The 2-meter amateur band remains one of the most active VHF bands, especially for FM, repeater operation, and local DX. A properly designed antenna can significantly improve both transmitted and received signals. The 5/4 wave VHF antenna using 450-ohm open ladder line is a simple yet effective design that offers higher gain than a standard ½-wave vertical while remaining easy to build with inexpensive materials.
This antenna is particularly attractive for radio amateurs who enjoy homebrew solutions and want a rugged, weather-protected vertical antenna for 145 MHz operation.
Why the 5/4 Wave VHF antenna design?
The 5/4 wave design is prized in the VHF community because it effectively compresses the signal toward the horizon. By using ladder line for capacitive coupling, you can create a “Slim Jim” style variant that is easy to tune and incredibly weather-resistant when housed in PVC.
A 5/4 wavelength vertical antenna provides a lower radiation angle compared to a ¼-wave or ½-wave antenna. This results in better signal strength at the horizon, making it ideal for VHF FM repeaters and simplex contacts. The additional electrical length helps concentrate radiation where it is most useful for terrestrial communication.
When properly matched, a 5/4λ antenna can offer noticeable gain without the complexity of collinear arrays or stacked elements.
Use of 450-Ohm Open Ladder Line
In this design, 450-ohm ladder line is used not only as a conductor but also as a capacitive coupling section. A short length of ladder line at the base of the antenna acts as a matching element, helping transform the feed impedance to better suit 50-ohm coaxial cable.
This method eliminates the need for coils or complex matching networks and keeps losses extremely low. Ladder line is inexpensive, widely available, and performs very well at VHF frequencies when used over short lengths.
5/4 Wave VHF Antenna Dimensions and Electrical Design
The antenna is designed specifically for 145 MHz, a common calling and repeater frequency in the 2-meter band.
The radiating element length is approximately 245 cm, which corresponds to a 5/4 wavelength after applying a velocity factor correction. The ladder-line coupling section is 40 cm, functioning as a quarter-wave capacitive coupler with an adjusted velocity factor.
The velocity factor (VF) is the ratio of the speed at which an electromagnetic signal travels through a cable or medium to the speed of light in a vacuum
These dimensions are derived using standard antenna formulas and practical correction factors to account for conductor diameter and dielectric effects.
Construction and Weather Protection
To ensure durability, the complete antenna assembly can be inserted into a PVC pipe with end caps. This protects the ladder line and radiating element from rain, dust, and UV exposure without significantly affecting RF performance at VHF frequencies.
PVC is RF-transparent at 145 MHz and makes the antenna suitable for permanent outdoor installation on rooftops or masts.
The 145 MHz 5/4 wave VHF antenna using 450-ohm ladder line is an elegant blend of simplicity and performance. With minimal components, easy construction, and excellent on-air results, it is a perfect project for both beginners and experienced radio amateurs.
For those seeking a cost-effective, high-gain vertical antenna for the 2-meter band, this design is well worth building and experimenting with.
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