Build a PAC-12 Style Portable HF Antenna
The PAC-12 is a compact base-loaded vertical antenna for portable HF operation from 40 m through 10 m using interchangeable loading coils and aluminium whip sections. It sets up on a tripod in under two minutes, requires no mast or trees, and handles 100 W from a base station transceiver as well as QRP field operation — making it the ideal urban portable or vehicle-mounted HF antenna.
Design Overview
The PAC-12 uses an inductive loading coil near the base to electrically lengthen a short whip to resonance. A full quarter-wave vertical on 40 m requires 10 m — completely impractical for portable operation. With a 2.4 m whip above the loading coil the antenna resonates on 40 m from a total height of approximately 2.5 m. The efficiency cost of this miniaturisation is real but manageable: approximately 35% on 40 m rising to 95% on 10 m as the whip becomes a larger fraction of the resonant length.
Base Loading
Coil at the bottom of the antenna, below the whip. Maximum loading effect per turn — fewest turns needed. Simplest construction. Most common commercial design (Buddistick, MFJ-1979). Efficiency lower than mid-loading but straightforward to build and tune.
Interchangeable Coils
One coil per band, each wound to the exact inductance needed for resonance on that band. Highest Q and efficiency for each band. Quick swap at the base connector. Adds 7 small coil pieces to the kit but gives the best performance of any loading approach.
Tripod Mounting
A camera tripod with standard 3/8" thread provides a stable, adjustable-height base. The antenna is self-supporting — no ropes, poles, or trees needed. Works on pavement, in parks, car parks, or on a balcony. Maximum operational flexibility for urban and mobile operation.
Loading Coil Theory
A shortened vertical is capacitively reactive — it looks like a capacitor to the feed point. The loading coil's inductive reactance cancels the antenna's capacitive reactance to restore resonance. The required inductance depends on the antenna's physical length relative to the resonant quarter-wave length and on the whip's self-capacitance.
X_L = |X_C| = 1 / (2 × π × f × C_ant)
L = X_L / (2 × π × f)
L (uH) = r² × n² / (9r + 10l) — r, l in inches, n = turns
| Band | Freq (MHz) | Inductance (µH) | Turns | Coil length (mm) | Efficiency |
|---|---|---|---|---|---|
| 40m | 7.100 | 28.5 | 52 | 42 | ~35% |
| 30m | 10.125 | 13.8 | 36 | 29 | ~48% |
| 20m | 14.175 | 6.8 | 25 | 20 | ~62% |
| 17m | 18.118 | 3.9 | 19 | 15 | ~74% |
| 15m | 21.225 | 2.5 | 15 | 12 | ~82% |
| 12m | 24.940 | 1.5 | 12 | 10 | ~89% |
| 10m | 28.500 | 0.8 | 9 | 7 | ~95% |
Efficiency context: on 40m, 65% of transmitter power is lost in the loading coil and ground system — equivalent to a 4.6 dB penalty vs a full-size vertical. At 100W the antenna radiates approximately 35W equivalent. This is a worthwhile trade for a 2.5m total height vs 10m for full size. On 10m the efficiency is near-perfect.
PAC-12 Coil and Whip Calculator
Complete PAC-12 portable HF system
Construction Sequence
Build the coil formers, wind and test each band's coil, assemble the whip and base, then field-test and tune. Allow a weekend for a full 7-band coil set.
Prepare PVC coil formers
Cut seven 80mm lengths of 50mm OD PVC pipe, one per band. Drill 3mm anchor holes through both walls at each end. Install two 4mm banana sockets through the pipe wall at each end — lower socket connects to the feed/coax, upper socket connects to the whip. Label each former clearly with its band: 40M through 10M.
Wind the coils
Thread wire through the lower anchor hole and bend over inside to secure. Wind the required turns from the table above in a tight single layer. Thread the exit end through the upper anchor hole and solder short pigtails to each banana socket. Start with the 10m coil (9 turns, 2 minutes) to build confidence before tackling the 40m coil (52 turns, 15 minutes).
Measure and adjust inductance
Measure each coil with an LC meter or NanoVNA in impedance mode. Compare to target inductance. Add or remove turns as needed — each turn changes 40m coil inductance by approximately 0.5 uH, and 10m coil by 0.1 uH. When correct, apply clear lacquer to lock the winding in place. Allow 24 hours to cure before outdoor use.
