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2 Meter Yagi Element Spacing

Discussion in 'General Technical Questions and Answers' started by KI4RDG, Jun 30, 2007.

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  1. KI4RDG

    KI4RDG Banned

    Can anyone tell me the proper element spacing for a 2 meter yegi antenna? I built one myself but the plans I used were for a 440 MHz yagi but I adjusted the element lengths to 2 meters but not the spacing.
     
  2. AE4FA

    AE4FA Ham Member

  3. VK2TIL

    VK2TIL Ham Member

    Element spacing, like element length, is measured in wavelengths; if you have not increased the spacing in the same way that you have increased the element lengths you have a problem.

    Spacing is critical to performance; it affects pattern and feedpoint impedance.

    Early Yagis had regular spacing of the order of 1/4 wavelength; modern design calls for log-taper spacing.

    Google DL6WU and you will find this kind of information;

    http://xe1mex.gq.nu/antenas/yagi.html

    A copy of the ARRL Antenna Book or VHF/UHF Antenna Classics will help you.
     
  4. W0LPQ

    W0LPQ Ham Member

  5. WA9SVD

    WA9SVD Ham Member

    If you didn't also scale element DIAMETER, you will also have to use different dimensions; if you used the same element material as specified for the 70 cm antenna, the elements will have to be slightly LONGER than if the are also scaled upward. (The elements don't all have to be the same diameter; many designs use a larger diameter driven element (DE) than for the parasitic elements. A "thicker" driven element will be usually provide a greater bandwidth than a thin element, but will also be slightly shorter. The use of larger diameter conductors for the driven element often leads to the DE being shorter than any of the other elements. Also, remember that because of "skin" effect, elements can be hollow tubing; they don't have to be solid material unless that's necessary for mechanical reasons or convenience.
    The ARRL Anrtenna book is a good reference, and you won't regret getting one. Hope the antenna works as well as you want. Good luck.
     
  6. K9STH

    K9STH Ham Member

    Go to the downloads section on this site and acquire the "YagiMax" suite. That program is very good to determine the characteristics of a yagi antenna. Also, the suite includes a program entitled "gamma" that will give the length of a gamma rod to match the antenna and for the value of the series capacitor.

    I use YagiMax all the time to determine the measurements of yagi antennas and the values of the gamma match. A little over a week ago I helped a local design a 3-element 10 meter yagi for use at Field Day (yes, the band did open into this part of the country) using YagiMax. Although I was not at the Field Day operations I understand that the antenna worked very well.

    Glen, K9STH
     
  7. N3ATS

    N3ATS Ham Member

    It's "generally" (depends on design and optimization) .2 wavelengths between elements. This should get you in the ball park.
     
  8. WA7KKP

    WA7KKP Ham Member

    Get the antenna books, and READ.

    Element spacing depends on several criteria:

    Front to back ratio
    Gain
    Bandwidth

    All of these interact, what is optimum for one is not necessarily optimum for all.

    Do your homework, build the beam, and come back and tell all of us . . . then you'll become an Expert.

    Gary WA7KKP
     
  9. KI4RDG

    KI4RDG Banned

    Well I removed 3 elements from the antenna. I used an online antenna calculater that told me that the correct spacing for 146 MHz was about 15" so because the boom I used was only 3 feet long I turned it into a 3 element but the antenna seems to be working better then before.

    I was able to hit a repeater 25 miles away with a full quieting signal from the left side of the antenna with it in the basement (the repeater was to the NW and the antenna was pointing to SW).I'm able to hit a number of the local repeaters including some I never was able to get to before.
     
  10. VK2TIL

    VK2TIL Ham Member

    Well done; you have now entered the ranks of amateur experimenters. [​IMG]

    Yagi performance is not measured by the number of elements; it's measured by boom length.

    The assumption is that element spacing is correct.

    So a "10-foot boom" yagi will not be as good as a "20-foot boom" one; it will still be pretty good, though.

    You can pile eleventy-seven elements on a given boom length and all you will end up with is a bad antenna.

    As I said earlier, about 1/4-wave spacing (closer to 0.2-wave as ATS said) is usual. It's still usual for short (in wavelengths, remember) yagis; the log spacing begins to make a difference in long (wavelength)-boom designs.

    Thinking in wavelengths rather than feet/metres/cubits is essential to understanding antennas.
     
  11. AG3Y

    AG3Y Ham Member

    Remember, FM = vertical polarization, SSB/CW = horizontal polarization.

    You will suffer over 20dB loss if you do not mount the antenna properly.

    When working with vertical polarized beams, it is best if you can put all the elements out there ahead of the tower/support pole. That can be don fairly easily with a short boom, by simply mounting the reflector, driven element and director ahead of the end of the boom, and then fastening the boom to the tower/support with a right angle bracket ! Not balanced, but if it is short enough, it will do well!

    Interaction between the active elements and the support may be one reason that experimenters chose horizontal polarization for "weak signal" work !

    Good luck! 73, Jim
     
  12. KA0GKT

    KA0GKT Ham Member

    A number of years ago (Back when NIST was called NBS [​IMG] ) the National Bureau of Standards published a white paper on Yagi design.  It is still available on-line as a PDF file.  If you are interested you may download it here: NBS Yagi

    A few editions ago, the ARRL antenna book had a section devoted to the NBS Yagi antenna with element length and spacing charts.  My 14th edition from 1982 has the information on pages 11-6,11-7 and 11-8.

    73 DE KAØGKT/7

    --Steve
     
  13. Guest

    Guest Guest

    As you can tell you have opened "a can of worms"! There are many different "opinions" on Yagi element spacing. My old 1943 antenna book, for VHF spacing, indicates the directors should be approximately 0.1 wavelength and the reflector spacing should be 0.15 wavelength for maximum forward gain from the driven element.

    I have run a simulation in EZNEC-ARRL and it indicates they had it right back in 1943.

    73,

    Frank [​IMG]
     
  14. KI4RDG

    KI4RDG Banned

    Right now the antenna is mounted to a beam on the ceiling and I can't run it vertical but I don't seem to be having any problems hitting the repeaters.
     
  15. N3ATS

    N3ATS Ham Member

    Outstanding. Thanks for posting that!
     
  16. N2CLB

    N2CLB XML Subscriber

    there is a program called "RF tool box" google it and try it out it will tell you the size of a ton of dif antennas
     
  17. KL7AJ

    KL7AJ Ham Member

    Glad you brought this up. And with smaller antennas, the diameter is (generally) a larger percentage, and hence, of greater consequence to the performance.

    Some people will tell you to "not re-invent the wheel", and this is good advice for things like high-gain Yagis, where folks a lot smarter than you or I have been working on this a LONG time.

    On the other hand, re-inventing the wheel is how you learn how a wheel works, and the way to learn how a Yagi works is to build a few that DON'T work.

    I will never forget my sophomore high school electronics class. Our teacher, Mr, Scott, had us actually designing and testing 432 MHz yagis made of brazing rod and balsa wood. It was such an education...and set me on a long career path in radio.

    What high school classes are doing that these days?

    [​IMG]
     
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