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Thread: 2 Meter (144- 146mhz) Yagi Antenna 8'

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  1. #1

    Question 2 Meter (144- 146mhz) Yagi Antenna 8'

    I am trying to find plans for a two meter Yagi antenna with the following specs and I have not found anything!


    * 2 Meter Yagi Antenna 8' or Less
    * I don't care how many elements are used... whatever is needed.
    * 9db of gain or more.
    * Boom construction Aluminum 8 feet in length or less
    * Driven Element (Folded Dipole)
    * Feed Point: Coaxial balun
    * Vertically Polarized Setup
    * Not for SSB use.


    I have found many variations of these specs but none seem to show exactly how they made the antenna. For instance I have found many portable units with wood construction in the 6 to 8 foot boom lengths. I wanted to use this as a permanent antenna on my roof so I really want to make it out of aluminum. (These are the supplies that I have.) I also have seen many computer designed layouts but I am really looking for plans that have already been implemented so I know that it will have a good SWR. I am very new to antenna building so I really want lots of detail. I have recently checked out older versions of the ARRL handbook (2005) and Antenna Book (2003). They seem to have very small yagis (tape measure build) and very long high performance antennas. Nothing in the middle. I was thinking of buying the latest ARRL Antenna Handbook but I wanted to know if they have actual plans for an 8 foot long yagi. I did find some ARRL handbooks from the 70's that have these designs but they dont seem to go into a lot of details about how it was feed and what kind of performance they were getting.

    Aside from all of the specs above I have tried to put a yagi together based off of a few different peoples designs. I have not had any luck with the SWR. One question that I have is when making the driven element... I am using a folded dipole design. Should the entire driven element be insulated from the boom? The boom is made of aluminum.

    I also wanted to know how I should place the balun after I solder it... Can I coil it around itself and zip tie it to the boom or should I let it hang down? Should I run my coax from the back of the antenna or should I let it hang down and attach it to the insulated mast? As you can see this is some of the detail that I am really looking for.

    If anyone has any ideas where I could find some detailed plans like this I would really appreciate it. I am willing to purchase plans or buy any book.

    Thanks,

    W8FGW
    Last edited by W8FGW; 07-07-2008 at 01:59 PM.

  2. #2
    Join Date
    May 2005
    Location
    Virginia
    Posts
    1,036

    Default

    There is a lot of information about 2 meter antennas on the web. So you should be able to find something to use as a starting point. Maybe it wont be a complete detailed set of plans, but probably you will find enough to make a decent antenna.

    I am wondering why you insist on folded dipole Driven element? This restricts the designs that you can use.

    For example there is a pretty good design of a wideband six element yagi here:

    http://www.dk7zb.fox28.de/2m-short/6-ele.htm

    This is not a complete set of plans, but if you look around at other antennas on this site, you will see how to build this.

    I am also wondering what kind of problem you had with your previous attempts. How high was the VSWR? At what frequency? Did it have good VSWR at some frequency? A folded dipole can be insulated from the boom or can be connected at the exact center of the middle of closed part of the loop opposite the feedpoint. (Not grounded at the feedpoints).

    To make a VHF yagi you have to use materials that match the design. If you change the element diameters you must compensate. If you change the method of mounting (Insulated/NonInsulated) Bracket style etc you must compensate. If you change the boom diameter you may need to compensate.

    So if you already have the materials on hand, and want to use them, you have to know how to modify the existing design to do these compensations.

    I'd be glad to help you out. Let us know what went wrong the first time and you will likely be successful on the next go around.

    As far as feedline routing, there are lots of options. Off the back of the boom is cleanest, but is kind of a mechanical hassle. As far as the balun, you can likely mount to the boom and zip tie it. I would not leave it swinging in the breeze as it will likely fail from wind induced vibrations.

    73, Harry WB3BEL

  3. #3

    Default

    Just aa a comment, since I haven't researched your "requirements" in detail...

    1. You specify a given boom length. Regardless of the element NUMBER, gain WILL be more dependant upon boom length than element number. Element number (and placement) will affect feedpoint (to some extent) impedance, and beamwidth (radiation pattern, including F/B ratio) more than actual gain for a given boom length.

    2. Be wary of designs that make unrealistic claims, or try to compare gainfigures that mix dBi and dBd gain figures. Remember, a gain figure based on a "dbi" figure will always have a 2.15 dB advantage over a figure based on a semi-real world dipole, real world fiigures can be much more complex.

    3. Radiation patterns are often shown with a clear "near and far" field horizon; obstructions (buildings, even trees, etc. of a conductive nature) can upset the radiation pattern in the far (and fare, far, DX) field.

  4. #4
    Join Date
    Jan 2000
    Location
    Indiana
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    4,961

    Default

    Another comment .. since you specify vertical ... you ARE aware that you don't want to mount the antenna by itself on the mast vertically..! That puts the elements in line with the mast which is a bad thing.

    Horizontal is a non issue, but vertical is.

  5. #5

    Default mast

    The mast is made of wood.... does that make a difference?

    thanks

  6. #6

    Default

    Use a simple gamma matching system. This allows all the elements to be bolted directly to the boom, no insulators are needed at all.

    K2WH

  7. #7

    Default

    I have the ARRL Antenna Book. It comes with a CD of programs ... one of particular interest to you is "YW" ... meaning Yagi for Windows. Using this program I have designed a 6 element 2 meter beam that is 7.7 feet in boom length.

    The design criteria you have posted is very similar to what I have developed using YW.

    The specs of the beam are (144 MHz, 146 MHZ and 148 MHz):

    Gain - 17.2, 17.5, 17.8 all dBi
    F/R - 20.7, 20.9, 21.8 all dB
    SWR - 1.1, 1.0, 1.2 to 1

    The driven element is a dipole length aluminum tube, 38" long and it is matched with a gamma match that YW calculates for you. You can get assembly tips from the Anntenna Book text under the Yagi section. I encourage you to get the book. BTW, a gamma match will provide a great SWR match over a large bandwidth ... easily covering the entire 2 meter band. The CD also has a scaling program which will allow you to take a proven antenna from another frequency range and rescale the components to the band you wish to construct the antenna for. That's exactly where I started when developing my antenna. HTH

    73,

    Buzz

    WB3fXW

  8. #8
    Join Date
    May 2005
    Location
    Virginia
    Posts
    1,036

    Default

    I doubt the gain is 17+ dBi must be a typo.

    The wood mast will be okay if varnished otherwise fiberglass would be better.

    I personally don't like gamma match and prefer direct coax feed. Less to go wrong and fewer places for water to get in.

  9. #9

    Default

    http://www.hamuniverse.com/2ssbyagi.html

    that will get you started, yes it exceeds your boom length by a little over 8".
    yes it is optimized for the ssb portion of the band, a simple matter of trimming to correct.

    The article has a link to the free software used to design it.
    if the 8'8" length is unacceptable you can redisgn it as a 5 element rather than 6.

    if you have a little extra cash pick up a fiberglass shovel (or any other long handled yard tool) handle and use that for your mast.
    Freedom isn't free, it carries with it the highest cost known to humankind, always has, always will.

    73, Ryan

  10. #10

    Question Mast Position on the boom

    Thanks everyone for the replies. I think that I have a good idea of where to start and the different options I have. One question I have is where I should position my mast if I want to use it vertically. Can it be located in the center of the boom or on one end behind the reflector? Does this vary with the design?

    Thanks,

    w8fgw

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