146MHz 5/8 ground plane tower mounted antenna. Install March 2026.

Due to recent damage to my ageing basic λ/4 ground-plane antenna, I required a new antenna for the 2-metre band to support local FM simplex and repeater communication. It was therefore decided to redesign a 5λ/8 ground-plane antenna that I had used many years ago with good success. Conveniently, several critical pieces of hardware from that earlier antenna were still available. Details of the 2010 5/8 ground-plane antenna project can be found here: 5-8_Ground_Plane.htm. The antenna would again be focused on the upper section of the 2-metre band, at approximately 146.5 MHz.

The reason for selecting the 5λ/8 ground-plane antenna is that, for a simple single-element design, it offers increased effective aperture compared with a standard λ/4 ground-plane antenna, and it produces a relatively low angle of radiation compared with similar antennas. The 5λ/8 ground-plane antenna has a slight gain advantage over a λ/4 ground-plane, typically around 1 to 1.5 dB. While this increase is small and almost insignificant on its own, every bit of additional gain is arguably worth having.

Photo 1 Fully installed 146 MHz 5/8-wave ground-plane antenna mounted on the tower.

The 5λ/8 ground plane antenna specifications:

A basic λ/4 ground-plane antenna, with the radiating element being 0.25 wavelength in physical length, has the advantage of presenting an impedance typically in the 25–40 Ω range, which is reasonably close to a 50 Ω feed line. An improvement to the match can be achieved by angling the radials downward at about 40 degrees, or by slightly shortening the main element so that the antenna operates slightly off resonance, thereby achieving a closer impedance match.

A λ/2 vertical antenna, while offering some gain advantage, presents a very high feed-point impedance, making it more difficult to match directly to a feed line.

The 5λ/8 ground-plane antenna offers a modest gain advantage over a λ/4 ground-plane and is generally easier to match than a half-wave vertical. However, by itself it does not provide a sufficiently good match to a 50 Ω feed line and therefore typically requires a matching inductor placed between the radiator and the feed point to achieve a good match.

The 5λ/8 radiator is electrically longer than its natural resonant length, which causes the current distribution along the element to shift upward. As a result, the feed-point impedance contains a significant capacitive reactance. In simple terms, the antenna behaves as though it is too long for resonance at the operating frequency, causing the feed point to appear capacitive. A series inductor placed at the feed point provides the necessary inductive reactance to cancel this capacitive component and bring the antenna system to resonance.

Design Modelling

MMANA-GAL Antenna Analyser predicted the following results

Fig 3 Radiation pattern of the 146.5 MHz ground-plane antenna. The radiation plot was produced using MMANA-GAL antenna analysis software. Predicted gain, impedance, and SWR are also shown.

Fig 4 A three-dimensional view of the radiation pattern of the 146.5 MHz ground-plane antenna. The radiation plot was produced using MMANA-GAL antenna analysis software.

Fig 5 Predicted SWR graph generated from MMANA-GAL analyser data.

Fig 6 Predicted impedance graph generated from MMANA-GAL analyser data. 

Inductor (Coil) Calculation

A 5λ/8 vertical element will typically present a base impedance within the range shown below. However, a number of factors - such as element diameter, the physical dimensions of construction hardware, materials used, and nearby structures - can affect this impedance value.

A typical 5λ/8 vertical element feed-point impedance is approximately:

Z ≈ 20–30 Ω − j(100 to 150) Ω

The negative j value indicates that the load is capacitive. Therefore, an inductive reactance of approximately 100 to 150 Ω (a positive j value) must be placed in series with the radiating element to cancel most of the capacitive reactance.

Determining the Inductance

For a typical 5λ/8 radiator with an impedance of approximately:

Z ≈ 25 − j120 Ω

the matching coil must therefore provide an inductive reactance of:

XL = +j120 Ω

Using:

Where f=146.5 MHz

 Target inductance (Maybe)

Inductor details to calculate the number of turns for the inductor (Coil).

Given:

• Coil former diameter = 20 mm

• Radius r = 10 mm

• Coil length = 8 mm (Estimation)

• Wire diameter d = 1 mm

Assume 1 to 2 mm spacing between turns.

Formula

Making N (Number of turns) the subject

Where

L = inductance (nH)

r = coil radius (mm)

l = coil length (mm)

d = wire diameter (mm)

N = number of turns

Rounded to 2.5 Turns.

Trial and error construction and testing resulted in a 3-turn inductor, with fine adjustments achieved by experimentally stretching or compressing the coil.

The size of the loading inductor is then adjusted until a sufficiently good match is obtained. Some minor trimming of the 5λ/8 radiator may also be required to achieve the best match. However, it is important to maintain a radiator length close to, or slightly less than, 5λ/8 (approximately 0.6λ).

