Build a 70cm Flower Pot Antenna
The flower pot antenna — a coaxial dipole with a sleeve balun, named for its appearance when built in a white PVC pipe — is one of the most effective and discreet UHF base station antennas a home operator can build. Designed by VK2ZOI (Fred Nachbaur, SK) for the 2m band and widely adopted for 70cm, the flower pot achieves 3–4 dBd gain from a coaxial construction that requires no external radiating elements — the antenna is entirely hidden inside its white PVC housing, indistinguishable from a plumbing pipe or fence post. At 70cm the complete antenna is under 18 inches tall, weighs a few hundred grams, and costs under $15 in materials. This guide covers coaxial dipole theory, the sleeve balun operating principle, exact dimensions for 432–440 MHz, step-by-step construction from RG-58 coax and PVC pipe, and SWR verification for a permanently mounted installation.
The Coaxial Dipole — What Is It?
The flower pot is fundamentally a coaxial dipole — a half-wave dipole where one element is formed by the coax inner conductor and the other by the outer surface of the coax shield. The critical innovation is the sleeve balun (also called a choke balun or bazooka balun) that forces the antenna current to flow on the correct surfaces rather than running down the feedline:
Why the Flower Pot Achieves Gain
A simple half-wave dipole provides 0 dBd gain (it IS the reference). The flower pot achieves 3–4 dBd gain through a combination of two effects that distinguish it from a simple dipole:
Dimensions — Velocity Factor is Everything
The flower pot's dimensions are set by the velocity factor of the coaxial cable used. All electrical lengths must be calculated using the actual VF of your specific coax — using the wrong VF produces an antenna resonant at the wrong frequency:
Flower Pot vs Slim Jim vs J-Pole — Choosing
All three are excellent 70cm base station antennas in the same gain range. The practical differences guide the choice:
- Flower pot: fully concealed inside PVC — the best choice for HOA-restricted installations, apartment buildings, rental properties, or any location where antenna visibility is a concern. The white PVC pipe is indistinguishable from a plumbing vent. Slightly more complex to build than the Slim Jim but uses readily available RG-58 coax rather than TV twin-lead.
- Slim Jim: built from 300 Ω TV twin-lead — cheaper and faster to build than the flower pot. Equally effective. Less stealthy (the twin-lead wires are visible through most PVC housing). Better choice when the goal is lowest cost and fastest build time.
- J-pole: most mechanically robust when built from copper pipe. Better suited to high-power operation. Slightly more complex tuning than the Slim Jim or flower pot. Less common on 70cm because the small copper pipe dimensions are harder to work with precisely.
- All three produce similar performance: the differences in gain are under 1 dB between any two of these designs. Choose based on construction preference and installation requirements, not marginal performance differences.
| Section | Length (inches) | Length (mm) | Material | Notes |
|---|---|---|---|---|
| Upper radiating element | 6.80 | 173 | Bare copper wire or coax inner conductor | Free-space λ/4; no coax jacket — bare conductor only |
| Lower radiating element | 4.49 | 114 | RG-58 coax outer braid (jacket stripped) | Coax λ/4; jacket removed, braid exposed |
| Sleeve balun section | 4.49 | 114 | RG-58 coax (jacket on) | Coax λ/4; shorted at bottom by soldering braid to bottom fitting |
| Total coax length needed | 8.98 | 228 | RG-58 | Lower radiator + balun; buy 12 inches for margin |
| Total antenna height | ~15.8 | ~400 | Upper element + coax sections; PVC housing adds 2 inches | |
| PVC housing length | 18.0 | 457 | 3/4-inch or 1-inch PVC pipe | 2 inches clearance above and below antenna structure |
| Junction between elements | At 6.80 inches from top | At 173mm from top | Solder junction | Inner conductor of coax soldered to base of upper element wire |
Materials for a 70cm flower pot antenna for 432–440 MHz
Building the 70cm Flower Pot
The flower pot is built from the top down — upper element first, then the coax assembly below it. Precision in the coax section lengths is important: at 435 MHz, 5mm of coax length difference shifts resonance by approximately 3 MHz. Measure each coax section with a caliper before cutting. Build and test before installing in the PVC housing.
Verify Your Coax Velocity Factor
Before cutting any coax, verify the velocity factor of your specific RG-58. Different brands of RG-58 have VF ranging from 0.65 to 0.67. A 0.01 VF difference produces a 3–4 MHz resonance shift at 70cm — worth measuring rather than assuming.
The simplest VF measurement uses the NanoVNA: cut a 20-cm piece of coax, short one end with a piece of wire, and connect the NanoVNA to the other end. Sweep until you find the frequency where the shorted stub shows minimum impedance (series resonance). At this frequency, the stub is exactly λ/4 long: VF = (4 × stub_length_m × f_resonance_Hz) / 3×10⁸. If your measurement gives VF between 0.65–0.67, use 0.66 for calculations. If outside this range, recalculate all section lengths using your measured VF before cutting.
