyl sided wood house so the inside wall could
      >even work.
      >
      >Thinking about the target frequencies, I think my interest lies at around
      >4Mhz up to around 15Mhz there bouts. Could this be done in the space
      >constraints?
      >
      >Been on ebay looking at capacitors and there are some really nice ones to
      >be had. Should I get one with only one tuning set of fins? Or should I get
      >one with three?
      >
      >Hope I make a little more sense this time around.


      Have fun with the experiments

      Jim
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