Line Isolator (Shack-Entry Choke)
A line isolator is a common-mode choke built and sized specifically for the point where coax enters the building — the last line of defense against RF in the mic, RF biting you at the rig, and computer/USB interference, whether or not a feedpoint choke is already doing its job further out. It uses the same core-and-winding physics as any current balun, but the sizing target, core choice, and grounding all differ from a feedpoint choke because of where and how it's installed. This guide covers those differences, a stacked-core turns calculator, and shack-entry-specific troubleshooting.
Same physics, different job
A line isolator uses the exact same core-and-winding principle as any 1:1 current choke — see the 1:1 Current Balun / Choke guide for the full derivation of how a ferrite-wound coax loop presents impedance to common-mode current while leaving the wanted signal untouched. What makes it a distinct build is its job: instead of stopping common-mode current at its source (the antenna feedpoint), a line isolator catches whatever current is still on the line by the time it reaches the building, whether that's because there's no feedpoint choke at all, the feedpoint choke isn't quite enough on its own, or the antenna's impedance swings widely enough across bands that no single feedpoint choke handles every case perfectly.
Why the sizing target is different
A feedpoint choke can often be optimized for the specific bands one antenna actually uses. A shack-entry line isolator is frequently shared duty — the last common point before multiple antennas' coax runs reach the operating position — so it needs to perform adequately across the full range you operate, often including 160m, where common-mode impedance is hardest to achieve with a compact winding. Because it's the last line of defense before the choke's failure shows up directly as equipment interference, many builders target a higher minimum impedance (700-1000Ω or more) than the roughly 500Ω that's adequate for a single-band feedpoint choke.
Mix choice: breadth over peak low-band performance
- #43 mix is usually the better default here: a shack-entry choke commonly needs to work across whatever the station actually operates — 160m through 6m in many cases — and #43's broad 2-200 MHz range covers that better than #31's narrower low-band-optimized range, even though #31 gives more impedance per turn specifically on 160/80m.
- Stacking a #31 core with a #43 core is a workable compromise for stations that spend real time on 160m and also want VHF coverage through the same entry point — the #31 boosts low-band impedance while the #43 keeps the upper end covered.
Grounding and bonding — a real difference from a feedpoint choke
A feedpoint choke is often mounted well up a mast or tower with no local ground reference at all. A line isolator, by contrast, sits right at the building's single-point entry, which is exactly where station grounding and lightning-protection bonding already needs to happen per standard practice. Mount and bond the line isolator's enclosure to the station's entry-panel ground bus with a short, direct strap — this is both a safety practice and a real contributor to the isolator's effectiveness, since a floating enclosure at this location can itself become part of an unwanted current path.
| Aspect | Feedpoint Choke | Shack-Entry Line Isolator |
|---|---|---|
| Primary job | Stop common-mode current before it starts down the coax | Catch whatever common-mode current is still present by the time it reaches the building |
| Typical target impedance | ~500Ω, tuned to that antenna's actual bands | 700-1000Ω+, sized for whatever the station operates |
| Typical core | Single FT-240 (mix chosen per antenna's bands) | Single or stacked FT-240-43, sometimes mixed with #31 for low-band boost |
| Grounding | Often none — commonly floating on a mast or tower | Bonded to the entry-panel ground bus as part of standard station grounding |
| Weather exposure | Full outdoor exposure at the antenna | Usually at or near the building entry point, sometimes indoors just past the bulkhead |
| Best paired with | A line isolator as a second line of defense | A feedpoint choke as the primary fix; the isolator alone is a good fallback when a feedpoint choke isn't practical |
Shack-Entry Line Isolator Turns Calculator
Materials for a station-entry common-mode choke
A shack-entry line isolator wound on two stacked FT-240-43 cores, bonded to the entry-panel ground bus.
Building a Shack-Entry Line Isolator
This build follows the same winding technique as a feedpoint choke — the differences are in target impedance, core count, and where you mount and ground the finished choke.
