Build a 2m/70cm Copper J-Pole Antenna
This project builds a conventional 1/2-inch copper-pipe J-pole centered near 146 MHz for the 2-meter band, then verifies its usable third-harmonic response on 70 centimeters. It uses one continuous long element, one continuous quarter-wave matching stub, two 90° copper elbows, a short copper crosspiece, an adjustable feedpoint, and one coax feedline. There is no split stub and no insulating break. The antenna is optimized for 2m; 70cm operation is secondary harmonic capability and should be verified with a NanoVNA before regular use.
Conventional J-Pole Geometry
A J-pole is an end-fed half-wave radiator matched to 50-ohm coax with a quarter-wave shorted parallel transmission-line section. One side is continuous from the bottom shorting section through the matching section and into the half-wave radiator. The shorter parallel leg forms the matching stub.
Why 70cm Can Also Show a Match
The 2m radiator is close to three half-wavelengths at 440–445 MHz, so a conventional 2m J-pole can also present a usable impedance on 70cm. However, the current distribution at the third harmonic produces additional elevation lobes, so low SWR does not automatically mean ideal UHF radiation.
- 2mPrimary band. Tune the antenna for minimum SWR near 146 MHz.
- 70cmSecondary harmonic operation. Measure the actual SWR across the frequencies you intend to use.
- Do not force a 70cm matchIf 70cm SWR is poor, do not radically retune the 2m geometry. Use a purpose-built dual-band or 70cm antenna instead.
Feedpoint Matching
The shorted bottom of the matching section is very low impedance and the open top is high impedance. Somewhere between them is the point that matches 50-ohm coax. Start with both feed connections approximately 1⅞ inches above the bottom short and move them together in small increments while watching SWR.
Mounting Matters
Keep the antenna vertical and clear of nearby conductive objects. A metal mast or clamp placed close to the matching section can detune the antenna and distort the pattern. Use PVC, fiberglass, wood, or a non-conductive standoff between the antenna and a metal support whenever practical.
| Part | Starting dimension | Purpose / notes |
|---|---|---|
| Long element | 57.75 in | Continuous long side; start slightly long if you want extra trimming margin. |
| Matching stub | 19.25 in | Continuous short side of the parallel matching section. |
| Element spacing | 1.75 in | Clear spacing between the two vertical copper sections. |
| Feedpoint | 1.875 in above bottom short | Starting position only. Adjust both feed connections together for lowest 2m SWR. |
| Bottom crosspiece | Cut to produce 1.75-in finished spacing | Exact cut depends on fitting insertion depth. Dry-fit and measure before soldering. |
| 70cm | Verify 440–450 MHz | Third-harmonic operation; actual SWR and pattern depend on the finished antenna and installation. |
2m / 70cm Copper J-Pole Calculator
Materials for the 2m-primary / 70cm-harmonic copper J-pole
Building the Copper 2m/70cm Harmonic J-Pole
Build the conventional J-pole first, tune it correctly on 2 meters, then measure what the finished antenna actually does on 70 centimeters.
Lay Out the Final Geometry Before Cutting
The finished antenna has two parallel vertical copper sections joined at the bottom by two 90° elbows and a short horizontal crosspiece. The long side is continuous; the short side is the matching stub. There is no insulated break anywhere in the matching stub.
Cut and Prepare the Copper
Cut the long element and matching stub to the starting dimensions. Cut the bottom crosspiece only after dry-fitting the elbows so the finished vertical spacing is approximately 1¾ inches. Deburr every cut and clean the last inch of every solder joint.
Solder the Bottom U
Flux and solder one 90° elbow to the base of each vertical pipe and join the elbows with the short copper crosspiece. Keep the two vertical sections parallel while the assembly cools. Solder the two top end caps after confirming the pipe lengths.
Install an Adjustable Feedpoint
Start approximately 1⅞ inches above the bottom short. Connect the coax center conductor to the long-element side and the shield to the matching-stub side. Use stainless hose clamps, movable straps, or another mechanically secure arrangement that lets both connection points move up and down together during tuning.
