Directional Antennas

Anatomy of a Yagi

Three kinds of element on a boom:

How do the lengths of a three-element Yagi reflector and director compare to that of the driven element?

The reflector is longer, and the director is shorter.

A useful mnemonic: the antenna points toward the short end. Directors are short and go in the direction of transmission; the single longer reflector is behind you.

Only the driven element is connected to anything. The others are parasitic — they re-radiate what they pick up, with a phase that reinforces the pattern forward and cancels it backward.

The two performance numbers

What is meant by the “main lobe” of a directive antenna?

The direction of maximum radiated field strength from the antenna.

What does “front-to-back ratio” mean in reference to a Yagi antenna?

The power radiated in the major lobe compared to that in the opposite direction.

Gain is what you gain forward. Front-to-back ratio is how well you reject what is behind you. They are different properties and they trade against each other, which is why:

Which of the following can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth of a Yagi antenna?

All these choices are correct — the physical length of the boom, the number of elements, and the spacing of each element along the boom.

You cannot maximise all three at once. A design optimised for gain has a poorer front-to-back ratio and narrower bandwidth than one optimised for pattern.

Making it bigger, and making it wider

What is the primary effect of increasing boom length and adding directors to a Yagi antenna?

Gain increases.

Boom length is the fundamental resource. More directors on a longer boom concentrate the pattern further; more elements crammed onto a short boom mostly add cost.

Which of the following would increase the bandwidth of a Yagi antenna?

Larger-diameter elements.

A fatter conductor has a lower Q, so it stays near resonance over a wider range. The same reason a fat dipole covers more of a band than a thin-wire one.

dBi and dBd

How does antenna gain in dBi compare to gain stated in dBd for the same antenna?

Gain in dBi is 2.15 dB higher.

$\mathrm{dBi} = \mathrm{dBd} + 2.15$

Because a half-wave dipole itself has 2.15 dBi of gain over an isotropic radiator. Referencing to the dipole therefore reads 2.15 dB lower for the same antenna — and manufacturers naturally quote dBi.

Always check which reference a gain figure uses. An “8 dB” beam is 8 dBd or 8 dBi, and those are meaningfully different antennas.

Stacking

In free space, how does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three-element Yagi?

Approximately 3 dB higher

Two identical antennas fed in phase double the power in the favoured direction: 3 dB, the familiar factor of two. And the pattern changes shape:

What is an advantage of vertically stacking horizontally polarized Yagi antennas?

It narrows the main lobe in elevation

Stacking vertically compresses the pattern vertically, pushing energy toward the horizon where distant stations are rather than up at the sky. On HF, controlling the elevation pattern often matters more than raw gain.

Feeding a Yagi

A Yagi’s driven element does not naturally present 50 ohms, so a matching arrangement is part of the design.

What is a beta or hairpin match?

A shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching

Which of the following is a characteristic of using a gamma match with a Yagi antenna?

It does not require the driven element to be insulated from the boom.

That is a genuine mechanical advantage: a driven element bolted straight to the boom is stronger, simpler, and needs no insulators to fail in weather. The gamma match takes its feed from a point along the element instead.

Check yourself

1. Which end of a Yagi does it transmit toward, and how do you tell from the element lengths?
2. An antenna is advertised at 10 dBi. What is that in dBd?
3. You stack two identical Yagis a half wavelength apart vertically. What do you gain, and what changes about the pattern?