Build a 70cm J-Pole Antenna
The 70cm J-pole is the copper pipe J-pole concept scaled to 432–450 MHz — the same proven design that produces reliable 2m FM performance, miniaturized to antenna dimensions that fit in a coat pocket and build from a single foot of 1/4-inch copper pipe. At 70cm wavelength everything shrinks to one-third of the 2m version: the radiating element drops from 40 inches to 13.3 inches, the matching stub from 19.5 inches to 6.5 inches, and the entire antenna fits in a section of pipe shorter than a ruler. What does not shrink is the importance of precise construction — at 430 MHz a 3 mm position error in the feedpoint tap shifts the SWR measurably, and a 5 mm error in element length moves the resonance off the top of the 70cm band. This guide builds the 70cm J-pole for the 70cm FM repeater segment (430–450 MHz) with the same soldering and tuning approach used for the 2m version.
Dimension Scaling from 2m to 70cm
Every electrical dimension of the J-pole scales with frequency. The ratio of 2m to 70cm center frequencies (146 MHz to 440 MHz) gives the scaling factor:
Construction Precision at 70cm
The 70cm J-pole is physically tiny — a 3 mm error that was inconsequential on the 2m version becomes a meaningful fraction of the antenna at 70cm:
Pipe Size Selection for 70cm
The 2m J-pole used 1/2-inch copper pipe. At 70cm the antenna is one-third the size, and 1/2-inch pipe becomes proportionally too large — the pipe diameter starts to become a significant fraction of the wavelength:
Connector Choice at 70cm
At 70cm, the feedpoint connector quality becomes operationally significant in a way it is not at 2m:
- N-type (recommended): the gold standard for 70cm. Low loss, weatherproof, rated to 18 GHz. Adds approximately $4–6 to the build cost but eliminates connector-related SWR and loss issues entirely. Use N-type if the antenna will be at height on a permanent installation.
- SMA: very compact and low loss at 70cm. Excellent for a portable or handheld-connected antenna. The small size makes direct connection to the pipe difficult — use a short SMA pigtail from the feedpoint tap to the radio coax connector.
- BNC: adequate for receive and low-power operation. Rated to 4 GHz, above 70cm, so electrically suitable. Less weatherproof than N-type — seal carefully for outdoor use. Common in portable and test applications.
- SO-239 / PL-259 (UHF type): despite the name "UHF connector," the SO-239/PL-259 is mechanically designed for HF and performs poorly above 300 MHz — high insertion loss, poor impedance consistency, and large physical size relative to the 70cm antenna. Avoid for any serious 70cm installation. If already on hand, acceptable for temporary use only.
| Section | Length (inches) | Length (mm) | Notes |
|---|---|---|---|
| Half-wave radiator | 13.5 in | 343 mm | Starting length — trim for tuning; add 0.5 in margin → cut 14.0 in |
| Quarter-wave matching stub | 6.5 in | 165 mm | Fixed — do not trim; cut to exact 6.5 inches |
| Gap between element tops | 0.38 in (3/8 in) | 9.5 mm | Critical — maintain with spacer during soldering; ±0.1 inch tolerance |
| Bottom elbow (short circuit) | Standard 1/4-inch 90° elbow | ~12 mm OD | Same soldering as 2m version; joins radiator base to stub base |
| Feedpoint tap — starting position | 0.56 in above elbow center | 14 mm | Adjust in 1/8-inch steps for minimum SWR; very sensitive at 70cm |
| Total height (radiator + gap + stub) | ~20.4 in | ~518 mm | Compact — fits in a 24-inch section of PVC conduit housing |
| Feedpoint connector | N-type preferred; BNC acceptable | — | SO-239/PL-259 not recommended at 70cm — excessive loss above 300 MHz |
Uhf Jpole 70cm Calculator
Materials for a 70cm J-pole from 1/4-inch copper pipe
The Gap Spacer — Critical at 70cm
At 2m the 1-inch gap between element tops is easy to set by eye during assembly and the 0.25-inch tolerance leaves room for error. At 70cm the 3/8-inch gap with a 0.1-inch tolerance must be physically enforced during soldering:
Feedpoint Connector Installation at 70cm
Installing the N-type connector on 1/4-inch copper pipe requires a different approach than the SO-239 on the 2m version — the N-type's center pin connection to a thin pipe wall is more involved:
Building the 70cm J-Pole
The 70cm J-pole follows the same build sequence as the 2m version with tighter measurements and a smaller gap spacer. Have the calipers and steel rule at hand throughout. Cut a gap spacer from plastic before starting — you will need it during the soldering step.