Photo 2 Installed redesigned matching coil using an insulated tube recovered from a commercial low-band VHF antenna.

Photo 3 Matching coil sealed and protected with heat-shrink tubing.

Construction

The antenna radiator is physically 5λ/8, or about 0.6λ at 146.5 MHz. The calculated length is typically reduced by about 5% to allow for the effect of the element’s physical diameter. In practice, however, the element is initially left slightly longer so that it can be experimentally trimmed to achieve the best performance and impedance match.

The antenna is constructed around a standard male N-type connector, which is mounted to a 100 mm diameter aluminium disc that serves as the attachment hub. Six λ/4 radials are attached to the disc and aligned horizontally, as shown in the assembly photograph.

The ground-plane radials are approximately 5–10% longer than a λ/4 radiating element, although they may be made longer if desired. The RSGB VHF/UHF Manual by G. R. Jessop, G6JP, suggests element lengths of 0.28λ to 0.30λ.

Figure 7  Basic dimensions for the 5/8-wave ground-plane antenna project.

The radials are attached using aluminium pop rivets. As is best practice for aluminium construction, all mating surfaces should be thoroughly cleaned before assembly. The pop rivets are dipped in aluminium jointing compound, or a small amount of grease, to help reduce oxidation and maintain good electrical contact at the joints.

Figure 8  Ground-plane radial element mounting plate. Note that the 2 mm radial attachment holes are 3.2 mm for the 5/8-wave ground-plane project.

Photo 6. Main attachment hub assembly. This assembly is from a previously constructed, similar antenna and was not used in this build. The threaded stud for the 5/8-wave ground-plane antenna uses a 5/16-inch, 24-TPI standard mobile antenna mount.

Figure 9 Ground plane radial element assembled.

A standard mobile antenna mount was disassembled, and the coax screen ground bracket was discarded. The antenna mounting threaded stud was then connected to the centre pin of the male N-type connector and secured within the connector body using two-part epoxy adhesive.

The weatherproof cone was then shortened, seated over the antenna mounting stud, and fixed in place using the standard nut. A small amount of windscreen silicone sealant was applied at the base of the cone and under the securing nut to provide an effective weatherproof seal.

The antenna mounting consists of a standard antenna mirror-mount bracket fitted with a female-to-female N-type bulkhead connector, allowing antennas constructed around male N-type connectors to be easily attached and replaced as required. This arrangement is suitable for most simple antennas, such as a λ/4 ground-plane, for frequencies as low as 50 MHz, and can also accommodate more complex antennas such as collinear designs operating into the low GHz bands. It therefore provides a convenient test platform for experimenting with various antenna designs. See: Generic Antenna Mount.

Photo 7 146MHz 5/8 ground plane antenna assembled.

Testing

Testing was carried out using a NanoVNA to measure SWR and impedance.

The SWR measurements showed a broad match, with an SWR of less than 2.6:1 from 144 MHz to 148 MHz, and a very good match at the target frequency of 146.5 MHz.

The Smith chart display indicates that the antenna does not reach resonance within the 2 m band and that it exhibits significant inductive reactance. This result is inconsistent with the MMANA-GAL modelling predictions.

Photo 8. NanoVNA display of an SWR of less than 2.6:1 from approximately 144MHz to 148MHz.

Conclusion

The 146.5 MHz 5λ/8 ground-plane antenna confirmed that its performance is generally in line with the MMANA-GAL model and is very similar to a previously constructed version.

Due to the one-day construction deadline, more detailed testing and optimisation were not carried out. It is likely that improved impedance matching could have been achieved through further adjustment of the loading inductor and additional trimming of the radiating element. However, in practical operation the antenna met all performance expectations.

The antenna provides reliable coverage for local FM simplex and repeater operation across the 2 m band and demonstrates the practicality of the 5λ/8 ground-plane design as a simple, single-element antenna with modest gain and a relatively low radiation angle. While the improvement in gain compared with a λ/4 ground-plane is small, the design offers a useful performance advantage while remaining mechanically simple and easy to construct.

The use of a standard N-type connector mounting system also allows antennas to be easily replaced or modified, providing a convenient platform for further antenna experimentation and development.

References

Details of the generic antenna mount. See: Generic Antenna Mount.

Makoto Mori. (n.d.). MMANA-GAL antenna modelling software: https://hamsoft.ca/pages/mmana-gal.php

RSGB. VHF UHF Manual - fourth edition by G.R.Jessop, G6JP

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Page initiated 03 March, 2026 

Page last revised 12 March, 2026