Antennas and measurements, from calculation to tuning
Antennas and measurements for radio amateurs: calculate starting lengths for verticals, end-feds and loops, coaxial lines and stubs, convert SWR and learn to measure your antenna with an analyzer or NanoVNA, with common problems and safety rules.
Wavelength, half-wave dipole, SWR from power and cable loss are in the Radio Lab Tools. Open the Radio Lab Tools →
Antenna and line calculators
Every result is a starting length: the real installation (height, nearby objects, insulators, wire diameter) moves the resonance. Cut a little long and trim while measuring.
Quarter-wave vertical (ground plane)
Quarter-wave vertical radiator with radials.
Formula: 234 / f feet = 71.32 / f metres (≈ 0.95 × λ/4)
End-fed half-wave (EFHW)
Half-wave wire fed at one end through a 1:49 transformer.
Formula: 468 / f feet = 142.6 / f metres (same as the dipole)
Full-wave loop
Closed loop of about one wavelength.
Formula: 1005 / f feet = 306.3 / f metres
Coaxial lines and stubs
Physical lengths of λ/4 and λ/2 cable sections.
Formula: L = VF × 299.79 / f / 4
Velocity factor from measurement
Measure the real velocity factor of a cable.
Formula: VF = 4 × L × f / 299.79
SWR converter
From SWR to return loss, reflected power and SWR at the antenna.
Formula: |Γ| = (SWR − 1) / (SWR + 1); RL = −20 log|Γ|; reflected = |Γ|²
Dipole and inverted V
The half-wave dipole calculator is already in the Radio Lab Tools.
For an inverted V start from the same length and trim while measuring: the angle between the legs and nearby objects shift the resonance. ARRL trimming rule: if SWR drops as frequency goes down the antenna is too long, if it rises it is too short.
Guided measurements with an analyzer or NanoVNA
The sequence for measuring an antenna repeatably. Menu names are the NanoVNA ones; antenna analyzers have equivalent functions.
Set the frequency range
STIMULUS → START and STOP: cover the whole band with some margin above and below.
Calibrate
CAL → RESET, then CALIBRATE with OPEN, SHORT and LOAD connected in turn (enough for one-port measurements), DONE and SAVE. Recalibrate whenever you change the frequency range.
Choose the measurement point
If you measure through a jumper cable, calibrate at the end of that cable, so the result refers to what comes after it.
Show SWR and the Smith chart
DISPLAY → FORMAT → SWR on one trace and SMITH on another; add RESISTANCE and REACTANCE to read R and X.
Find resonance
Move the marker to where reactance X crosses zero: that is the resonant frequency. Minimum SWR is often close, but not always the same.
Trim and repeat
Shorten or lengthen a little at a time, symmetrically on both legs of a dipole, and measure again. Note every step.
How to read the results
What an SWR value really means and the most common myths.
| SWR | |Γ| | Return loss | Reflected | Mismatch loss |
|---|---|---|---|---|
| 1.2:1 | 0.091 | 20.8 dB | 0.8% | 0.04 dB |
| 1.5:1 | 0.200 | 14.0 dB | 4.0% | 0.18 dB |
| 2.0:1 | 0.333 | 9.5 dB | 11.1% | 0.51 dB |
| 3.0:1 | 0.500 | 6.0 dB | 25.0% | 1.25 dB |
| 6.0:1 | 0.714 | 2.9 dB | 51.0% | 3.10 dB |
“Shortening the coax improves SWR”
On a 50 Ω line connected to an antenna, SWR does not depend on the cable length. If SWR changes with length, there is usually current on the shield (no choke) or the cable or the meter is not 50 Ω.
“SWR 1:1 means a perfect antenna”
SWR only tells how close the impedance is to 50 Ω. A dummy load has SWR 1:1 and does not radiate; a long lossy cable lowers the SWR read in the shack.
“At SWR 2:1 half the power comes back”
At SWR 2:1 about 11% of the power is reflected; mismatch loss is about 0.5 dB, little more than a tenth of an S-unit.
Common problems
The most frequent cases when installing and tuning an antenna.
Resonance outside the band
Below the band the antenna is too long, above it is too short. Correct a little at a time: shortening by 1% raises the resonance by about 1%.
SWR changing with rain or over time
Water in connectors or cable, dirty insulators, damp transformers or baluns. Check and seal outdoor connectors; a cable with water inside must be replaced.
SWR changing when touching or moving the cable
Common-mode current on the shield: add a choke near the feed point. The same problem can bring RF into the shack.
RF in the shack: “biting” microphone, USB disconnecting
Chokes and ferrites on coax, USB and audio cables, good station grounding. More in QRM Troubleshooting. Go to QRM Troubleshooting →
The NanoVNA gives strange readings
Calibration done on another frequency range or without the test cable, loose connectors, strong signal picked up by the antenna during the measurement (for example from a nearby transmitter).
Safety
Principles to follow at all times; for exposure limits and installation rules, your country’s regulations apply.
RF exposure
Exposure grows with power and transmit time and drops with distance: keep antennas away from where people are. International guidelines are the ICNIRP ones; legal limits depend on the country.
Power lines
Never install antennas where, if they fall, they can touch power lines. It is the most serious cause of accidents when installing antennas.
Lightning and grounding
Disconnect antennas from the radio when not in use and during thunderstorms. For lightning protection and installation grounding, use a qualified installer.
Working at height
Never climb roofs or towers alone; use fall protection and work in good weather.
Sources: NIST/CODATA — speed of light, ARRL — Single-band dipoles (468/f, trimming), Palomar Engineers — Loop antennas (1005/f, ~100–120 Ω), WiMo — EFHW antennas (1:49, 2500–5000 Ω, choke), AA5TB — The End Fed Half Wave Antenna, Rohde & Schwarz — VSWR and return loss, NanoVNA — Calibration, NanoVNA — Start measurement, ICNIRP — RF EMF (100 kHz–300 GHz). Sources consulted on 18 October 2026. Links open on the publisher’s website.
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