Non-Resonant TX Antennas in RX Mode: The Hidden Noise Problem

It's a common assumption among hams and shortwave listeners that you can use any transmitting antenna for receiving without consequence. And while this is mostly true from a sensitivity perspective, it completely ignores one key problem: how non-resonant transmitting antennas behave during reception — especially in terms of common mode noise pickup.

Same Physics, Different Direction

Whether you're transmitting or receiving, the antenna system is still a bidirectional interface with the environment. If your transmitting antenna is non-resonant, it may still be functional for RX — but if there's no defined return path and no choke, the coax shield can become active, just like if you would transmit over these antennas.

Why Non-Resonant TX Antennas Cause Noise During RX

  • When the antenna isn't resonant, it presents a complex impedance to the receiver.
  • This usually leads to higher voltage at the feedpoint, especially for reactive loads.
  • If the antenna lacks a proper ground or counterpoise, that voltage is also seen between the center conductor and the shield.
  • The coax shield now sits at an RF potential relative to the environment — and begins to act like an unintended antenna.

Because the potential between the shield and the surrounding environment is now shifted, the shield forms an elevated structure with undefined reference, allowing it to pick up common mode signals riding on that imbalance. The higher the feedpoint voltage and the poorer the return path, the more pronounced this effect becomes. The system is now more susceptible to near-field electric noise, because the shield behaves like a floating pickup wire with no clear outbound current return path.

This is identical in principle to what happens in these antennas when transmitting — the only difference is that in this case you're receiving.

Will a Transmatch or Tuner Help?

Only partially:

  • A transmatch (tuner) at the shack end does not change the feedpoint voltage or the imbalance at the antenna.
  • It provides a good match for the receiver but does nothing to stop coax shield current caused by poor return paths.
  • A tuner cannot eliminate common mode noise — it operates on differential signals only.

If you place the tuner at the antenna feedpoint, then you can achieve a better match there, which can reduce voltage across the shield. But unless you also add a choke, the shield is still exposed.

The Real Fix: Choking the Shield

  • A common mode choke near the antenna feedpoint or at the RX input is the real fix.
  • It ensures that only differential current flows inside the coax.
  • It prevents the shield from becoming part of the antenna system.

Summary

  • Non-resonant transmitting antennas used for receiving can cause common mode noise pickup.
  • The coax braid becomes active when there's no proper return path.
  • Tuners don't help with common mode problems unless placed at the antenna and used with a choke.
  • A choke is always recommended, even in RX-only use of a TX antenna.

Whether it's for QRO or QRM-free reception, balanced current paths and shield control still matter — in both TX and RX directions.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.