Yesterday I received an inquiry on how to construct a VHF Coaxial Dipole antenna (known is some circles as the Flowerpot Antenna) for a non-commercial, community two-way radio project operating at 160 MHz.
How to work out the antenna dimensions for 160 MHz?
Assumption: We know the wavelengths for 144 MHz and 160 MHz, 2.03m and 1.875m respectively, therefore any method of calculating the dimensions for 160 MHz must produce element lengths shorter than those for 144 MHz. With an increase in frequency of 16 MHz the new antenna electrical half-wave length must be physically shorter.
I used a known antenna design for 144.2 MHz and frequency scaled the new 160 MHz antenna based on the known dimensions for 144.2 MHz.
Known antenna dimensions here – https://vk1nam.wordpress.com/2m-coaxial-dipole-antenna/
- F1 – known antenna resonant frequency
- F2 – target antenna resonant frequency
F1/F2 = antenna scaling factor, F1 144.2 MHz / F2 160 MHz = 0.90125
Known 144.2 MHz Top Section = 460 mm.
Known 144.2 MHz Braid Section = 450 mm.
New 160 MHz antenna dimensions:
- Top Section 460 mm x 0.90125 = 414.5 mm, round up to 415 mm
- Braid Section – 450 mm x 0.90125 = 405.5 mm, round up to 406 mm
Results of frequency scaling
Construction – 160 MHz antenna assembled in a 1 metre length of non-conductive plastic pipe. You should avoid using carbon fiber tubes or reinforced carbon composite tubes. Carbon fiber or graphite fiber will detune the antenna’s resonant frequency.