Set your target impedance and lowest band
Use the calculator above with your lowest operating frequency and a target impedance in the 700-1000Ω range for general multiband duty — go higher if this isolator is the only common-mode protection your station has (no feedpoint chokes in place) or if you operate 160m regularly.
Stack the cores if the calculator recommends it
Place two (or more) identical cores directly against each other, aligned so a single winding passes through both as if they were one thicker core. Stacking is what lets you hit a high target impedance without needing an impractical number of turns of thick entry-run coax.
Wind the coax and secure it
Pass the intact coax through the stacked cores for the calculated turn count, keeping turns snug but not tight enough to kink the cable — thicker entry-run coax has a larger minimum bend radius than the thinner coax often used at a feedpoint, so a stacked-core approach genuinely helps here. Secure with cable ties or self-amalgamating tape.
Mount at the entry point and bond to ground
Install the finished choke at or near where the coax enters the building, and bond its enclosure to the entry-panel ground bus with a short, direct strap. This ties the isolator into the station's existing single-point grounding and lightning-protection scheme rather than leaving it as a separate, floating item.
Verify across your full operating range
Measure common-mode impedance with a NanoVNA across every band you actually operate, not just the design frequency — a shack-entry isolator earns its keep by working broadly, so confirm it stays above your target on the highest band you use as well as the lowest.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| RF-in-the-shack symptoms persist even with the line isolator installed | The common-mode current source is at the feedpoint and needs to be addressed there, not just caught at the entry | Check whether a feedpoint choke exists; a shack-entry isolator alone is a fallback, not always a complete fix on its own | Add a feedpoint choke (see the 1:1 Current Balun / Choke guide) as the primary defense, keeping the line isolator as a second layer |
| Isolator works on some bands but not others | Core mix doesn't cover the full range you're testing, or the target impedance wasn't set for the worst-case band | Re-run the calculator using your actual lowest operating frequency, and check the core mix's rated range against the affected band | Add turns, stack an additional core, or add a second mix (e.g. #31 alongside #43) to extend low-band coverage |
| Noise or interference changes when you touch the isolator's enclosure | Enclosure isn't properly bonded to the entry-panel ground, leaving it as a floating conductor | Check continuity from the enclosure to the station ground bus | Add a short, direct bonding strap from the enclosure to the same ground system as the rest of the entry panel |
| Station has multiple antennas and the problem only shows up on one | Only one coax line has a line isolator; the others are unprotected at the entry | Confirm whether every antenna's coax run has its own isolator at the entry point | Build a separate line isolator for each coax line — one isolator does not protect lines it doesn't carry |
| Coax shows wear or cracking where it enters the stacked cores | Bend radius too tight for thick entry-run coax on the chosen core size | Inspect the jacket at the winding entry/exit points for stress marks | Use fewer, more evenly spaced turns (compensating with an extra stacked core to hit the target impedance) or a larger-ID core |
Is a line isolator a substitute for a feedpoint choke?
Not really — it's a second line of defense. A feedpoint choke stops common-mode current at the source, which is generally the more effective single fix. A line isolator catches whatever gets past that (or covers the gap if a feedpoint choke isn't practical for that antenna), and the two together handle the widest range of real-world cases.
Do I need one isolator per antenna, or can one cover my whole station?
One isolator only protects the coax line it's actually wound around. If you run multiple antennas on separate feedlines into the shack, each line needs its own isolator at the entry point.
Why is #43 usually recommended here instead of #31?
A shack-entry isolator typically needs to work across whatever bands the station actually operates, and #43's broader 2-200 MHz range covers that better than #31's low-band-optimized range — even though #31 gives more impedance per turn specifically on 160/80m. Stacking a #31 core with a #43 core is a reasonable compromise if you spend significant time on 160m and also want VHF coverage.
How many turns do I need for 160m coverage?
Use the calculator above with 1.8 MHz as the frequency — reaching a high target impedance that low typically calls for either more turns than fit comfortably on a single core, or (more practically) stacking two cores as shown in the worked example, which reaches the same impedance target with far fewer turns of thick entry-run coax.