Lowering The Boom On Yagi Element Isolation

Antenna design can be confusing, to say the least. There’s so much black magic that goes into antennas that newbies often look at designs and are left wondering exactly how the thing could ever work. Slight changes in length or the angle between two elements result in a vastly different resonant frequency or a significant change in the antenna’s impedance. It can drive one to distraction.

Particularly concerning are the frequent appearances of what seem to be dead shorts between the two conductors of a feedline, which [andrew mcneil] explored with a pair of WiFi Yagi antennas. These highly directional antennas have a driven element and a number of parasitic elements, specifically a reflector behind the driven element and one or more directors in front of it. Constructive and destructive interference based on the spacing of the elements and capacitive or inductive coupling based on their length determine the characteristics of the antenna. [Andrew]’s test antennas have their twelve directors either isolated from the boom or shorted together to the shield of the feedline. In side-by-side tests with a known signal source, both antennas performed exactly the same, meaning that if you choose to build a Yagi, you’ve got a lot of flexibility in what materials you choose and how you attach elements to the boom.

If you want to dive a little deeper into how the Yagi works, and to learn why it’s more properly known as the Yagi-Uda antenna, check out our story on their history and operational theory. And hats off to [andrew] for reminding us that antenna design is often an exercise in practicality; after all, an umbrella and some tin cans or even a rusty nail will do under the right circumstances.

18 thoughts on “Lowering The Boom On Yagi Element Isolation

  1. This is an odd comment in the article:

    “Particularly concerning are the frequent appearances of what seem to be dead shorts between the two conductors of a feedline”

    Antennas are *designed* for a specific frequency. Many matching networks are literally DC short circuits, and the impedance of an antenna outside its design frequency is of little concern. What matters is that it looks like the correct impedance *at* its design frequency.

  2. It interesting that the original article notes no difference between the insulated and non insulated elements.

    From my reading on the subject many years ago I was under the impression the difference came down to a dissymmetry in the radiation pattern when non insulated elements were employed. In most situations this is not a significant issue.

    1. Yagi-Uda calculator. Put in desired target frequency and it can calculate everything else.
      Supports bonded or non bonded booms, any boom size, any element size, square or round booms. Generates the dipole along with the impedance matching balun for various coax types.
      Everything you need in one program.
      Windows only, but it was worth installing Wine for. :)

  3. It would be so great if we could see radio waves…

    I wonder if this will be possible in the future with cheaper scanning grids. Similar to how we can now see temperature with cheap FLIR cams.

  4. If you are going to use a conductive boom, it’s far more important to insure that the element-to-boom connection has no chance of becoming corroded. The commercial antennas I have seen have welded joints and, of course, no dissimilar metals.

    Believe the conductive boom compensation value is very small compared to boom width, but there is a very slight difference in element length and perhaps spacing as well. But not a factor until you get up above 900MHz..

  5. black magic, really? I thought we had science now.

    In case you have not noticed it, science is actually under political attack. It would be a good time to champion science and not affront it.

    If you want to design an antenna there are countless books, the ARRL is a good starting point with their antenna handbook. If you know the type of antenna you want to design there are countless calculators available for use on the net.

    Not long ago I wanted to built a high gain yagi for a wifi project. I did not need to sacrifice a goat to do it. I had to hand it the center frequency around where I wanted to work and how many elements I wanted it to have. It did not even require any math like it did when I was a kid.

    1. As some one who’s been doing it for years at or near the bleeding edge. The mystique is gone. If youre good with 3D cad and have 40k for a software license you can simulate god near anything and nail the performance to within .1dB of the manufactured design. No more black magic, you can plot full 3D field vectors and see everything live.

  6. Yagi antennas are garbage. Its essentially a dipole with a terrible sub reflector ( a metal rod / rods thats reflecting a small portion of the forward energy) then it gets reflected back to a terrible main reflector. You could get more gain with a pringles can with a flare on the end. Its a 1 dimensional cassegrain lmao.

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