Build an Isotron-Style Compact Loaded HF Antenna
An Isotron-style antenna packs an HF radiator into a flat disc barely two or three feet across by combining a large capacitance plate with a loading coil, mounted directly on a mast arm with no radials or counterpoise needed at all. Commercial units run $150-250 per band; this build gets you the same self-contained, compact design from a plywood or PVC disc form, wound wire, and a slug-tuned coil. It's built for balconies, small yards, and HOA-restricted situations where even a compact vertical's footprint is too much — and it comes with the biggest efficiency tradeoff of any design in this series, which this guide states plainly rather than glossing over.
A resonant LC circuit that happens to radiate
Instead of relying on a long conductor's own length for resonance, this design pairs a large flat capacitance plate with a loading coil to form a resonant circuit at your target frequency — the same relationship that sets the frequency of any LC tank circuit. The size of the plate and the coil trade off against each other; a bigger plate needs less coil, and vice versa.
plate size and coil inductance trade off to hit the same target frequency
The most compact — and least efficient — design in this series
Packing an HF antenna into two or three feet means an even more extreme size-to-wavelength ratio than the screwdriver or Hamstick builds, and the same base-loading efficiency penalty described in the AX1-style guide applies here too: the coil sits where current is highest, so a meaningful share of your power turns into coil heat rather than radiation.
Why no radials or counterpoise
Unlike the vertical and random-wire designs elsewhere in this series, this antenna is a self-contained resonant structure mounted on a mast arm — it doesn't rely on a ground-plane return path the way a Buddistick or random wire does. That's the design's real selling point for restricted-space installs, not raw performance.
- Mast-mounted vertical/random-wire designs: need a counterpoise or radial system for a return path.
- This design: self-contained, no ground system required, at the cost of efficiency.
Why nearby objects matter more here than usual
The capacitance plate works by coupling to free space, and anything conductive nearby — the mast, gutters, siding — changes that coupling more noticeably than it would for a larger antenna, because the plate's own capacitance is a smaller, more easily disturbed quantity to begin with. Final tuning always has to happen in the exact mounted position you'll actually use.
| Section | Size (inches) | Size (mm) | Notes |
|---|---|---|---|
| 40m capacitance disc | ~36 in | 914 mm | Larger plate and coil than higher bands; starting size, not a guaranteed-resonant spec |
| 20m capacitance disc | ~24 in | 610 mm | Moderate plate and coil size |
| 15m/10m capacitance disc | ~14-18 in | 356-457 mm | Smallest plate and coil in this design |
| Loading coil | slug-tuned | — | Same continuously-variable coil concept as the AX1-style build — no fixed turns table, tuned live against an analyzer |
| Mast mounting arm | 12-18 in | 305-457 mm | PVC or fiberglass boom from the mast clamp to the feedpoint |
Isotron-Style Compact Loaded HF Antenna Dimension Calculator
Materials for Isotron-Style Compact Loaded HF Antenna
Building the Isotron-Style Compact Loaded HF Antenna
This build is mostly patient winding and careful tuning rather than heavy construction — budget real time for the final tuning step, same as the AX1-style build.
Choose your band and cut the disc form
Cut a plywood or PVC disc to the diameter in the dimensions table for your target band.
Wind the capacitance plate
Wind insulated wire or aluminum foil tape in a flat spiral across the disc, working from the center outward, and secure it so it can't shift.
Build or source the slug-tuned loading coil
Use a purchased slug-tuned mini coil or wind your own on a threaded ferrite/brass slug form, following the same approach as the AX1-style build.
Connect the disc to the coil
Wire the center of the capacitance plate's spiral to one side of the loading coil, with a solid soldered connection.
Build the mounting arm and feedpoint
Assemble a PVC or fiberglass arm from a mast clamp to an SO-239 feedpoint bracket at the base of the coil.
Mount to the mast
Clamp the arm to your mast, keeping the disc clear of the mast itself and any other nearby metal as much as your installation allows.
Add the common-mode choke and connect coax
Install a common-mode choke near the feedpoint and connect your coax feedline.
Initial tune-up
With the antenna in its final mounted position, sweep SWR and adjust the loading coil slug to bring the resonant dip toward your target frequency.
Fine-tune resonance
Continue adjusting the slug in small increments, re-sweeping after each change, until the dip centers on your working frequency.
Weatherproof the disc
Coat the finished spiral winding with exterior varnish or resin to protect it from rain and UV exposure.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Can't reach resonance at all on the target band | Disc undersized or coil range insufficient for that band | Confirm the slug is at the end of its range and the disc matches the size table | Rebuild with a larger disc or a coil with more inductance range |
| Extremely narrow SWR bandwidth | Normal high-Q behavior for this design | Compare against the expected behavior described in the theory section | Center the tune on your most-used frequency; expect to retune for big frequency jumps |
| SWR changes noticeably after mounting near the mast or other metal | Nearby conductive material coupling into the compact capacitance plate | Compare a sweep with the disc close to vs. further from nearby metal | Increase clearance from the mast where possible, then retune in the final position |
| Good SWR but weak signal reports | Normal efficiency tradeoff for this compact design | Compare against the -6 to -10 dB expectation set earlier | This is the size tradeoff, not a fault — a larger antenna will always outperform it |
| Disc winding cracked or wire exposed after weather exposure | Sealant breakdown from UV and moisture | Inspect the winding for cracked coating or bare wire | Clean, re-coat, or rebuild the affected section |
| Resonance drifts noticeably between dry and wet weather | Moisture changing the effective capacitance of the disc | Compare sweeps in dry vs. damp conditions | Improve the weatherproofing seal on the winding |
Why doesn't this need radials or a counterpoise?
The capacitance plate and loading coil form a self-contained resonant circuit rather than relying on a ground-plane return path, which is what makes this design usable in spots where running radials just isn't practical.
How compact can this really be?
Roughly two to three feet across depending on band, which is dramatically smaller than any wire or full-size vertical design in this series — at a real efficiency cost.
Does a good SWR mean good performance?
No — the same distinction from the AX1-style guide applies here. A clean match confirms the feedline is happy, not that you're radiating efficiently.
Can one antenna cover multiple bands?
Not with this fixed-disc design — like the Hamstick, each unit is built and tuned for one band. Covering multiple bands means building multiple units.
How close can it be to the mast or other metal?
Closer than ideal will detune it more noticeably than it would a larger antenna, so give it as much clearance as your installation allows and always do final tuning in the actual mounted position.
Is this a good match for HOA/stealth installs?
Yes, that's the design's real strength — small, mast-mountable, and radial-free — as long as you go in accepting the efficiency tradeoff described throughout this guide.