Quagi antenna

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KA0EIV, Nov 2, 2016.

ad: L-HROutlet
ad: l-rl
ad: Left-2
ad: Left-3
ad: K5AB-Elect-1
ad: L-MFJ
ad: Subscribe
ad: MessiPaoloni-1
  1. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    I've been hopeful that "AutoEZ" a complimentary program to EZNEC, has the capability to optimize the Quagi designs. W7EL himself recommends it. I have not taken the time to study it, and I don't know its capabilities. Dan, the author posts her on the Zed. Perhaps we can get some comment from him.

    *****
    AutoEZ Program Information

    AutoEZ is a Microsoft Excel application developed by Dan Maguire, AC6LA. AutoEZ works in conjunction with EZNEC and allows you to use variables to control diverse aspects of your antenna model. You can then run multiple EZNEC test cases while automatically changing one or more variables between runs, or AutoEZ can automatically optimize your model. AutoEZcan take full advantage of all advanced EZNEC features such as virtual segments, realistic transmission lines with loss, transformers, L networks (which can be combined to form T and Pi networks), and parallel connected loads.

    A free demo version is available for download which may be used with any of the EZNEC v. 5.0 or 6.0 program types.

    See the AC6LA website AutoEZ User Guide (opens in a new window) for more information.
     
  2. WB3BEL

    WB3BEL Ham Member QRZ Page

    Yes, the 154" antenna is a Yagi design.

    There are lots of Yagi designs that are easy to build. Many of them easier than the Quagi. Many of them will outperform the Quagi.
    The Quagi was a good antenna for it's time. Many experienced VHF ops have a fondness for them that will last forever.

    I personally prefer the Yagi. It is easier to transport and use rapidly. Having done a fair amount of VHF/UHF rover operations, it is a pain to unload the antenna from the roof racks and try to find a place to set it down without bending the loops. A Yagi is easy, you can lay it on the ground without worrying about having to straighten things out again. Also, rover operations often need to navigate where antenna-eating trees are drooping over the road or trail and love to devour 3D antennas. The 2D Yagi gets bent up as well, but seems to be easier to repair than a beaten up loop Yagi or Quagi. Those of you who have driven up winding mountain roads in the search of the ideal VHF horizon know all about this and have lots of twisted metal stories to swap.

    There are antenna modeling programs that can perform optimization on these kinds of antennas. Most of these have two main limitations.
    1. Most designers will have a difficult time figuring out what metrics and weighting to use to compare designs. If you have a design where you are concerned with Gain and sidelobe levels and bandwidth and VSWR and mechanical concerns and polarization purity and so forth. You can say parameter A is more important than parameter B. But you can not ignore B entirely. When you get 4 or 5 or 6 variables, the definition of which antenna is better than another in an evolution of designs gets blurry fast.

    2. These kinds of antennas are often pretty narrowband designs and the slopes of convergence for some of the variables is very sensitive to small changes and have strong interactions with each other. So you have to be pretty close to what you eventually want when you start the optimization. If you don't understand basic antenna theory, it is hard to get to this trailhead unless you copy something someone else has done who has that knowledge. There will be lots of local maxima and minima and you might have to go through some designs that look worse than the best on your way to the one that optimizes your design objectives. Maybe eventually using brute force computing, you could toss everything into the pot and see what cooks out.

    If you really have your heart set on making a better Quagi. Here are a few hints. Look at VE7BQH's G/T table for antennas. Figure out which one is closest to what you think is a good antenna. Study the element spacing. Use it as a foundation and optimize the element lengths without straying too far from the spacing of the original design for those parasitic elements that are doing most of the work. You can get quite a bit of insight by observing the magnitude of the current distribution of the antenna elements.
     
    KA0HCP likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    With the original N6NB design, we in the Southern California Contest Club have been using Quagis for years while operating mobile as rovers -- that's a great many rover stations.

    The driven element and reflector loops are attached to the boom using a machine screw/lockwasher/wingnut assembly. To "flatten" the Quagi, you loosen the wingnut and fold the loop along the boom, so now it all lays flat. Takes about five seconds per element. Reverse the process to install and use the antenna.

    Use the same process for the triband quads Wayne builds for roving (144-222-432 MHz on a single boom, using all quad loops).

    However for "rover" ops, 99% of the time the antennas remain in place and operational, and rotatable, above the vehicles while driving so other than for initial installation before the contest or removal afterwards, there's no reason to ever touch any of the antennas. And that's for 10 or 11 bands (50 through 10.3 GHz or 50 through 24 GHz).
     
  4. WB3BEL

    WB3BEL Ham Member QRZ Page

    Steve,
    Yes, you guys seem to have a whole lot of fun pack roving out on the west coast. Any antenna that helps get more folks excited about weak signal VHF/UHF is a good antenna in my book.
     

Share This Page