Build a 2m/70cm Collinear Antenna
The collinear antenna is the standard high-gain base station antenna for VHF and UHF — a vertically stacked array of half-wave or 5/8-wave elements fed in phase that compresses the radiation pattern toward the horizon, trading high-angle coverage for increased gain where it matters most for terrestrial communication. A well-built 5/8-wave over 5/8-wave collinear for 2m delivers 5–6 dBd gain from a simple coaxial or pipe structure. Adding dual-band capability for 70cm from the same antenna requires a coaxial collinear design where the phasing sections are electrically transparent on both bands. This guide covers collinear theory, the coaxial collinear construction method using phasing sleeves, element and phasing coil dimensions for both 2m and 70cm, housing, mounting, and SWR verification for a permanent base station installation.
How In-Phase Stacking Produces Gain
A collinear antenna achieves gain by stacking multiple vertically oriented radiating elements end-to-end, all fed with currents that are in phase. When multiple elements radiate in phase, their fields add constructively in the horizontal direction and cancel in the vertical — compressing the radiation pattern toward the horizon and increasing gain where terrestrial communication occurs:
Phasing Methods — Coil, Sleeve, and Coaxial
The phasing section between each collinear element ensures the current arrives at the next element in the correct phase. Three construction methods are commonly used for amateur collinear antennas, each with different practical implications:
Dual-Band Collinear Design — 2m and 70cm Together
Achieving simultaneous dual-band operation on 2m and 70cm from a single feedpoint requires a collinear design where the phasing sections are electrically transparent on both frequencies. The coaxial collinear approach achieves this naturally:
The 5/8-Wave over 5/8-Wave Collinear
The most popular homebrew 2m base station collinear uses two or three 5/8-wave elements separated by phasing coils. The 5/8-wave element is used instead of a half-wave because it presents a higher impedance that, when combined with the phasing coil, is easier to match to 50 Ω coax:
| Parameter | 2m (144 MHz) | 70cm (435 MHz) | Notes |
|---|---|---|---|
| Quarter-wave (λ/4) | 20.3 inches (51.7 cm) | 6.7 inches (17.1 cm) | Radial and ground plane length reference |
| Half-wave (λ/2) | 40.6 inches (103 cm) | 13.5 inches (34.3 cm) | Phasing section reference length in air |
| 5/8-wave element | 51.2 inches (130 cm) | 16.9 inches (43.0 cm) | Standard collinear element length |
| λ/2 phasing coax (VF=0.66) | 26.8 inches (68.0 cm) | 8.9 inches (22.7 cm) | RG-58 phasing section physical length |
| λ/2 phasing coax (VF=0.82) | 33.3 inches (84.5 cm) | 11.1 inches (28.2 cm) | Foam coax phasing section length |
| Phasing coil (2m, #14 AWG) | 6 turns, 1/2-inch form, 1-inch long | N/A (use coaxial method for dual-band) | Approximate — adjust for resonance |
| Total 2-element collinear height | ~9 ft (2.7 m) | ~3 ft (0.9 m) | Including phasing section between elements |
| Total 3-element collinear height | ~14 ft (4.3 m) | ~4.5 ft (1.4 m) | Practical for most installations on a mast |
Materials for a dual-band coaxial collinear covering 2m and 70cm — 3-element coaxial design
Building the Dual-Band Coaxial Collinear
This guide builds a 3-element dual-band coaxial collinear for 2m and 70cm using RG-58 coax sections inside a PVC housing. The coaxial collinear method is the most precise and produces the best dual-band performance. Measure every coax section twice before cutting — phasing section length errors directly degrade gain.
Understand the Coaxial Collinear Structure
Before cutting any coax, understand the complete assembly. A coaxial collinear is a series of coax sections where alternate sections have their inner conductor and outer shield connections reversed. This reversal, combined with the precise half-wave phasing length, causes each element to radiate in the same phase as its neighbours:
Cut and Prepare the Coax Sections
Cut the RG-58 coax sections to the following lengths. Precision is critical — cut to within 2mm of the target length. Use a vernier caliper to verify each cut length before soldering:
Build the Phasing Sleeves
The phasing sleeves are the most distinctive part of the coaxial collinear. Each sleeve is a λ/4 section of coax where the outer shield is folded back over the outer jacket of the adjacent radiating section, creating a shorted stub that acts as a choke. This sleeve maintains the phase relationship between sections:
Assemble and Solder the Collinear Sections
Working from the bottom feedpoint upward, connect the three radiating sections and two phasing sections. At each junction, make clean, mechanically secure solder joints. The RF current at 144 MHz and 435 MHz flows on the outer surface of conductors — any high-resistance joint creates a local hot spot and degrades gain:
Test Before Housing — Bench SWR Check
Before inserting the collinear into the PVC housing, connect the NanoVNA to the feedpoint and verify SWR on both 2m and 70cm. This is the most important step — once the antenna is inside the PVC housing it is difficult to access for repairs:
If the bench SWR looks reasonable on both bands, proceed to housing. If either band shows SWR consistently above 3:1, identify and fix the issue before housing — once inside the PVC pipe the antenna is sealed and repairs are much harder.
