This design is a highly practical dual-band RW4HFN antenna that allows amateur radio operators to transmit and receive on both the 2-meter (145 MHz) and 70-centimeter (430 MHz) bands using one compact assembly.
This design functions as a center-fed element on the lower band and exhibits multi-mode characteristics on the higher band, providing excellent versatility for fixed or portable operation. It is intended to be fed with a 50 Ohm Coaxial Cable.
Key Construction Specifications
The following dimensions are critical for the successful construction and proper performance of this antenna :
- Operating Frequencies: 145 MHz (VHF) and 430 MHz (UHF).
- Element Material: The elements are typically constructed using metal with a diameter of 6 mm.
- Radiating Element Length: The top and bottom vertical radiating sections are both 43.5 cm long.
Feed System and Choke
The dual-band RW4HFN antenna utilizes a specific folded section that acts as a matching transformer, efficiently coupling the dual-band element to the coaxial cable.
- Horizontal Feed Section: The horizontal length of the feed-stub section is 9 cm.
- Vertical Separation: The vertical gap within the feed-stub section is 4 cm.
Essential Component: The RF Choke
A crucial part of this design is the RF Choke located at the feed point. This choke is essential for preventing common-mode currents from flowing down the outside of the coaxial cable shield, which would distort the antenna’s radiation pattern and lead to RFI.
An RF choke is a specialized inductor designed to block high-frequency alternating current (AC) or radio frequency (RF) signals while allowing low-frequency or DC signals to pass
Choke Construction – The choke is made of Several Ferrite Rings (or beads) placed directly over the coaxial cable near the connection point.
Building the RW4HFN antenna to these precise dimensions can provide a reliable, high-performing vertical antenna for your VHF/UHF amateur radio communications.
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