Monday, October 27, 2008

Antennas: The Yagi, Bob, Week 23

Tonight we will cover another of the basic ham antennas, the Yagi. This is the most popular rotatable antenna as it is a good compromise between, cost, durability, manageability and performance.

Let me start by saying I fully expected to get a neat simple explanation of the theory of Yagis from the ARRL Antenna Book but it is not there. This seems to be one of those black magic designs that just work. Don't misunderstand, this can be modeled by the popular computer programs and you can see how it works but I didn't find any simple explanation of why it works. With that said, let's dive in anyway and at least describe it and what it does, along with some of the compromises in design.

It consists of a horizontal boom with two or more horizontal elements that are perpendicular to the boom. I believe most of you have seen several Yagis by now so I won't go too much into the appearance. The two necessary elements are the driven element, which is essentially a horizontal dipole like the kind we have covered previously, and the reflector. The reflector, as you might guess is placed "behind" the driven element, that is to say, the radiation of the antenna is primarily in the direction opposite the reflector. A Yagi has only one reflector. Any more elements after the driven element and the reflector are directors and are on the opposite side of the driven element from the reflector. So, going from back to front the elements are: reflector, driven element, director, director, etc.

Early designs of Yagis had all of the elements equally spaced at around 0.15 wavelengths between each one. Optimal designs now have the reflector, driven element and first director more closely spaced (about 0.1 wavelength or less) and the directors spaced farther apart. So, lets look at what we mean by an optimal design. First, what are the design trade-offs of a Yagi?

Even though we wouldn't all agree on what the perfect Yagi was we can agree on three things we would want:
  • 50 Ohm Impedance at the feedpoint; pure resistive (no reactance)
  • Zero gain at the back and sides
  • Maximum possible gain at the front
We might disagree on how narrow we want the gain pattern to be but lets forget that for now and look at the big three.

If we design for a maximum gain antenna what we get is an antenna that has a very narrow range of frequencies with a usable SWR. The ARRL Antenna Book has a nice set of graphs showing this which I will use to share some numbers. The example I've chosen is for a 10 Meter, 3 element Yagi. For those that have the book it is on page 11-5. The table below shows three Yagi designs: maximum gain antenna, the maximum gain per SWR antenna and the optimal antenna.

ValueMax Gain DesignMax Gain per SWR DesignOptimized Design
SWR at 28.4 MHz222
SWR at 28.0 MHz722
SWR at 28.8 MHz1022.2
Gain at 28.4 MHz8.47.67.2
Gain at 28.0 MHz7.97.57.1
Gain at 28.8 MHz8.27.87.4
F/R at 28.4 MHz132222
F/R at 28.0 MHz201520
F/R at 28.8 MHz61823

From this table you can see that you get a modest improvement in gain for the maximum gain design but at a cost of both SWR and the Front to Back gain ratio. The Gain per SWR gives you good SWR across the band and better gain than the optimized but at a cost of decreased Front to Back gain ratio. The optimized Yagi design sacrifices a bit of overall gain but gives you a good SWR across the band and a consistently good Front to Back ratio as well.

There are design considerations for adding more directors as well but I will leave that for another time. This is more specifically for VHF/UHF antennas and we may have a session just on that.

The ARRL Antenna Book rates two element Yagis well and indicates that the increased gain drops off as you start adding directors.

EZNEC Antenna Software by W7EL
Here is the promised link to get the EZNEC antenna modeling software. I have the free version now which limits you to 20 segments. Each wire should have several segments to allow for accurate modeling so 20 segments won't go very far on a multiple element antenna. I will probably end up buying the full version which is $89 for a web purchase and direct download or $99 to get the CD.

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