Field Guide · term

Also known as: feedpoint impedance, feed impedance, antenna impedance

Feedpoint impedance is the impedance an antenna presents at the terminals where the feedline connects — the load the transmitter or receiver actually “sees.” It is a complex quantity, Z = R + jX, whose real part combines the useful radiation resistance with ohmic loss resistance, and whose imaginary part is the reactance that appears when the antenna is not at resonance.1 Matching this impedance to the feedline — almost always 50 Ω in radio work — is what allows power to transfer without reflection, and a mismatch shows up as an elevated standing-wave ratio.

50 Ω feedline feedpoint Z = R + jX R = R_rad + R_loss X = 0 at resonance
The feedpoint impedance is the complex load the feedline drives; matching its real part to the line and cancelling its reactance transfers maximum power.

How it works

Current flowing into the antenna does two things: it radiates energy into space, and it dissipates a little as heat in the conductors and nearby ground. Modelled as a resistance, the radiated part is the radiation resistance R_rad and the wasted part is the loss resistance R_loss; the ratio R_rad / (R_rad + R_loss) is the antenna’s radiation efficiency. On top of this resistive part, the antenna generally stores energy in near-field electric or magnetic fields, which appears as reactance X — capacitive (negative) when the antenna is electrically short, inductive (positive) when it is long.

At resonance the reactance passes through zero and the feedpoint looks purely resistive. The value of that resistance depends on the antenna type and the feed location:

  • A half-wave dipole in free space is resonant near 73 Ω resistive — close enough to 50 Ω that it is often fed directly.
  • A quarter-wave monopole over a good ground is about half that, near 36 Ω.
  • A folded dipole steps up to roughly 300 Ω. Feeding a dipole off-centre or at the end raises the impedance dramatically, into the thousands of ohms at a voltage maximum.

In practice

Getting maximum power transfer requires two conditions: the resistive parts must be equal and the reactance must be cancelled (conjugate match). Techniques include:

  • Trimming to resonance so X → 0, then relying on the natural R being near 50 Ω.
  • Matching networks — a gamma match, hairpin, or transformer at the feed, or an external antenna tuner that synthesizes the conjugate.
  • A balun, which does not itself transform 50 Ω but ensures a balanced antenna is fed by an unbalanced coax without common-mode current corrupting the pattern and shifting the apparent impedance.

Feedpoint impedance is also affected by everything nearby — height above ground, adjacent metal, the mast — so the free-space textbook value is only a starting point; the installed impedance is what a vector network analyzer or antenna analyzer measures.

Relevance to SDR

For a receive-only SDR the consequences of a mismatch are milder than for a transmitter — no power is being reflected back into a PA — but they are real: a badly mismatched antenna reflects signal away from the receiver, lowering the delivered signal level and worsening the noise figure of the front end. A wideband scanning antenna is deliberately a compromise, presenting something near 50 Ω over a broad band rather than a perfect match at any one frequency. GopherTrunk never sees impedance directly — it processes the IQ samples the SDR produces — but a well-matched feedpoint maximizes the signal-to-noise ratio reaching the decoder, which is what determines whether a marginal P25 or DMR signal locks.

Sources

  1. Antenna impedance — Wikipedia, for the decomposition into radiation resistance, loss resistance, and reactance. ↩

See also