Install in PVC Housing and Mount on Mast
Insert the completed coaxial collinear into the 1-inch PVC pipe housing. The PVC protects the antenna from weather and provides a rigid mounting structure. The coax radiating sections pass through the PVC; the stripped inner conductor tips protrude at the junctions. Thread the coaxial collinear assembly into the PVC from the bottom, feeding the assembly upward section by section:
Final SWR Verification and Performance Check
With the antenna installed on the mast, perform a final NanoVNA sweep on both 2m and 70cm from the shack end of the coax. The installed SWR will differ slightly from the bench measurement due to the vertical installation and height above ground. Both bands should show SWR well below 2:1 across the amateur allocations:
The Phasing Coil Collinear — Simpler Single-Band Build
For operators who want a 2m-only collinear and prefer a simpler construction method, the phasing coil approach using aluminium or copper tubing elements is more straightforward than the coaxial method:
- Element material: 3/8-inch or 1/2-inch aluminium rod or tubing for the radiating elements. Each element is a 5/8-wave length (51.2 inches on 2m). Two or three elements provide 5–6 dBd or 7–8 dBd gain respectively.
- Phasing coils: small inductors wound on 1/2-inch PVC pipe between elements. Each coil provides the phase delay to maintain in-phase radiation. For 2m, approximately 5–7 turns of #14 AWG wire at 1-inch winding length provides the correct phasing — verify with NanoVNA and trim turns if SWR is off target.
- Matching at the base: a small series inductance at the feedpoint transforms the 5/8-wave element's high impedance to 50 Ω. This can be built as a small coil in series with the feedpoint, or as a gamma match using a small capacitor and parallel conductor.
- Housing: enclose the complete assembly in 1.5-inch or 2-inch PVC pipe, with end caps sealed. The phasing coils fit inside the PVC; the aluminium elements are centred in the pipe with insulating spacers.
- Dual-band limitation: this design works on 2m only — the phasing coil dimensions are specific to 2m and do not provide correct phasing on 70cm. For dual-band, use the coaxial collinear design described in the main guide.
Commercial Collinear vs Homebrew — When to Buy
Several commercial manufacturers produce excellent dual-band collinear antennas that are worth considering alongside a homebrew build:
- Diamond X50A / X200A: the most widely used commercial dual-band collinears for 2m/70cm. Weatherproof fibreglass housing, N-type connector, well-specified gain. X50A is 4.5 ft (7 dBi on 2m, 10 dBi on 70cm); X200A is 8.5 ft (9 dBi on 2m, 12 dBi on 70cm). Both are widely available and relatively inexpensive ($60–90).
- Comet GP-3 / GP-6: similar specifications to the Diamond series; popular alternatives with slightly different gain claims. Read actual user reviews rather than manufacturer gain specs — many collinear gain claims are optimistic.
- Homebrew advantage: a homebrew coaxial collinear costs under $35 in materials and teaches the operator exactly how the antenna works. Commercial units cost $60–90 and are ready to mount. For operators interested primarily in getting on the air, a commercial unit is pragmatic. For those interested in antenna construction, the homebrew coaxial collinear is a rewarding and educational project.
