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40 to 10 antenna - design needed

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by CT2FZI, Dec 21, 2009.

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  1. AI3V

    AI3V Ham Member QRZ Page

    Luis,

    The "magic" length of a loop is 1 wavelength.

    As long as there is AT LEAST 1 wavelength of wire in the loop it will work OK.

    Try for as much AREA inside the loop as possible, The more AREA inside the loop the better, circle=Best.

    Rege
     
  2. CT2FZI

    CT2FZI Ham Member QRZ Page

    Thanks for your message Rege :)

    I was thinking that a full wavelength could work very well, so if I want to use 40M and up I can make a square with 33 foot (10 meters) each side?

    Should the feed point be in one of the sides or in one of the square corners?

    I guess that if I can use this loop it would also work in 80M band [half wavelength] ?


    73,

    Luis CT2FZI
     
    Last edited: Dec 23, 2009
  3. W5DXP

    W5DXP Ham Member QRZ Page

    You would need to install an insulator (or stub) at the halfway point to make it a 1/2WL dipole on 80m. Short out the insulator for operation above 7 MHz.
     
  4. CT2FZI

    CT2FZI Ham Member QRZ Page

    Thank you!

    Now I understand :)

    I wish you a merry QSO, and a happy new antenna ;)

    Luis, CTFZI
     
  5. CT2FZI

    CT2FZI Ham Member QRZ Page

    touching the roof or in the air?

    I am sorry,

    One more important question... I live in a 12 story building and the roof is plane, not in triangle.

    Will be possible for me to place the loop wires touching the roof or do they really need to be elevated on the air?

    If the answer is "elevated on the air" I will have to use another type of antenna... Inverted V type antennas is my best option so far, because on top of the roof I have the "machinery house" where the elevator machines are.

    73

    Luis, CT2FZI
     
  6. K9ZMD

    K9ZMD Premium Subscriber QRZ Page

    Horizontal "Loop Skywire" Antenna

    Luis,

    The brief descriptions given for the horizontal loop have so far been correct. The ARRL Antenna Handbook calls this type of antenna a "Loop Skywire". For more detail as well as some drawings & construction hints, go to this page where a Loop Skywire is described.

    I built a horizontal loop for 80 meters and installed it around the perimeter of my roof. It was very irregular in shape. It was also just 2-3 centimeters off the concrete roof tiles. It worked, but I would not recommend such a low installation to anyone due to signal attenuation and RFI issues. "High and in the clear" is always better; even a couple of feet off the roof surface would be better than what I had. However, my compromised installation still allowed all-band operation, including DX contacts using digital modes & CW.

    From your description of the roof, I am picturing it flat like the top of a box, with a smaller box near the middle to house the elevator machinery.

    Can you put a short post (support) at each corner of the roof? If so, would the elevator house obstruct any corner-to-corner run of wire? If not obstructed, and the distance between supports is 36 feet or more, then you can install this antenna. The only question is how tall do you dare to make those supports? The taller the better, of course.

    I should mention that a perfectly square roof is not required, although problems develop if the shape of your loop becomes a very long, thin rectangle (decreasing the area enclosed). It is also not necessary to have the loop perfectly horizontal, so I'll inquire about the position (and possible utility) of that elevator house.

    If a side of the loop is obstructed by the elevator house, can you use the house as a support to further elevate that part of the wire? Configuring that run of wire more like an inverted Vee would be even better than a low corner-to-corner run. Does the elevator house extend full width of the roof, obstructing two sides of the loop? Oh joy! That means you can configure two sides of your loop like an inverted Vee. It would no longer be a flat horizontal loop, but go for the elevation! I'd expect a better take off angle on 40 meters because of the sloping wire (and hope that it's in a desirable direction)

    The feed point is usually at a corner of a horizontal loop, which is the best practical support for the feed line's weight. In truth, the feed point of a horizontal loop can actually be anywhere you find it convenient to make the connection. It makes no difference for loading, and probably no significant difference to the radiation pattern.

    80 meter operation would not be possible while configured as a closed loop. However, as Cecil mentioned, you can open the 40 meter loop 180 degrees from the feed point and work it like an 80 meter half-wave. Or, you can insert a 1/4 wave stub 180 degrees from the feed point; this would require a bit more work up-front, but would allow 80 meter operation without any further fooling around.

    Let us know how this all fits into your real-world antenna environment. 73

    Gary, K9ZMD/6
     
  7. CT2FZI

    CT2FZI Ham Member QRZ Page

    Photo of the roof

    Hello Gary K9ZM,

    Thank you so much for your answer.

    I have attached a photo of my roof.

    I guess it will be more like a try and buy thing, I have the length, go up the roof and mount it the best way I can...

    Double inverted V sounds good :)

    Nevertheless I still have one question:

    " Or, you can insert a 1/4 wave stub 180 degrees from the feed point "

    How can this be done? Never heard of it before...

    Luis, CT2FZI
     
  8. W5DXP

    W5DXP Ham Member QRZ Page

    A 1/4WL shorted stub exhibits an open-circuit on 80m thus opening the loop = a 1/2WL shorted stub exhibits a short-circuit on 40m thus closing the loop. Just remember that 1/4WL of ladder-line stub is about 58 feet on 3.8 MHz.
     
  9. K2ER

    K2ER Ham Member QRZ Page

    Luis, that's a helpful picture. What are the dimensions of the roof? I see it's a rectangle with the long sides much longer than the short ones.

    The formula for a loop is 1005/Mhz, which gives you feet. Convert to metric of course.

    If this were my situation, I think I'd try to make a loop the entire perimeter of the roof. I'd find a way to raise it by a meter or so on the four corners, maybe by making simple "mastlets" - a piece of PVC pipe attached to a wooden base and affixed to the roof somehow.

    Next, I'd just toss the wire over that obstructing elevator thing. I would use insulated wire and possibly further insulate the section that is going over the elevator box (plastic tubing? Etc.).

    Just a few ideas for you.
     
  10. CT2FZI

    CT2FZI Ham Member QRZ Page

    Thanks, lots of good ideias!

    I guess that I have room for a 80M loop, so I can use it 80M and up right? :)

    It will be rectangular, with some elements making an inverted V but its better than nothing... :)

    Cheers and happy 2010!

    Luis, CT2FZI
     
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