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Article · Antennas

Question of the Month: How Do Long Dual Band Verticals Develop Gain?

Lengthening a vertical past 5/8 wavelength should send the signal skyward — yet commercial dual band antennas are longer than that and still claim gain at the horizon. Here is how they do it, and how to read their gain figures.

By Bill Rinker, W6OAVWritten in 20156 Min Read

The Question

Replacing a 1/4 wavelength vertical antenna with a longer 5/8 wavelength vertical antenna increases the signal gain towards the horizon. However, I understand that lengthening a vertical antenna beyond 5/8 wavelength is detrimental to signal gain towards the horizon. If this is true, then how do commercial dual band verticals longer than 5/8 wavelength develop signal gain towards the horizon?

The Answer

This response will discuss what happens when a 5/8 wavelength vertical antenna is lengthened, the categories of commercial dual band antennas, how commercial dual band antennas physically longer than 5/8 wavelength increase gain towards the horizon, and how to analyze antenna gain figures.

5/8 versus 3/4 Wavelength Vertical Antennas

An EZNEC elevation plot comparing a quarter wavelength vertical with a five eighths wavelength vertical. The five eighths antenna's main lobe is lower and stronger toward the horizon.
Figure 1. Patterns of 1/4 and 5/8 wavelength verticals. The 5/8 wavelength antenna concentrates more signal in a narrower lobe toward the horizon.
EZNEC plot: Bill Rinker, W6OAV

Figure 1 compares the radiation patterns of both 1/4 wavelength and 5/8 wavelength vertical antennas. The 5/8 wavelength vertical antenna has greater gain towards the horizon than the 1/4 wavelength vertical antenna. This gain occurs because, due to the current pattern on a 5/8 wavelength vertical antenna, more of the signal is concentrated in a narrower lobe towards the horizon.

An EZNEC elevation plot comparing a five eighths wavelength vertical with a three quarter wavelength vertical. The three quarter wavelength antenna sends most of its signal at a high angle.
Figure 2. Patterns of 5/8 and 3/4 wavelength verticals. Most of the longer antenna’s radiation is concentrated at higher angles above the horizon.
EZNEC plot: Bill Rinker, W6OAV

Increasing the length of a 5/8 wavelength vertical antenna to a 3/4 wavelength vertical antenna decreases the gain towards the horizon. Figure 2 compares the radiation patterns of both 5/8 wavelength and 3/4 wavelength vertical antennas. Most of the latter’s radiation, due to the current pattern on the antenna, is concentrated at higher angles above the horizon. This may be OK if one wants to work aircraft!

Two EZNEC current plots for a 19 inch vertical: on 2 meters the current forms a quarter wavelength pattern, and on 70 centimeters it forms a three quarter wavelength pattern.
Figure 3. RF current patterns on a 19 inch vertical, on 2 meters and on 70 cm.
EZNEC plot: Bill Rinker, W6OAV

Sometimes one will hear a ham say that he can use a 2 meter 1/4 wavelength vertical antenna on both 2 meters and 70 centimeters with relatively low SWR. This is true. A 1/4 wavelength antenna can be considered as a 3/4 wavelength antenna on the third harmonic of its fundamental frequency. For example, as shown in Figure 3, a 19.1 inch vertical will look like a 1/4 wavelength antenna on 147 MHz and like a 3/4 wavelength antenna on 443 MHz. The antenna input impedance will be relatively close to the same on both frequencies. As shown in Figure 2, this is a compromise dual band antenna due to the high angle of radiation on 70 cm. However, it is an inexpensive and easy way to construct a temporary dual band antenna.

Based on the above discussion, how do the manufacturers make long dual band VHF/UHF antennas that avoid the above discussed issues? As described below, there are three basic categories of VHF/UHF dual band antennas.