Cut Pipe Sections
From the 1/4-inch copper pipe, cut two sections using a mini pipe cutter for clean, square ends:
Solder Bottom Elbow with Gap Spacers in Place
The bottom elbow soldering is the most critical mechanical step. The gap spacers must be inserted between the element tops before the elbow is soldered, to hold the separation while the solder sets:
Solder End Caps
Solder 1/4-inch end caps on both element tops — the top of the radiator and the top of the stub. Clean, flux, and solder as usual. The end caps seal the element tops against water ingress and finish the antenna mechanically.
After the end caps cool, verify the gap between the two element tops one final time with calipers. If the gap has shifted during end cap soldering (heat can soften the previous solder joints), gently press the tops to restore the 3/8-inch spacing while the solder is still slightly soft — immediately after torch removal, in the first 10 seconds of cooling.
Install the Feedpoint Connector
Install the N-type (or BNC) feedpoint connector on the stub at 0.56 inch (14 mm) above the center of the bottom elbow. This is the starting tap position — fine adjustment happens during tuning:
Initial NanoVNA Measurement
Hold the antenna vertically at arm's length, well away from metal surfaces. Connect the NanoVNA directly at the feedpoint connector (not at the shack end of a long coax). Sweep 400–480 MHz:
Tune Feedpoint Tap and Trim Radiator
The same two-variable tuning as the 2m J-pole applies, but with finer steps at 70cm:
Weatherproof and Install in PVC Housing
The complete 70cm J-pole is only 20 inches tall — it fits neatly inside a 24-inch section of 1-inch OD PVC conduit, which provides weatherproofing and a rigid mounting structure identical in concept to the Flower Pot PVC housing:
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| SWR minimum 20+ MHz below target | Radiator much too long, or gap too small (elements electrically coupled) | Measure radiator length (target 13.5 in); measure gap (target 3/8 in) | Trim radiator 3 mm at a time; if gap is under 0.30 in, carefully open gap with small file or blade |
| Correct resonant frequency but minimum SWR above 3:1 | Feedpoint tap position significantly wrong | Move tap in 3 mm steps up and down; note SWR at each position to find minimum | Iterate tap position in 3 mm increments; optimal position for 440 MHz is typically 12–18 mm above elbow center |
| SWR varies dramatically when hand or body is near antenna | No ferrite choke — common-mode current on feedline | Move body away from antenna during measurement — if SWR changes, common-mode is present | Install 5 type-31 ferrite beads on feedline immediately at feedpoint; keep feedline perpendicular to antenna for first 6 inches |
| Good SWR but very short range on 70cm FM repeaters | Coax loss — wrong coax type or damaged connector | Check coax type — RG-58 loses 6 dB/25 ft at 440 MHz; inspect all connectors | Replace feedline with LMR-400 minimum; inspect and replace any N-type or BNC with signs of corrosion |
| SWR minimum shifts 5–10 MHz after mounting in PVC housing | Normal — PVC dielectric slightly detuning the antenna | Compare SWR bare vs in housing — shift confirms PVC dielectric effect | Trim 1–2 mm from radiator to compensate for upward frequency shift induced by PVC enclosure |
| Element gap bridged by solder — SWR high everywhere | Excess solder flowed into gap during elbow joint soldering | Check for continuity across the gap — should be open circuit | Use a thin jeweler's saw or rotary tool to cut the solder bridge; verify gap is open; re-test SWR |
Can I tune one J-pole to cover both 432 MHz weak-signal and 440 MHz FM?