Cut the Upper Radiating Element
Cut the upper radiating element from #14 AWG solid copper wire or a brass rod. This element is 6.80 inches (173mm) long — this is a free-space quarter-wave length because the upper element is bare conductor, not inside coax. Straighten the wire completely — any bend or kink changes the effective electrical length slightly. File both ends clean and square:
Prepare the Coax Sections
The antenna uses one continuous piece of RG-58 coax, processed in three sections from top to bottom. Cut the RG-58 to 9.5 inches total — slightly more than the 8.98 inches needed for the lower radiator plus balun, with margin for the bottom connection hardware.
Working from the TOP of the coax downward, prepare the sections:
Solder the Upper Element to the Coax Inner Conductor
At the top end of the coax, the inner conductor is exposed by stripping 1 inch of both jacket and braid. Solder the bottom of the upper radiating element wire directly to this inner conductor. The joint must be mechanically strong — this is the structural connection between the upper and lower halves of the antenna:
Close the Sleeve Balun and Connect the Feedpoint
The sleeve balun is formed by the exposed coax braid of the lower radiating element section acting as one conductor, and the inner conductor (connecting to the upper element) as the other. The sleeve must be short-circuited at its BOTTOM end (the junction between the lower radiating element and the balun section). This short circuit is made by soldering the exposed braid to the outer jacket of the balun section at the junction point:
Test SWR Before Housing
Before installing the antenna in the PVC housing, verify the SWR with the NanoVNA. Hold the antenna vertically in free space — at arm's length, away from metal surfaces and your body. Connect the NanoVNA to the SO-239 at the bottom and sweep 420–450 MHz:
Install in PVC Housing, Final Test, and Mount
Insert the complete antenna assembly into the PVC pipe from the bottom. The SO-239 installed in the bottom end cap serves as the mechanical anchor for the whole assembly — the coax attaches to the SO-239, holding the antenna in position inside the pipe. The upper element tip should end up 1–2 inches below the top of the pipe for protection.
With the antenna inside the unsealed pipe, connect the NanoVNA and verify SWR has shifted down to the target range (432–436 MHz). Once confirmed, cement the top cap in place and seal the SO-239 installation in the bottom cap with silicone sealant. Mount on the mast with the antenna vertical and perform a final sweep from the shack end of the coax:
Stealth Installation Options
The flower pot's concealed design opens up installation locations that are impossible for visible antennas:
- Plumbing vent disguise: a white PVC flower pot mounted on a rooftop is indistinguishable from a plumbing vent pipe. Add a standard plumbing vent cap to the top and a cleanout fitting at the bottom to make it look more like actual plumbing. In HOA-restricted neighbourhoods or rental properties, this approach passes casual inspection without revealing an antenna.
- Fence post installation: a flower pot inside a hollow PVC fence post — common fence styles use 4-inch square white PVC posts — is completely invisible. The PVC wall is thin enough that antenna performance is minimally affected. Connect the coax through a small hole at the base of the post, painted over to match the post colour.
- Flag pole installation: a flower pot inside a thin-wall aluminium or fibreglass flag pole works if the pole is non-conductive. Aluminium flag poles act as a short circuit around the antenna — use fibreglass or PVC flag poles only. Some commercial flag pole antennas use exactly this approach.
- Apartment building rooftop: a single white PVC pipe strapped to a rooftop air conditioning conduit or drain pipe is often entirely ignored by building management. The 18-inch height of the 70cm flower pot is unobtrusive. Always get explicit written permission before installing on property you do not own — this advice applies regardless of how concealed the antenna is.
Dual-Band Version — 2m and 70cm
A dual-band flower pot for simultaneous 2m and 70cm operation can be built by stacking two coaxial dipole sections on the same coax feedline. The original VK2ZOI design describes this configuration in detail — the key principle is that the 70cm sleeve balun dimensions are chosen to be electrically transparent on 2m while working correctly on 70cm:
- Design principle: the 70cm sleeve balun (λ/4 at 435 MHz) is approximately 3λ/4 at 145 MHz — an odd multiple of λ/4, which presents high impedance on 2m just as it does on 70cm. This means a single sleeve balun works on both bands simultaneously.
- Construction: add a 2m upper radiating element (19.5 inches for 145 MHz) above the 70cm element. The 70cm element serves as a sleeve balun for the 2m element on both bands. The complete antenna has three sections: 2m upper element (bare wire), 70cm lower element (braid exposed), and balun (jacket on).
- Feedpoint: one SO-239 feeds both bands simultaneously — no switching required. The antenna works on whichever band is active.
- Housing: a 36-inch PVC pipe houses the dual-band version. It is still visually very similar to the 70cm-only version and easily passes as plumbing infrastructure.
