Legal Limit HF Station: Where to Put Your QRO Chokes

Building a Legal Limit HF Station (1.5 kW) – How Much Choking You Really Need and Where to Put It

Running a true legal limit 1.5 kW HF station on SSB and heavy-duty digital modes like FT8 puts serious stress on your entire RF chain – not just the amplifier. The difference between a quiet, stable station and a noisy, “mysterious RF gremlin” station is usually how well you control common-mode current on the coax.

For a robust 1.5 kW station, a good engineering target is:

• High bands (30–10 m): aim for at least ≈ 6 kΩ of common-mode choking impedance.
• Low bands (40–160 m): aim for at least ≈ 10 kΩ of common-mode choking impedance.

Two different problems: TX stray RF vs RX noise pickup

Common-mode current is really two separate problems that need different “viewpoints”:

• On transmit – stray common-mode RF: part of your PA output leaves on the outside of the coax shield and flows over the shack, tower, control cables and even your body. That is what causes hot mics, RF burns, lockups and RFI.
• On receive – common-mode noise pickup: the coax shield behaves as a long random antenna that picks up switching hash, SMPS noise, PLC, solar noise and whatever your neighbors radiate, and brings it straight into the radio.

A good high-power choking strategy must deal with both sides: keep stray transmit RF where it belongs (on the antenna) and keep received noise out of the front-end.

Step 1 – Right behind the PA: wideband quad-core choke

The first line of defense sits immediately behind your power amplifier. Here we recommend a rugged, wideband current choke with serious ferrite volume and thermal margin, such as our 10 kW quad-core 160–10 m wideband QRO current choke.

• Location: as close as possible to the PA RF output connector.
• TX role (stray RF): keeps the bulk of the transmit common-mode current from flowing into the shack cabling and rack.
• RX role (noise): helps prevent shack wiring and PA cabinets from acting as “noise antennas” coupling back into the feedline.
• Why QRO-rated? A 10 kW choke on a 1.5 kW station runs cool and linear, even with long FT8 sessions.

This choke greatly reduces the chance that PA cabinets, control cables, or the operator become part of the RF circuit. It also gives your tuner and any downstream switching hardware a much cleaner, more predictable environment.

Step 2 – Outside the shack: dual-band station entry stack (160–10 m + 30–10 m)

The second strategic location is the building entry point – just after your coax leaves the shack and before it runs across the garden or towards a tower. Here we recommend a dedicated stack of QRO chokes in series:

• One wideband 160–10 m QRO current choke, such as the 10 kW ICAS wideband 160–10 m 20 dB CMR current choke.
• In series with a high-bands quad-core 30–10 m choke, such as the 5 kW ICAS quad-core high bands 30–10 m QRO 1:1 current balun/choke.

The practical layout:

• Location: outside the shack, where all feedlines exit the building or where they arrive at the first outdoor junction box.
• Configuration: place the 30–10 m quad-core in series with the 160–10 m choke, with the 160–10 m unit as the last one going towards the antennas.
• TX role (stray RF): forms a strong RF “firewall” that stops common-mode RF from using the building and garden wiring as a return path.
• RX role (noise): blocks noise picked up outside from flowing back into the shack and also stops shack noise from being re-radiated along the feedline.

If you use a coax switch: place this choke stack on the common output of the switch, not one stack per port. This way you only need the chokes that are actually in use at any given time.

With a quad-core choke right behind the PA and a stacked pair (160–10 m + 30–10 m) at the building entry, you already have a very strong suppression of both transmit stray RF and received noise across 30–10 m and 40–160 m. But we can still improve things at the antenna itself.

Step 3 – At the antenna: band-specific high-CMR feedpoint choke

The final – and often most important – choke sits as close as mechanically possible to the antenna feedpoint. This is where coax often doubles as a hidden counterpoise, especially with EFHW, OCF, and end-fed vertical designs.

For this position we recommend a band-specific, high-CMR, dual-core feedline isolator, such as our 9 kW band-specific dual-core 30 dB CMR coaxial line isolator choke.

• Location: as close as practical to the antenna feedpoint or the defined counterpoise connection point.
• TX role (stray RF): keeps the feedline from acting as a second unintended radiator, improving pattern and reducing RF-in-the-shack.
• RX role (noise): stops the coax shield from picking up near-field junk (house noise, solar controllers, Ethernet) and feeding it straight into the antenna port.
• Why band-specific? By focusing on a narrow frequency range, we can push the common-mode impedance much higher than with a generic wideband choke.

If your antenna design uses the coax shield as part of the counterpoise (for example, several EFHW and OCF designs):

• Respect the manufacturer’s recommended distance along the coax before placing the choke.
• If the counterpoise is effectively “buried” in the coax, the choke must go at the end of that defined counterpoise length, not directly at the balun box.

Placing a choke in the wrong spot on such systems can actually detune the antenna. Always follow the antenna manufacturer’s guidance about where the coax stops acting as a counterpoise and where it should start behaving like a clean feedline.

Putting it all together – a complete 1.5 kW legal limit choking strategy

For a modern 1.5 kW HF station running SSB and high-duty digital modes, a solid baseline configuration looks like this:

• Behind the PA (inside the shack): one wideband quad-core QRO choke (10 kW quad-core 160–10 m wideband current choke).
• Outside the shack (station entry): a series stack consisting of:
  • one 10 kW ICAS wideband 160–10 m 20 dB CMR current choke (dual-core 160–10 m), and
  • one 5 kW ICAS quad-core high bands 30–10 m QRO 1:1 choke (quad-core 30–10 m).
  Arrange them with the 160–10 m choke as the last device going towards the antennas.
• At each antenna feedpoint: one band-specific high-CMR dual-core isolator (9 kW band-specific 30 dB CMR isolator choke) placed where the actual feedline starts.

In combination, these chokes provide the multi-kΩ common-mode impedance you need on both high bands (30–10 m) and low bands (40–160 m), while keeping each choke within its comfortable thermal and voltage limits.

Remember: FT8 at 1.5 kW is not “1.5 kW like SSB” – it is closer to a continuous-duty service. That is why we recommend QRO chokes with plenty of headroom, even if your amplifier is “only” legal limit.

What you gain by doing it this way

• On transmit – less stray RF: the shack, tower and control cables stay out of the RF circuit, so you get fewer RF hot spots, fewer lockups and more predictable amplifier behavior.
• On receive – lower noise floor: the coax shield stops acting as a random “noise antenna”, so you often see a cleaner S-meter and can actually hear weak DX instead of local hash.
• More predictable antenna behavior: with common-mode currents under control, your antenna pattern and SWR are determined mostly by the antenna itself, not by the route of the coax.
• Better coexistence: less RF coupling into neighbors’ equipment and your own household electronics – good engineering and good neighbor relations in one go.

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