The Choke That Saves Your Multiband Antenna
Updated: December 2025
Why controlling unwanted currents keeps your SWR stable and your radiation pattern where you expect it.
Multiband antennas promise simplicity: one antenna, one feedline, many bands. In practice, many installations behave nothing like the model. SWR changes when the coax is rerouted. The noise floor rises or falls depending on where the feedline hangs. The strongest DX direction seems to drift from band to band.
A very common cause is RF current flowing on the outside of the coax shield. The cure is simple and fundamental: a properly placed 1:1 current balun (common-mode choke).
Why RX and TX got mixed together
Modern discussions often label everything as common-mode current. Historically, receive and transmit problems were treated as different issues, even though they involve the same physical conductor: the outside of the coax shield.
Keeping that distinction makes troubleshooting clearer and avoids false assumptions.
On receive: common-mode current means noise pickup
On receive, the problem truly is common-mode current.
Household noise couples into the environment. If the outside of your coax shield is electrically active, it behaves like a long receiving antenna. Noise induces current on the outside of the shield and delivers it straight into your receiver.
- Same phase
- Same direction
- Flowing on the outside of the coax
A choke placed at the feedpoint or shack entrance presents a high impedance to that current, breaking the noise path while leaving wanted signal untouched.
On transmit: stray return currents distort the antenna
On transmit, the problem is better described using older terminology: stray or unintended return currents.
If a defined return path is missing or frequency-dependent, RF current finds alternatives. One of the easiest is the outside of the coax shield.
When this happens, the feedline becomes part of the radiating structure. This is not a noise problem — it is a pattern and tuning problem.
- Radiation pattern shifts
- Lobes and nulls move
- SWR changes with coax routing
Same physics, different consequences
- Receive: common-mode current increases noise
- Transmit: stray return current alters radiation and SWR
Same conductor. Same mechanism. Different direction of energy flow.
Why the choke fixes both
A proper 1:1 current balun makes the outside of the coax shield electrically expensive for RF current.
- On RX, noise currents are blocked
- On TX, the feedline is no longer a return path
The bottom line
Without a choke, the feedline quietly becomes part of the antenna. With a choke, the antenna is forced to stand on its own.
- Stable SWR
- Repeatable radiation patterns
- Lower noise and less RF in the shack
Mini-FAQ
- Is this only relevant for end-fed antennas? — No. Any antenna can develop stray return or common-mode currents.
- Does a tuner replace a choke? — No. A tuner matches impedance, not current paths.
- Do I need more than one choke? — Often yes. Feedpoint first, shack entrance second.
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