Flower Pot Antenna Length Calculator
Use this tool to get practical starting dimensions for a single-band flower pot antenna.
What is a flower pot antenna?
A flower pot antenna is a compact vertical antenna design often made from coax cable and housed in a plastic tube or flower pot-style enclosure. It is popular with amateur radio operators because it is inexpensive, weather-friendly, and easy to build with common tools.
While there are multiple variants, most builds use a half-wave radiating section combined with a quarter-wave coaxial matching section. This calculator provides practical starting lengths based on your selected operating frequency and coax velocity factor.
How to use this calculator
- Frequency: Enter your target frequency in MHz.
- Velocity factor: Use the VF from your coax datasheet.
- Correction factor: Start around 0.95, then trim in small steps during tuning.
- Calculate: The tool returns radiator and matching section lengths in both cm and inches.
What the outputs mean
The results include free-space wavelength references plus practical cut lengths:
- Half-wave radiator (adjusted): The main radiating section to begin with.
- Quarter-wave coax matching section: A VF-adjusted electrical length in coax.
- Total active length: Radiator + matching section (excluding extra pigtail/service slack).
Build notes and best practices
Before you cut
- Measure carefully and leave a little extra for final trimming.
- Use heat-shrink or weatherproof tape for any exposed joints.
- Keep metal hardware away from the upper radiating section.
- If installing outdoors, add a drip loop and seal feedline connectors.
During tuning
Tune with a reliable SWR meter or antenna analyzer. Trim very small amounts (1 to 2 mm at a time), then re-measure. Environmental factors, nearby objects, mounting mast effects, and coax routing can all shift resonance.
Example: 146.52 MHz (2 meter calling frequency)
With VF = 0.66 and correction factor = 0.95, a common starting point is:
- Adjusted half-wave radiator: approximately 97.1 cm (38.23 in)
- Quarter-wave coax matching section: approximately 33.8 cm (13.31 in)
These are starting dimensions. Real-world tuning is normal and expected.
Frequently asked questions
Can I use this for VHF and UHF?
Yes. The calculator supports 30 to 1000 MHz, which covers many common VHF/UHF applications.
Do I need the exact velocity factor?
Close is usually fine for a first cut, but using the manufacturer value improves first-pass accuracy.
Is this a guaranteed final cut length?
No. Think of this as a strong baseline. Final resonance always depends on real installation conditions.
Final reminder
This calculator is intended for educational and hobby use. Follow local regulations, use proper RF safety practices, and verify your design before transmitting at power.