Build a Dual-Band J-Pole Antenna
The dual-band J-pole covers both 2m (144–148 MHz) and 70cm (430–450 MHz) from a single antenna and a single feedpoint — the go-to fixed station antenna for operators using a dual-band FM radio. Built from copper pipe with a unique twin-stub design that presents correct matching on both bands simultaneously, it requires no duplexer, no switching, and no tuner. The antenna's construction takes one weekend, costs under $25, and produces an antenna that performs noticeably better than the rubber duck on any handheld on both bands. This guide builds the classic copper pipe dual-band J-pole using the well-proven N9TAX-style split-stub design, tuned for the full 2m and 70cm FM bands.
The Dual-Band Challenge
A single J-pole antenna works on one band — the band where its matching stub is a quarter-wave and its radiator is a half-wave. Getting the same antenna to work on 70cm (three times the frequency of 2m) seems impossible at first glance, but the harmonic relationships of J-pole physics make it achievable with a clever stub modification:
The Split-Stub Design Explained
The split stub (also called the N9TAX design after the amateur who popularized it) works by creating two independent matching sections within a single stub assembly:
Practical Dual-Band Performance
A well-built dual-band J-pole provides useful performance on both bands simultaneously from a single feedpoint and single coax run to the radio:
Tuning the 70cm Center Frequency
A key feature of the dual-band J-pole is that the 2m and 70cm center frequencies can be independently adjusted:
- 2m center frequency: controlled by the radiator length and the 2m feedpoint tap position on the lower stub. Trim the radiator tip to raise the 2m center frequency.
- 70cm center frequency: controlled primarily by the lower stub section length and the 70cm feedpoint tap position. Trim the lower stub section to raise the 70cm center frequency.
- For 70cm FM repeater use (440–450 MHz): the lower stub section should be tuned slightly shorter than the standard 6.4 inches to shift the 70cm minimum SWR toward 440 MHz rather than 432 MHz weak-signal.
- For both weak-signal and FM use: a compromise center frequency around 433–436 MHz keeps SWR below 1.8:1 from 432 to 450 MHz — acceptable for both weak-signal CW/SSB and FM repeater operation.
| Section | Length | Notes |
|---|---|---|
| Radiator (long element) | 40.0 inches (1016 mm) | Starting length — trim for 2m tuning; serves as 3λ/2 on 70cm |
| Lower stub section | 6.4 inches (163 mm) | Quarter-wave at 70cm; tune 70cm center frequency by adjusting length |
| Non-conductive break between stubs | 0.5–1.0 inch (13–25 mm) | PVC or fiberglass rod — mechanically joins, electrically isolates stubs |
| Upper stub section | 12.6 inches (320 mm) | Together with lower stub, total = 19.5 inches (quarter-wave at 2m) |
| Total stub length (lower + break + upper) | ~19.5–20.0 inches (495–508 mm) | Electrically 19.5 inches; physically slightly longer due to break |
| Gap between radiator and stub tops | 1.0 inch (25 mm) | Same as single-band J-pole |
| 2m feedpoint tap (on lower stub) | 1.5–2.0 inches above bottom short bar | Adjust for minimum SWR on 2m |
| 70cm feedpoint tap (on lower stub) | Same SO-239 as 2m — shared feedpoint | Both bands share one SO-239 at the 2m tap position |
| Total antenna height | ~62 inches (1575 mm) | Radiator + gap + stub (including break) + bottom short bar |
Vhf Jpole Dual Calculator
Materials for a dual-band 2m/70cm J-pole from 1/2-inch copper pipe
Building the Dual-Band J-Pole
The dual-band J-pole build follows the same sequence as the single-band version with the addition of the stub split. Read through the complete build sequence before starting — the non-conductive break between stub sections is the most critical and least forgiving part of the build. Plan where it goes before cutting any pipe.
Plan the Stub Assembly Before Cutting
The stub section of the dual-band J-pole consists of three parts: the lower stub (copper pipe, 6.4 inches), the non-conductive break (PVC or Delrin rod, 0.75 inches), and the upper stub (copper pipe, 12.6 inches). These three parts must be mechanically rigid and aligned while being electrically disconnected at the break:
Cut All Pipe Sections
Cut three copper pipe sections and the non-conductive break piece:
Solder the Bottom Assembly
The bottom of the dual-band J-pole is identical to the single-band version — the 90° elbow connects the bottom of the lower stub to the bottom of the radiator, forming the J shape. Solder this joint as described in the single-band J-pole guide: clean, flux, heat with the torch, apply lead-free plumbing solder.
After the bottom elbow cools, solder end caps on: the top of the radiator (long element), and the bottom of the lower stub (if the elbow has a third port — use a tee fitting instead of an elbow if you want the bottom to be more substantial). Solder the upper stub's top end cap as well.