- Performance comparison: a well-built homebrew coaxial collinear performs within 0.5–1 dB of a commercial unit of equivalent element count. The commercial unit's advantage is manufacturing consistency and weatherproofing quality — both hard to guarantee on a first homebrew build.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| High SWR on 2m, acceptable on 70cm | Phasing section length slightly wrong for 2m — correct length at 70cm (3×) but off at 2m (1×) | Measure each phasing section with vernier caliper; compare to calculated VF-corrected length | Disassemble and re-cut phasing sections to exact length; VF error of even 0.01 shifts 2m SWR significantly |
| High SWR on both bands simultaneously | Wiring polarity reversal error at one or more junctions — inner not swapped with outer | Check continuity as described in Step 4; look for continuity where there should be an open circuit | Identify the incorrectly wired junction; disassemble and reverse the inner/outer connection at that junction |
| SWR minimum shifted above or below band | Radiating section lengths slightly off; or velocity factor of coax different from assumed value | Measure one radiating section with NanoVNA as a standalone element to find its actual resonant frequency | Adjust radiating section lengths — add small wire stub at tips to lower resonance, or trim to raise; recalculate using actual measured VF |
| Gain noticeably lower than expected — sounds like a simple vertical | One phasing section incorrect length causing one element to be out of phase — cancellation not constructive addition | Build a 2-element test version and compare signal strength to single element; should be clearly stronger | Recheck and recut all phasing sections; verify junction polarity reversals; rebuild if necessary |
| SWR degrades significantly after installation in PVC housing | PVC's dielectric constant is slightly above 1.0 — it slightly loads the exposed inner conductor tips | Compare bench SWR (bare) to housed SWR; shift of 5–10 MHz upward is normal | This is normal and expected — the PVC housing slightly raises the resonant frequency. Cut radiating sections 2–3% longer to compensate when building for housing. |
| SWR fine initially but worsens after months outdoors | Moisture ingress at end caps or feedpoint connector; corrosion at solder junctions | Inspect end cap seals; check feedpoint connector body for moisture; open housing and check junctions | Re-seal all housing entries with silicone; replace corroded connector; re-solder any discoloured junctions; fill housing with foam to exclude moisture |
How much gain does a collinear actually deliver?
A properly built 3-element coaxial collinear delivers approximately 5–6 dBd on 2m and 7–8 dBd on 70cm. In S-meter terms, this is roughly one to one-and-a-half S-units better than a simple quarter-wave ground plane. The gain is real and significant for accessing distant repeaters and working portable stations at range. However, commercial collinear gain claims often use dBi (isotropic) rather than dBd (over dipole reference) — a gain of 7 dBi is only 5 dBd. Always compare on a consistent basis when evaluating gain claims.
Why does more gain hurt nearby stations?
A collinear's gain comes from compressing the vertical radiation pattern toward the horizon — it radiates less energy straight up. Stations very close to you (within 1–2 miles) may actually be harder to work on a high-gain collinear because they are overhead rather than at the horizon. This is the trade-off of a low-angle antenna: optimised for distance, less good for nearby stations. Most urban and suburban ham radio operation involves repeaters or stations at distances where the collinear's low-angle gain is an advantage. If nearby local simplex communication is your primary use, a simple quarter-wave ground plane or J-pole provides more uniform coverage including high-angle paths.
Can I use the collinear for satellite operation?
No — satellite communication on 2m and 70cm requires a directional antenna (Yagi or beam) that can be pointed at the satellite's position as it crosses the sky. A fixed omnidirectional collinear cannot track a moving satellite. Additionally, most amateur satellite transponders use linear polarisation (vertical or horizontal) or circular polarisation — a vertically polarised omnidirectional antenna like the collinear experiences severe polarisation loss when the satellite's polarisation rotates relative to the ground station. For satellite work, a pair of crossed Yagis with circular polarisation is the appropriate antenna — covered in the satellite crossed Yagi build guide on hamradiobase.
How long will a homebrew PVC-housed collinear last?
A well-built and properly sealed homebrew coaxial collinear in PVC housing should last 10–15 years or more in most climates. The PVC itself is highly UV-resistant and does not corrode. The weak points are the solder junctions inside (susceptible to moisture if sealing fails) and the feedpoint coax connector (susceptible to corrosion at the threads). Annual inspection of the feedpoint connector and end cap seals, combined with re-sealing if any deterioration is found, maintains the antenna in good condition long-term. The coax wire inside degrades much more slowly than exposed wire because it is protected from UV and mechanical stress.
What feedline coax should I use with a 70cm collinear?
At 70cm (435 MHz), feedline loss is significant and the choice of coax matters much more than on HF. RG-58 loses approximately 6 dB per 100 feet at 435 MHz — a 50-foot RG-58 run would cancel nearly all the gain of the collinear. For 70cm, use LMR-400 (1.5 dB per 100 ft at 435 MHz) or LMR-240 (2.5 dB per 100 ft) as a minimum. For runs over 75 feet to the shack, LMR-400 or 7/8-inch hardline is strongly recommended. Keep the feedline as short as practical, and use N-type connectors (not PL-259/SO-239) for all 70cm connections — PL-259 connectors show significant insertion loss above 300 MHz.
Is the collinear better than a Yagi for repeater use?
For a fixed base station communicating with a repeater, an omnidirectional collinear is generally preferable to a Yagi because you don't need to point it at anything — you can work any station in any direction without adjusting the antenna. A Yagi provides more gain in one direction but requires pointing. The exception is when your target repeater is at the edge of range and you need maximum gain in that specific direction — in that case, a 5-element 2m Yagi pointed at the repeater will outperform any collinear. For general operation including simplex, APRS, and multiple repeater access, the omnidirectional collinear is the right tool.