Add the Feedline Choke
Place Type 31 ferrite beads or an appropriate common-mode choke immediately below the feedpoint. Route the coax away from the antenna rather than alongside the matching section.
Tune 2m First
Mount the antenna vertically in its intended test configuration, clear of metal objects, and sweep approximately 130–170 MHz. First move both feedpoint connections together to minimize SWR. Once the impedance match is good, make small corrections to the long element only if the resonance needs to move toward 146 MHz.
Verify 70cm — Do Not Assume It
Without changing the tuned 2m antenna, sweep approximately 400–480 MHz and inspect the 430–450 MHz region. Record the SWR at your repeater and simplex frequencies. Some builds will show a very usable 70cm match; others may not.
Final 2m Recheck
After every feedline, choke, or mounting change, re-sweep 2m. If you experimented with small feedpoint changes to improve 70cm SWR, confirm that you did not degrade the primary 2m match. Prioritize 2m because that is the band the physical J-pole is designed around.
Weatherproof the Feedpoint
Once tuning is complete, seal the PL-259/SO-239 junction and exposed feed hardware with self-amalgamating tape and RTV where appropriate. Do not permanently seal movable feed hardware until the antenna has passed both sweeps.
Mount on a Non-Conductive Stand-Off
Mount the antenna vertically with the matching section kept clear of metal. If the main support is a metal mast, use a PVC or fiberglass extension or stand-off so mast hardware does not sit directly beside the matching stub.
| Symptom | Likely cause | What to do |
|---|---|---|
| No useful 2m SWR minimum | Wrong lengths, feedpoint wired incorrectly, poor solder joint, or nearby metal | Verify dimensions and continuity; center conductor goes to the long side, shield to the short side; move the antenna into the clear. |
| 2m resonance correct but SWR is high | Feedpoint impedance mismatch | Move both feed connections together up or down the matching section in small increments. |
| SWR changes when coax is moved | Common-mode current | Add or improve the feedline choke immediately below the feedpoint. |
| 2m works, 70cm SWR is poor | Normal variation in third-harmonic behavior | Do not destroy the 2m tune trying to force a UHF match. Use a purpose-built dual-band or 70cm antenna if UHF performance matters. |
| 70cm SWR is good but range is disappointing | Third-harmonic radiation pattern and/or coax loss | Check feedline loss and antenna height; remember that low SWR does not guarantee a low-angle UHF pattern. |
| SWR changes after mounting | Metal mast or clamps coupling to the matching section | Increase non-conductive stand-off distance and re-test. |
Is this a true independently tuned dual-band J-pole?
No. This is a conventional 2m copper J-pole that may also provide a usable match on 70cm through third-harmonic operation. The previous split-stub design has been removed from this guide.
Why not keep the insulated split stub?
Because electrically isolating the upper and lower pieces means they cannot simply be treated as one continuous quarter-wave 2m matching stub. The earlier explanation was internally inconsistent and has been retired rather than patched.
Do I need a duplexer?
No duplexer is required merely to use one antenna alternately on 2m and 70cm from a normal dual-band radio with one antenna port. A duplexer or diplexer is only required for specific multi-radio or simultaneous signal-routing applications.
Why can 70cm SWR be low while performance is weaker?
SWR describes impedance match, not radiation pattern. At the third harmonic the radiator has multiple current maxima and minima, producing additional elevation lobes and less energy at the low angles normally desired for terrestrial FM work.
What should I use if 70cm performance is important?
Use a purpose-built 70cm antenna, a commercial dual-band collinear, or a proven dual-band design engineered to correct the third-harmonic pattern. Keep this copper J-pole primarily as a rugged 2m antenna with secondary 70cm capability.
Can I clamp this directly to a metal mast?
It may work, but conductive hardware near the matching section can change the impedance and pattern. A non-conductive mast extension or stand-off is the safer approach and makes tuning more repeatable.