Assemble the Non-Conductive Stub Break
With the lower stub soldered to the bottom elbow and both end caps in place, assemble the non-conductive break between the lower stub top and the upper stub bottom:
- Mix a small amount of two-part epoxy (JB Weld or equivalent).
- Apply epoxy to one end of the PVC break rod and insert it into the drilled socket in the lower stub top end. Push firmly to seat fully. Wipe excess epoxy from the stub surface.
- Apply epoxy to the exposed end of the break rod (the end protruding from the lower stub).
- Slide the upper stub over the protruding rod end, pushing until the two copper sections are 0.75 inches apart (the rod is fully engaged in both sockets).
- Align the upper stub so it is parallel to the lower stub. Hold or prop in position while the epoxy cures — typically 5–10 minutes for working time, 24 hours for full cure.
- After cure, verify with an ohmmeter that lower and upper stub sections are electrically isolated (open circuit between them).
Install the Feedpoint SO-239
The dual-band J-pole uses a single SO-239 mounted on the lower stub section, exactly as in the single-band version. The single feedpoint serves both bands simultaneously because at both frequencies the impedance at this tap point on the lower stub is close to 50 Ω:
Initial NanoVNA Sweep — Both Bands
Hold the antenna vertically away from metal surfaces and connect the NanoVNA. Sweep 130–170 MHz for 2m and then 400–480 MHz for 70cm. Both bands should show SWR minima:
Tune 70cm First — Adjust Lower Stub Length
Tune 70cm before 2m — the lower stub length affects 70cm resonance directly and 2m resonance only slightly. Adjusting the lower stub first allows 2m fine-tuning as the last step:
Tune 2m — Adjust Radiator Length and Tap Position
With 70cm confirmed, tune 2m. The 2m tuning follows the same procedure as the single-band J-pole:
Weatherproof and Mount
Apply self-amalgamating tape over the PL-259/SO-239 coax connection. Seal the SO-239 mounting hole in the stub with RTV sealant applied around the connector body. The epoxy joint at the stub break is mechanically secure but should also be sealed against moisture — a wrap of self-amalgamating tape over the break region provides additional protection.
Mount the antenna vertically at the highest practical outdoor location. The dual-band J-pole's single feedpoint and single coax run is one of its most practical advantages — one coax to the shack, one connector at the radio, and both bands are covered. Add 5 ferrite beads (type 31) to the coax immediately below the feedpoint to prevent common-mode current from distorting the pattern on either band.
vs Two Separate Single-Band J-Poles
The most common alternative to a dual-band J-pole is running two separate single-band J-poles — one for 2m and one for 70cm — each with its own coax to the shack where a duplexer combines them to the single antenna port on a dual-band radio:
- Performance: two separate J-poles with a duplexer typically outperform the dual-band J-pole by about 0.5–1 dB on each band (no stub-break losses, optimized matching on each band). The difference is small and rarely operationally significant.
- Cost: two J-poles + a duplexer costs approximately $50–80 vs $24 for the dual-band J-pole. The duplexer alone is $30–50.
- Installation: two separate coax runs vs one; two mast mount points vs one. For rooftop installations, one coax through the wall is a significant practical advantage.
- Conclusion: for most operators the dual-band J-pole's simplicity and lower cost outweigh its small performance deficit. The two-separate-antenna approach is worthwhile only for a serious weak-signal or contest station where the last 0.5 dB matters.
vs Commercial Dual-Band Vertical
Commercial dual-band vertical antennas (Diamond X50, Comet GP-3, etc.) offer a direct comparison to the homebrew dual-band J-pole:
- Performance: commercial antennas specify 3–5 dBd gain on 2m and 5–7 dBd on 70cm. These specifications use collinear element designs that genuinely outperform a J-pole by 1–3 dBd. The higher gain is real but requires a larger physical antenna.
- Cost: commercial dual-band verticals cost $60–200, vs $24 for a homebrew J-pole. The cost difference is significant for a new operator.
- Size: commercial collinears are typically 4–6 feet tall and use aluminum tube construction. The J-pole at 5 feet is comparable in size.