Build the base and assemble the whip
Mount the SO-239 connector in a small aluminium plate that threads onto the camera tripod. The plate also carries the radial terminal ring. Join the three 800mm and one 600mm aluminium tube sections with 10mm push-fit overlaps secured by M3 screws. Total whip length: approximately 3.0m. Insert the whip upper banana plug into the coil top socket, mount coil bottom socket to the base SO-239 terminal.
Field test and fine-tune each coil
Set up outdoors on the tripod with three 10m counterpoise wires laid on the ground. Insert the 20m coil and sweep 13-16 MHz with a NanoVNA. SWR minimum should appear near 14.175 MHz. If high: add 1-2 turns. If low: remove turns. Repeat for each band, working from highest frequency (10m, fewest turns) to lowest (40m, most turns). Record the final turn counts for each coil.
Prepare counterpoise wires
Cut band-specific counterpoise wires: 10.1m (40m), 7.1m (30m), 5.1m (20m), 3.9m (17m). Pre-wound on small card bobbins with a crocodile clip at one end. In the field, clip the matching-band counterpoise to the SO-239 outer terminal and trail it on the ground away from the tripod. For 15m-10m a single 5m wire is adequate.
Counterpoise and Ground
A loaded short vertical critically depends on its ground system. Without any counterpoise the coax braid becomes the return path — causing RF on equipment and unpredictable impedance. Use at minimum a single resonant counterpoise wire (λ/4 for the active band) clipped to the coax braid terminal. Three or four radials improve efficiency noticeably.
| Antenna | 40m eff. | 20m eff. | Setup | Packed size | Best use |
|---|---|---|---|---|---|
| PAC-12 (2.4m whip) | ~35% | ~62% | 2 min | 90×10cm | Urban, vehicle, balcony |
| SOTA linked vertical | ~85% | ~92% | 5 min | 60×8cm | Summit, park, field |
| EFHW + 49:1 UNUN | ~78% | ~88% | 6 min | 20×8cm | SOTA, POTA, portable |
| Full QW + 4 radials | ~95% | ~98% | 15–20 min | 10m wire+stake | Fixed or semi-fixed |
How much power can the PAC-12 handle?
Using 0.8-1.0mm enamelled copper wire and keeping coil Q above 100, the antenna handles 100W SSB continuously and 100W CW at moderate duty cycle. For 200W or higher, use 1.5mm wire and a 75mm former. The limiting factor is I2R heating in the loading coil, which is highest on 40m where current through the coil is greatest relative to radiated power.
Do I need a separate coil for each band?
The standard PAC-12 uses one interchangeable coil per band for maximum Q per band. Alternatives include a single tapped coil with alligator clips selecting the correct tap (slightly lower Q but fewer pieces), or a motorised screwdriver antenna that varies inductance continuously for maximum convenience at the cost of weight and complexity.
Is the PAC-12 better than a random wire with ATU?
For urban operation without an ATU, the PAC-12 provides direct 50 Ohm connection without additional matching hardware. With an ATU, a random wire is equally convenient. In terms of efficiency, both are broadly comparable when a proper counterpoise is used. The PAC-12 wins on tidiness and portability; the random wire wins on simplicity and lowest total cost.
Can I use the PAC-12 for mobile HF from a vehicle?
Yes — mount the base on a mag-mount or boot-lip mount on the vehicle. The vehicle bodywork provides a large ground plane replacing separate radials and gives much better efficiency than a counterpoise on open ground. A car-roof mag-mount with the PAC-12 vertical gives excellent mobile HF performance on 20m through 10m and usable 40m performance.
Why is 40m efficiency so much lower than 20m?
On 40m, the 2.4m whip is only 24% of a full quarter-wave — requiring a large loading coil with high loss resistance relative to the low radiation resistance of the short element. On 20m the same whip is 34% of quarter-wave needing a much smaller coil. On 10m it is 68% and barely needs loading. Efficiency improves rapidly as the whip becomes a larger fraction of the resonant length.
What is the difference between PAC-12 and Buddistick?
The Buddistick uses a single tapped coil with adjustable-tap selection rather than separate coils per band. Both designs are excellent portable HF antennas. The PAC-12 home-build approach gives higher Q per band with separate optimised coils; the Buddistick commercial design gives quicker band changes at the cost of slightly lower Q. Build cost under 25 pounds vs approximately 100-150 pounds for a commercial equivalent.