Categories of Vertical VHF/UHF Dual Band Antennas

Drawing comparing three small VHF and UHF whip antennas of increasing length, each with its loading coil, labelled 4a, 4b and 4c with the fraction of a wavelength each represents on VHF and UHF.
Figure 4. Three small dual band antennas. They look alike; electrically they are not. Drawn for OneShack

**Category 1 — antennas under 19 inches in length.** These antennas are small and provide no gain on VHF and minimal gain on UHF. Figure 4(a) shows a relatively simple 12 inch antenna that is a short loaded 1/4 wavelength antenna on VHF and an end fed 1/2 wavelength antenna on UHF. Gain specs are 0.0 dBi on VHF and 2.15 dBi on UHF.

**Category 2 — antennas approximately 19 inches in length.** This category includes simple dual band antennas that are approximately 19 inches in length. Caution must be taken with this category. Depending upon their construction, some of these antennas function as 1/4 wavelength antennas on VHF and as 1/2 wavelength antennas on UHF. Others function as 1/4 wavelength antennas on VHF and as 3/4 wavelength antennas on UHF. As discussed earlier, the latter group has a high angle of radiation on UHF. The former group has a nice low angle of radiation on UHF. Figures 4(b) and 4(c) show examples of both types of antennas.

How does one tell to which group one of these similar looking antennas belongs? Some antenna manufacturers do not indicate the configuration of these antennas. Gain figures can be deceiving. For example, one antenna of the 4(b) type is advertised as having 2.1 dBi of gain on VHF and UHF. Due to its construction, the radiation angle is about 20 degrees above the horizon for both bands. An antenna of the 4(c) type is advertised as having 2.1 dBi of gain on VHF and 3.0 dBi gain on UHF. The problem, as discussed earlier, is that the UHF angle is about 70 degrees above the horizon! A good place to find out dual band antenna configurations is a dealer that publishes the details, such as DX Engineering.

**Category 3 — antennas greater than 19 inches in length.** These antennas are usually between 27 and 50 inches and provide good low angle gain on both VHF and UHF. They are made up of phasing networks and matching networks which create collinear antennas, sometimes just on UHF and sometimes on both VHF and UHF. A collinear antenna is an array of electrical half wave dipoles in line and fed in phase to achieve gain.

How do long vertical antennas develop gain?

Diagram of a collinear dual band antenna: a matching network at the base, a half wavelength section, a phasing network in the middle and another half wavelength section above, with the VHF and UHF current distributions drawn alongside.
Figure 5. An example of a VHF/UHF collinear design: a matching network at the base and a phasing network between the two half wave sections.
Drawing: Bill Rinker, W6OAV

Figure 5 shows an example of a collinear antenna and a schematic showing how low angle gain is achieved. The antenna is an end fed loaded 1/2 wavelength VHF antenna and a two half wavelength collinear UHF antenna. This 27.5 inch antenna has gain specifications of 2.15 dBi on VHF and 4.9 dBi on UHF. The phasing network provides two functions. The first function is that it acts as a loading coil at VHF to electrically lengthen the antenna since the antenna isn’t a physical 1/2 wavelength on VHF. The second function is that it reverses the phase of the UHF signal by 180 degrees as it passes through the network. Without the network, the upper half wave would be reversed and become a full wave sine wave. The matching network matches the low impedance of the coax to the high impedance of the antenna. There are many ways of designing these antennas. The longer the antenna, the more the networks and the higher the resultant gains.

Drawing of five collinear VHF and UHF whip antennas of increasing length, each showing the phasing sections along its length and a matching network at the base.
Figure 6. High gain VHF/UHF antennas. The longer the antenna, the more phasing sections, and the more gain toward the horizon. Drawn for OneShack

Figure 6 shows some of the various available high gain VHF/UHF antennas. Note the number of phasing networks. Some antennas have collinear 1/2 wavelength sections and some have collinear 5/8 wavelength sections. Others have a combination of both.

Analyzing Antenna Gain Figures

When analyzing antenna gain figures, verify that the figures are specified in either dBi or dBd. Gain figures such as “dB” are meaningless.

If possible, locate the manufacturer’s radiation patterns as shown in Figure 1. It’s important to know where the specified gain occurs relative to the horizon.

References

© Bill Rinker, W6OAV. Shared on OneShack with his permission. Not to be copied or reused without his permission.

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