Not with acceptable SWR on both segments simultaneously. The 70cm J-pole has a 2:1 SWR bandwidth of approximately 15–18 MHz. The distance between 432.1 MHz (weak-signal SSB) and 446 MHz (FM simplex) is 14 MHz — right at the edge of the antenna's bandwidth, meaning SWR will be above 2:1 at whichever end you tune away from. The practical solution is to build two separate 70cm J-poles — one tuned to 432 MHz for weak-signal use and one tuned to 440 MHz for FM — or to use the 10-element 70cm Yagi from the Yagi guide for weak-signal work and the J-pole for FM. The Yagi covers a smaller frequency range than the J-pole and its bandwidth at 70cm similarly does not span both segments.
Why does the 70cm J-pole need N-type instead of the SO-239 used on the 2m version?
The SO-239/PL-259 connector family — marketed with the confusing name "UHF connector" — was designed in the 1930s for HF and VHF use. Its characteristic impedance is not controlled and varies significantly at frequencies above 200–300 MHz. At 440 MHz, a PL-259/SO-239 junction introduces 0.5–1.5 dB of additional loss and an unpredictable SWR contribution from the connector geometry itself. For a 2m J-pole at 146 MHz the effect is minimal. For a 70cm J-pole at 440 MHz the connector is a significant source of degradation. N-type connectors are precision 50 Ω devices rated to 18 GHz — they add zero measurable loss at 440 MHz. BNC connectors are a reasonable compromise: they are precision 50 Ω to 4 GHz and adequate for 70cm use, though less weatherproof than N-type.
How does the 70cm J-pole compare to the 10-element Yagi for repeater use?
The comparison depends on operating style. The 70cm J-pole is omnidirectional with approximately 2 dBd gain — ideal for FM repeater use where you need to access repeaters in multiple directions without rotating the antenna. The 10-element Yagi provides 14–16 dBd of directional gain — excellent for reaching a specific repeater at maximum range or for satellite work, but requiring pointing toward each target. For a home station with one or two nearby repeaters, the J-pole is more convenient. For a portable station at a site trying to reach the most distant repeater possible, the Yagi wins decisively. Many operators have both: the J-pole as the everyday omnidirectional antenna and the Yagi available for special operations.
Can I mount the 70cm J-pole alongside the 2m J-pole on the same mast?
Yes — with adequate vertical separation. The 70cm antenna should be at least 12 inches (approximately λ/2 at 70cm) above or below the 2m antenna to minimize electrical coupling between them. In practice, a 12–18 inch separation produces negligible interaction, and both antennas can share the same mast with individual feedlines to a dual-band radio or a duplexer. Physical clearance is the primary concern: the 2m J-pole is 60 inches tall, so stacking it above the 70cm J-pole (20 inches tall) on the same mast with a 12-inch spacing bar produces a total antenna assembly of approximately 95 inches — less than 8 feet, manageable on a standard push-up mast. The resulting two-antenna station covers 2m and 70cm independently with optimal performance on each band.
Is the 70cm J-pole suitable for APRS on 144.390 MHz?
No — the 70cm J-pole operates at 430–450 MHz and has no usable SWR at 144 MHz. APRS in North America uses 144.390 MHz, which is a 2m frequency. For APRS you need a 2m antenna — the 2m J-pole, Slim Jim, Flower Pot, or any other 2m vertical from this guide series. The 70cm band does have digital modes including APRS-like digipeating in some regions using protocols like LoRa and AFSK at 433 MHz, but this is a different system from the standard APRS network. If you need both 2m APRS and 70cm FM simultaneously from a single feedline, the dual-band J-pole guide covers that specific combination.
Can I use 3/8-inch pipe instead of 1/4-inch pipe?
Yes — 3/8-inch OD copper pipe works for a 70cm J-pole. The larger diameter means a slightly lower velocity factor on the pipe, which shifts the resonant frequency slightly lower than the calculated dimensions. Shorten all element lengths by approximately 2–3% from the table values to compensate — start the radiator at 13.0 inches instead of 13.5 inches, and the stub at 6.3 inches instead of 6.5 inches. The NanoVNA tuning step will correct any remaining offset. The gap between element tops should remain at 3/8 inch. The 3/8-inch pipe is more readily available at hardware stores than 1/4-inch, and produces a slightly more robust mechanical assembly — a worthwhile trade if you cannot source 1/4-inch pipe locally.