What “resonant” means on a trace
The antennas lessons define resonance as the frequency where the reactance is zero and the feedpoint looks purely resistive. On an instrument it appears as:
- A minimum in SWR — ideally close to 1:1, in practice 1.2 to 2:1.
- A maximum in return loss — the deepest point of the dip.
- The X trace crossing zero on a VNA that shows R and X separately.
The third is the real definition and the other two follow from it. A VNA showing X crossing zero at 14.2 MHz has told you the answer directly; SWR is a derived quantity.
Calibrate first, always
A VNA measures what is at the end of its cable, so it must be told where that end is. Skip this and every number is wrong.
Open, Short, Load — the three standards, at the point you want to measure from. Usually that is the far end of the coax, not the instrument’s connector, because you care about what the transmitter sees.
Calibrate over the frequency range you intend to sweep, and recalibrate when you change that range. Calibration is per-sweep, not permanent.
The sweep
- Set the range wide first — the whole band and well beyond it. An antenna is rarely resonant where you expect, and a narrow sweep can miss the dip entirely.
- Find the dip. Note the frequency, not just the SWR.
- Narrow the sweep around the dip and read:
- Resonant frequency — where the dip is.
- Minimum SWR — how good the match is there.
- Bandwidth — the range where SWR stays below 2:1.
Those three numbers describe the antenna completely for practical purposes.
Cutting a dipole to frequency
The classic exercise, and the one that teaches the most.
- Start with the formula from the antennas lesson:
For 14.2 MHz that is 33 feet, or 16.5 feet per leg.
- Cut it long — add 5%. You can always remove wire; adding it back is another matter.
- Sweep it. The dip will be below your target frequency, because the antenna is too long.
- Trim both legs equally, a few inches at a time, and re-sweep after each cut. Resonance moves up as the antenna gets shorter.
- Stop when the dip sits where you want it.
What you will notice, and what no formula tells you: the resonant frequency depends on height above ground, on nearby objects, and on the wire’s insulation. The 468 figure is a starting point. Measure the antenna where it will actually hang, not on the lawn.
That is precisely why measuring beats calculating, and why the velocity-factor correction exists.
Reading a bad result
| What you see | Usually means |
|---|---|
| No dip anywhere in the sweep | open or short in the feed line, or the antenna is far outside the swept range |
| Dip at a much lower frequency than expected | antenna too long — or you are seeing a coax resonance, not the antenna |
| Very broad, shallow dip | lossy system. Loss flattens SWR and looks like a good match |
| Dip at the right place, SWR still 3:1 | resonant but wrong impedance — a matching problem, not a length problem |
The third row is the important one. A dummy load has a perfect SWR and radiates nothing. A very broad, very good match on a small antenna usually means something is absorbing power — a lossy balun, water in the coax, or a corroded connector.
Without a VNA
An SDR plus a wideband noise source and a directional coupler measures return loss directly:
- Noise source into the coupler’s input; antenna on the output; SDR on the coupled reflected port.
- Record the reflected level with the antenna connected.
- Record it again with the antenna replaced by a short — that is full reflection, your 0 dB reference.
- Return loss is the difference. Convert to SWR if you want:
Sweep the noise source’s output with the SDR across the band and the dip appears in the reflected level. It is the same measurement a VNA makes, done manually, and doing it once makes clear what a VNA is actually doing.
Check yourself
- Your 20 metre dipole dips at 13.6 MHz. Too long or too short, and by roughly how much?
- A friend’s antenna shows 1.1:1 across the entire 40 metre band. Impressive?
- Why calibrate at the far end of the coax rather than at the instrument?
Answers
- Too long. Resonance is 0.6 MHz low out of ~14.2, about 4%, so shorten by roughly 4% — around 8 inches per leg on a 33 foot dipole. Trim in stages and re-sweep.
- Suspicious. A full-size 40 m dipole is not that broad. A perfectly flat SWR across a whole band usually means loss is masking the mismatch — check for a bad connector, water in the coax, or a lossy balun.
- Because you want to measure what the transmitter sees through the actual feed line. Calibrating at the instrument leaves the coax inside the measurement, and coax transforms impedance with length.