- Conclusion: a homebrew dual-band J-pole is an excellent starting antenna for a new operator who wants to learn construction while producing a genuinely functional result. Once on the air and comfortable with VHF operation, upgrading to a commercial collinear is a clear next step if more gain is desired.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| 2m SWR correct but no 70cm SWR minimum visible | Non-conductive break is conducting — upper stub is in circuit at 70cm | Measure resistance between lower and upper stub copper sections — should be infinite | Disassemble break; verify no copper filings or solder bridging the gap; rebuild with fresh epoxy |
| 70cm minimum visible but very high SWR (5:1+) at minimum | Lower stub length significantly wrong or feedpoint connection fault | Measure lower stub from bottom elbow to break — should be 6.4–6.5 inches | If too long: trim lower stub; if too short: recut and re-solder; check SO-239 center pin contact |
| Both minima present but SWR on 2m affected by 70cm operation | Intermodulation from duplexer (if using one) or coax routing issue | Test antenna without duplexer — connect directly to a single-band radio on each band separately | This antenna does not need a duplexer; connect directly to dual-band radio single antenna port |
| SWR varies when coax is moved on either band | Common-mode current — coax radiating on either or both bands | Move coax while watching SWR — variation confirms common-mode current | Add ferrite beads (type 31) or wound coax choke to feedline immediately below SO-239 |
| 70cm SWR minimum significantly above 450 MHz | Lower stub was cut too short | Measure lower stub — if under 6.2 inches, it is too short for the 440 MHz band | Cannot easily extend copper pipe — cut new lower stub to correct length and resolder elbow assembly |
| 2m works well but 70cm performance is poor — range very short | Coax loss on 70cm — wrong coax type used | Check coax type — RG-58 loses 6 dB/50 ft at 440 MHz; this kills 70cm performance | Replace feedline with RG-8X minimum; LMR-400 preferred for runs over 25 ft at 70cm |
Do I need a duplexer to use this antenna with a dual-band radio?
No — this is one of the key advantages of the dual-band J-pole. The antenna presents acceptable SWR on both 2m and 70cm simultaneously from a single feedpoint. A dual-band radio's single antenna port connects directly to the SO-239 on the J-pole via a single coax run. No duplexer, no switching, no tuner. The antenna handles both bands simultaneously from the single connection. Duplexers are needed only when you have separate single-band antennas that must be combined to feed a single-port radio — with this dual-band J-pole, that combination is built into the antenna itself.
Can the antenna transmit on 2m and receive on 70cm at the same time?
Not from the same radio port simultaneously without a duplexer — even though the antenna covers both bands, the radio's transmitter and receiver share the single coax connection. The antenna's dual-band coverage means you can use it for 2m and 70cm at different times without switching antennas, but simultaneous transmit/receive on different bands requires a duplexer between the antenna feedpoint and the radio. For the casual operator using a dual-band radio, this limitation is essentially never a practical issue — you operate on one band at a time and just switch the radio's band selector when needed.
What if I want to use this antenna for APRS on 144.390 MHz and voice on 146.520 MHz simultaneously?
The dual-band J-pole's 2m coverage is broad enough to cover APRS (144.390 MHz), the national simplex calling frequency (146.520 MHz), and all FM repeater inputs and outputs (146–148 MHz) simultaneously — the SWR is below 1.5:1 across this entire range from a well-tuned antenna. Running APRS and voice simultaneously from the same radio port and antenna is a radio limitation, not an antenna limitation — most dual-band radios can operate cross-band (APRS on 2m while monitoring 70cm) simultaneously, which this antenna handles natively.
How do I know which end of the antenna is the top?
The long element (radiator) is always vertical with the closed cap pointing straight up. The short stub assembly is below and parallel to the lower portion of the radiator, connected at the bottom by the 90° elbow. The SO-239 feedpoint is on the lower stub section near the bottom elbow. The complete antenna looks like the letter J — the long vertical portion (radiator) with the J-hook (elbow and stub) at the bottom. The coax exits from the feedpoint near the bottom of the antenna and runs down the mast or building wall. If the antenna is mounted upside-down by mistake, the radiation pattern will be identical — only the mounting mechanics change.
Can I build a ladder-line version of the dual-band J-pole?
Yes — the dual-band J-pole has been successfully built from 450 Ω ladder line by inserting a small non-conductive break (a piece of plastic cut from a credit card, for example) in the correct position on the stub conductor. The procedure is the same as the copper pipe version — cut the stub conductor at the 6.4-inch mark from the bottom, insert a 0.75-inch non-conductive spacer, and reconnect the mechanical structure with electrical tape or a small plastic clip. The feedpoint connection uses alligator clips or small binding posts at the appropriate tap position. The ladder line dual-band J-pole is lighter and cheaper than the copper pipe version but less mechanically durable — ideal for portable or emergency kit use.
How high should I mount the dual-band J-pole for best results?
Higher is always better, with the greatest gains coming from the first 20–30 feet above local obstructions (rooftops, trees, buildings). A J-pole at 25 feet above the roof of a two-story house is effectively at 45+ feet above grade — adequate for reliable regional repeater access and occasional simplex contacts. For APRS, the higher the antenna the larger the coverage footprint — a J-pole at 50+ feet fills in the iGate coverage area significantly. For serious simplex weak-signal work, 30+ feet above the local clutter gives access to the horizon and enables contacts that are impossible from inside a building. Even 10–15 feet above a second-story roofline produces a dramatic improvement over the rubber duck on a handheld inside the house.