- Reference: the VK2ZOI flower pot design page provides exact dimensions for the dual-band version — search for "VK2ZOI flower pot antenna" for the original documentation.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| High SWR across entire sweep — no resonance visible | Open or short at wrong location; coax inner conductor not connected to upper element; or sleeve balun short circuit missing | Ohmmeter from SO-239 centre to upper element tip should show continuity; check sleeve balun solder joint at section boundary | Rebuild junction between inner conductor and upper element; verify sleeve balun short circuit solder joint is present and solid |
| Resonance well below 420 MHz even before housing | Coax sections are too long; or VF was overestimated | Measure each coax section length with caliper; compare to 4.49 inches; calculate actual VF with test stub method | Rebuild with shorter coax sections using accurately measured VF; do not trim the upper element first — the coax sections are the primary determinant of frequency |
| Resonance above 460 MHz even before housing | Coax sections too short; or VF underestimated | Measure coax section lengths; verify total is 8.98 inches | Rebuild with longer coax sections; measure VF with test stub before cutting |
| SWR minimum at correct frequency but value is 3:1 or higher | Sleeve balun short circuit not forming a clean RF short; or inner conductor shorted to braid at wrong location | Verify the sleeve balun boundary solder joint makes a clean connection between braid and jacket edge; check that inner conductor is not shorted to braid except at the SO-239 connection | Resolder sleeve balun boundary solder joint; verify electrical isolation between inner and outer at all points except SO-239 |
| Performance good initially but degrades over months outdoors | Moisture ingress at top cap solder joint between upper element and coax; or corrosion at SO-239 | Disassemble and inspect; look for green corrosion at the inner conductor / upper element solder junction | Rebuild corroded junction with fresh solder; apply self-amalgamating tape over junction before re-housing; ensure top cap is fully sealed with PVC cement |
| PVC housing is now sealed and resonance is wrong — can I fix it? | Upper element needs trimming (too low frequency) or extending (too high frequency) | Measure SWR minimum frequency; calculate how much frequency correction is needed | For trimming: carefully drill a small hole in the top cap and use a file through the hole to shorten the upper element tip — difficult but sometimes possible. For extending: thread a short wire extension through a sealed eyelet in the top cap. Prevention is far better — always test before sealing. |
Can I use RG-8X instead of RG-58 for the flower pot?
Yes — RG-8X works well and its velocity factor is typically 0.82 rather than 0.66 for RG-58. Using RG-8X requires different coax section lengths: λ/4 in RG-8X at 435 MHz is (300/435 × 0.82) / 4 = 0.141 m = 5.56 inches, compared to 4.49 inches for RG-58. An RG-8X flower pot has longer coax sections and is slightly larger in diameter — it may need a 1-inch PVC housing rather than 3/4-inch. The principle and construction procedure are identical; only the calculated section lengths change. Always use the actual measured VF of your specific cable.
Why is it called a flower pot?
The antenna was originally described by VK2ZOI with the coax and upper element housed in a commercial white PVC pipe. When mounted on a balcony railing or rooftop with the antenna pipe pointing upward and a garden flower pot placed inverted over the bottom to hide the SO-239 connector, the complete installation looks like a decorative element — a pipe with a flower pot. The name stuck in Australian amateur radio circles and spread worldwide with the design. Many builders now omit the flower pot prop entirely and just use a sealed PVC pipe, but the name remains universal for the design.
Does the PVC colour matter for antenna performance?
White and grey PVC have essentially identical dielectric constants and losses — both produce the same antenna performance. The only significant colour-related consideration is UV resistance: white PVC reflects UV more effectively and maintains its mechanical properties longer in direct sunlight than unpainted grey PVC. Both are adequate for the 5–10+ year lifespan of the antenna. If painting the PVC to blend with the environment (fence colour, building colour), use standard spray paint compatible with PVC — the paint layer is thin enough that it does not measurably affect antenna performance.
How does the flower pot compare to a commercial 70cm antenna?
A well-built 70cm flower pot performs comparably to commercial quarter-wave ground plane antennas (which it outperforms by ~3 dBd) and similarly to commercial J-poles and slim jims in the same gain class. Commercial collinear antennas like the Diamond X50 or X200 series outperform the flower pot by 2–4 dBd because they use multiple in-phase stacked elements rather than a single dipole. For the price difference ($15 homebrew vs $60–90 commercial collinear), the flower pot delivers excellent value. If maximum performance is the goal and HOA stealth is not required, the 2m/70cm collinear guide describes a higher-gain alternative.
Can I transmit at 50W into the flower pot?
Yes — an RG-58 flower pot handles 50W continuous without concern. RG-58 is rated for approximately 100–200W at 435 MHz depending on the specific cable grade, and the antenna's thin inner conductor (14 AWG copper wire) handles 50W at 70cm with minimal heating. At 100W the antenna still works correctly but the coax sections experience modest warming under sustained carrier — digital modes at 100W for extended periods may cause the coax inner insulation to soften slightly at the solder junctions. For stations regularly running 100W digital modes, rebuild the antenna using RG-8X (heavier coax with larger conductor) rather than RG-58.
What is the radiation pattern of the flower pot?
The flower pot radiates an omnidirectional pattern in the horizontal plane — equal gain in all directions around the antenna axis. In the vertical plane, the pattern is compressed toward the horizon compared to a simple quarter-wave vertical: maximum radiation occurs at approximately 8–12° elevation rather than 20–25°. This pattern compression is what produces the 3–4 dBd gain relative to a dipole or ground plane — not more total power, but more power directed toward the horizon where it is most useful for terrestrial communication. Directly overhead (90° elevation) the flower pot has a null, and directly below the antenna there is also a null — both of which are the normal behaviour for a vertically polarised end-fed half-wave dipole.