Build a 2-Meter Flower Pot Antenna

Flower Pot structure from top to bottom: Section 1 — Half-wave radiator (top): The coax center conductor AND braid both radiate. Length: ~19.3 inches (λ/2 at 146 MHz, VF 0.66) This section is the primary radiating element. The top end is left open (center and braid not joined). Section 2 — Coaxial choke (middle): A specific length of coax wound into a coil, OR a straight section with the exact length to provide high impedance on its outer surface. Length: ~19.3 inches (λ/2 at 146 MHz) The choke isolates the upper radiating section from the feedline below — preventing the feedline from radiating and distorting the pattern. Section 3 — Feedline (bottom): Standard coax from the choke bottom to the radio. The SO-239 or BNC connector is at the bottom of the choke section. Note: unlike the J-pole, the Flower Pot has no separate matching stub. The feedpoint impedance is determined by the choke design and typically falls in the range of 40–60 Ω — close enough to 50 Ω for direct coax connection with no tuner.

Coaxial choke options for 146 MHz Flower Pot: Option 1 — Coiled choke (most common): Wind 6–7 turns of the coax into a coil, approximately 3–4 inches in diameter, at the junction between the radiating section and the feedline section. The coil's inductance at 146 MHz creates high impedance to common-mode current. Tape the coil securely to prevent uncoiling. Option 2 — Straight λ/2 choke section: A straight section of coax exactly λ/2 long (at the operating frequency, considering VF) presents a virtual short circuit from the OUTSIDE of the braid — similar to a choke. Length: 19.3 inches for 146 MHz, VF 0.66 Simpler than winding a coil but requires more precise length for effective choking. Option 3 — Ferrite bead choke: 10–15 type-43 ferrite beads on the coax at the junction. Provides broadband choking. Most compact solution; slightly less effective than a resonant choke at the design frequency. This guide uses the coiled choke (Option 1) — most forgiving of length errors and provides excellent choking at 146 MHz.

Flower Pot gain mechanism: The coaxial choke creates a phase reversal between the radiating section above it and the feedline below. Both sections radiate in-phase toward the horizon, adding approximately 3 dB of collinear gain compared to a simple half-wave dipole. Pattern comparison at 146 MHz: Quarter-wave vertical: 0 dBd, modest low-angle J-pole (half-wave): 2 dBd, slightly lower angle Flower Pot (collinear): 3 dBd, lower angle still Commercial 5/8-wave: 3–4 dBd The Flower Pot's low-angle pattern enhancement is particularly valuable for: — Repeater access from greater distances — Simplex contacts over flat terrain — APRS coverage with a larger footprint — Linking sites where low-angle gain matters Vertical beamwidth: ~25–30° (narrower than J-pole) This concentrates more energy at the horizon.

Velocity factor of common coax types: RG-58: VF = 0.66 (solid polyethylene) RG-8X: VF = 0.82 (foam polyethylene) RG-213: VF = 0.66 (solid polyethylene) LMR-400: VF = 0.85 (foam polyethylene) RG-174: VF = 0.66 (solid polyethylene) Length of λ/2 at 146 MHz for different coax: VF 0.66: (984 × 0.66) / (2 × 146) = 2.22 ft = 26.7 in VF 0.82: (984 × 0.82) / (2 × 146) = 2.76 ft = 33.1 in VF 0.85: (984 × 0.85) / (2 × 146) = 2.86 ft = 34.3 in This guide uses RG-58 (VF 0.66) — most widely available, lowest cost, and the VF is well-established. The completed antenna uses approximately 5 feet of RG-58. If using a different coax, recalculate ALL section lengths using the actual VF of your coax. Mixing different coax types in one antenna causes impedance steps that produce SWR and pattern issues. Use the SAME coax throughout the entire antenna.