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Rectangular loop shape factor

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VY1JA, Oct 13, 2017.

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  1. VY1JA

    VY1JA Premium Subscriber QRZ Page

    I am examining the use of a side center fed vertical loop and looking for maximum gain. Bob Haviland's book mentions SF, which I believe stands for shape factor) of .875 to .9 as optimum, but I could not find the definition. Does anyone know the where I can find data for an 80 and/or 160 meter squashed vertical loop with center feed in one vertical side? If you have definitions for it I can model it in Llewellen's (SP?) version 6, if I have a starting point. Any help would be greatly appreciated by the guys in CW SS and DX and myself.
    73,
    J., VY1JA
     
  2. K4SAV

    K4SAV Ham Member QRZ Page

    As you make the loop more rectangular (less height, more width) the gain increases, but it gets that gain by increasing the take-off angle and by making the loop less omnidirectional. Max peak gain occurs at a shape factor of somewhere around 2.7 (or 0.37 depending on which way you want to express it), ignoring all the other changes. This may not be what you want. That's for a bottom wire remaining at the same height above ground. The answer may be a function of antenna height and ground quality too. I didn't look at that. Run a few iterations using EZNEC and you can see this.

    Jerry, K4SAV

    edit: Don't try to use a vertical 160 loop on 80.
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Since you have EZNEC, you could answer your own question if you also had AutoEZ. Keeping the total loop length the same, you could vary the length of the sides of the rectangle and look for maximum gain. In fact, if I can get away from my honey-do tasks for 10 minutes, I'll run it for you. I have no monetary connection to AutoEZ (except that I paid for it on ac6la.com) and am a very satisfied customer. I'm still learning useful things that used to take a lot of time but AutoEZ will complete it automatically while I get a fresh cup of coffee.
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    To run AutoEz, you have to have genuine Microsoft Excel, correct?
     
  5. K8JD

    K8JD Ham Member QRZ Page

    Circular loops are the most efficient.
     
    NH7RO likes this.
  6. K8JD

    K8JD Ham Member QRZ Page

    Count ground reflection for a gain factor on a horizontal plane loop when you WANT high angle for the NVIS bands !
     
  7. VY1JA

    VY1JA Premium Subscriber QRZ Page

    In answer: the work was already done by the author, Bob. I do not remember how to operate EZNEC 6 which I purchased, but plan to get back into it later. That is why I need the definitions to start with. I do not think I have the add-in(?) for AutoEZ and I use the free office software rather than Excel proper. I can only assume the suggestion for a 160M circular loop was humor, John. :eek:). Jerry, what is shape factor? height over width or vice versa or is it defined relative to the feed point? My loop is going to be VERTICALLY polarized with the feed in the center of one of the vertical legs. The height will be significantly less than the width. bottom wire at 10 ft top wire at 60 ft and the rest of the one wave loop distance between the two 50 ft vertical sides It is intended to be aimed (figure 8) toward Europe. I want to see if it is worth doing. This would be like taking two "C" or "Double-L" antennas a feeding it as a quad loop. The C antennas work well on 160 for short tower height. I am getting older and am keeping towers to what I can climb. Soon they will be 3 steps high. :eek:) My current antenna is 1/2 of this planned antenna... It is a "C" or "Double-L", but I have a 70ft pole which is exactly the right distance to set up relays and make the C into a low rectangular quad loop.

    See this site for info on the k2KQ 160/80 C: https://www.yccc.org/Articles/double_l.htm
     
    Last edited: Oct 17, 2017 at 7:57 PM
  8. K4SAV

    K4SAV Ham Member QRZ Page

    If you have only 60 ft of height, there isn't a lot of options for the shape of the loop. That also means you have at least 222 feet between the supports, because that's what the top and bottom wires need to be. So sides = 50 ft, top and bottom = 222 ft to resonate at 1.82 MHz, using bare wire. Bottom wire 10 ft above ground. Fed in the center of a vertical side the feedpoint impedance should be 44 ohms. Good match for 50 ohms coax. Two to one SWR bandwidth should be 34 kHz, not including the coax.

    Plots below are for average ground. The blue trace, included for comparison, is for a full size 160 vertical with 7 ohms ground loss. Nulls in the plane of the loop are down 10.5 dB from the forward direction (at 24 degrees elevation).

    Jerry, K4SAV

    Edit: Squashing the loop adds peak gain but the loop becomes less omnidirectional. The bandwidth also decreases for the squashed loop.

    160 loop.gif
    160 loop az.gif
     
    Last edited: Oct 17, 2017 at 10:01 PM
  9. AF7XT

    AF7XT Ham Member QRZ Page

    @K4SAV So Jerry, what does 121' x 57' x 125' x 33' horizontal loop get me besides a bunch of pissed off squirrels ? I mailed but it might as well be shared here .
     
  10. K4SAV

    K4SAV Ham Member QRZ Page

    I got your email. Sounds like you don't have enough horizontal space for the vertical loop.

    A horizontal loop of 121 x 57 x 125 x 33 including the skew of approximately what is in the picture shows that it resonates on about 3.0 MHz for a height of 60 ft.. The feedpoint impedance varies a lot depending on where you feed it. At 3 MHz, if fed in the center of the west wire, the feedpoint impedance is about 42 ohms, center of south wire, 221 ohms, center of east wire 43 ohms, and center of north wire 217 ohms.

    Feeding at a short wire results in a much lower impedance and a much narrower bandwidth. The azimuth patterns at low elevation angles also change depending on where you feed it. The elevation pattern changes a small amount. At the lowest resonant frequency, max gain is straight up. I didn't include patterns because this is probably not the configuration you would like. I don't know what all that metal buried in your ground is going to do, but it probably won't hurt the gain.

    You do have enough height and horizontal space to put up an inverted L for 160. That's a good antenna for 160. If you can't bury radials or use elevated radials consider the K2AV method of solving this problem.
    http://k2av.com/

    Your 160 noise problem is not likely to be solved with any 160 transmit antenna. Most of the guys that have been on 160 for a while now have antennas only for receiving purposes. After you get a transmit antenna going for 160, you will be looking for something that allows you to hear something besides static and miscellaneous electronic noise polluting gadgets.

    Learning how to use one of the NEC tools can be a bit frustrating because none of them are very user friendly. I think EZNEC is more friendly than the others and you already have that. G3TXQ wrote a bunch of tutorials for helping people learn this program.
    http://www.hamradiodeals.co.uk/forums/viewforum.php?f=46
    There is more stuff here:
    http://www.arrl.org/antenna-modeling
    Don't forget the help file within EZNEC. There is information in there on just about everything the program can do.

    Jerry, K4SAV
     

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