Ohmforge RF Calculator: Antenna, Link Budget & VSWR
Dipole and quarter-wave lengths, free-space path loss and link margin, VSWR ↔ return loss.
Try an example: Wi-Fi link: 2.4 GHz over 100 m · Dipole for the 20 m band (14.2 MHz) · VSWR 2:1 → return loss and mismatch loss · 75 Ω antenna on a 50 Ω radio
How it works
Three everyday radio calculations in one place: antenna element lengths for a frequency (half-wave dipole, quarter-wave monopole, coax stubs), a Friis link budget with free-space path loss, received power, margin and the maximum range for the fade margin you want, and the conversions between VSWR, return loss, reflection coefficient and mismatch loss — also straight from a load impedance.
- Wavelength λ = c/f with c = 299 792 458 m/s. Half-wave dipole = λ/2 × k × VF and quarter-wave monopole = λ/4 × k × VF, where k is the end-effect factor (0.95 reproduces the ARRL rule 468/f(MHz) ft within 0.2 %) and VF the velocity factor of the element. Coax stubs use the cable's velocity factor. All lengths are starting points: trim the real antenna with an analyser.
- Free-space path loss FSPL = 20·log10(4π·d·f/c) between isotropic antennas (ITU-R P.525). Friis link budget: received power = Ptx − Ltx + Gtx − FSPL − Lother + Grx − Lrx; margin = received power − sensitivity. The maximum range is the free-space distance at which the margin equals the fade margin you ask for.
- The first Fresnel zone radius at mid-path is ½·√(λ·d); keeping 60 % of it free of obstacles is the usual rule for line-of-sight links.
- Mismatch: |Γ| = (VSWR − 1)/(VSWR + 1) = 10^(−RL/20); Γ = (ZL − Z0)/(ZL + Z0) for a load impedance; mismatch loss = −10·log10(1 − |Γ|²); reflected power = |Γ|².
Free-space values: real links lose more to the ground, buildings, foliage and multipath, and the received power fluctuates (fading). Transmit power and antenna gain limits are set by your country's radio regulations — this calculator gives no licensing or legal advice.
Sources
- H. T. Friis: A Note on a Simple Transmission Formula, Proc. IRE 34 (1946) (IRE / IEEE)
- ITU-R P.525: Calculation of free-space attenuation (ITU-R)
- D. M. Pozar: Microwave Engineering, 4th ed., Wiley 2012 — §2.3 (VSWR, return loss), §3.7–3.8 (stripline, microstrip), §5.1 (L-section matching) (Wiley)
- ARRL Antenna Book: half-wave dipole length 468/f(MHz) feet (end effect) (ARRL)
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Antenna lengths, link budget and VSWR in one RF calculator
Work out dipole and quarter-wave lengths, a free-space link budget with margin and range, and convert between VSWR, return loss and mismatch loss.
What it is for
Three questions come up whenever you build something that transmits: how long should the antenna be, will the signal arrive with enough margin, and how bad is the mismatch the SWR meter shows? This calculator answers all three with one Calculation switch.
Antenna lengths turns a frequency into a half-wave dipole, a quarter-wave monopole and coax stub lengths. Link budget and path loss adds up transmit power, antenna gains and losses against the free-space path loss and compares the result with your receiver's sensitivity. VSWR, return loss and mismatch loss converts between the four ways people describe a mismatch, either from one of them or straight from a load impedance R + jX.
It does not predict real-world coverage. Free space is the best case: buildings, terrain and fading only make it worse. It also gives no advice on legal power limits, which depend on your country and band.
Worked example
A LoRa sensor at 868 MHz should reach a gateway 2 km away. Both sides use 2.15 dBi dipoles with 0.5 dB of cable loss, the transmitter puts out 14 dBm, the receiver's datasheet promises −120 dBm, and you add 15 dB under Other losses for a wall and some trees. You want a 10 dB fade margin.
The tool reports a free-space path loss of 97.24 dB and an EIRP of 15.65 dBm. The receiver gets −94.94 dBm, a link margin of 25.06 dB, so the 10 dB fade margin is covered. With that margin kept in reserve, the free-space range is about 11.33 km; at 0 dB margin it would be 35.82 km. The first Fresnel zone at mid-path has a radius of 13.14 m, so keep at least 7.885 m around the line of sight clear.
For the same band, Antenna lengths at 433.92 MHz gives λ = 690.9 mm, a 328.2 mm dipole (164.1 mm per leg) and a 164.1 mm quarter-wave whip.
How to read the result
In link mode the Link budget table walks the signal from the transmitter to the receiver, one gain or loss per row, so you can see which line costs the most. Received power at other distances shows how the margin shrinks: each tenfold increase in distance costs 20 dB.
In antenna mode, the table lists each length with its basis; the classic 468/f rule is listed for comparison.
In mismatch mode, Return loss and VSWR describe the same thing. Mismatch loss is the power that never reaches the load: 0.51 dB at VSWR 2:1, as the reference table shows.
Tips and common mistakes
- Enter return loss as a positive number: 14 dB of return loss is a VSWR of 1.499:1.
- Use the sensitivity for the data rate you actually run. LoRa, Wi-Fi and BLE sensitivities change by 10 dB or more between rates.
- The end-effect factor depends on wire thickness and surroundings. Cut the element long and trim it while you watch the SWR minimum.
- Coax stubs use the cable's velocity factor, not the antenna's. Foam cable is near 0.8, solid PE 0.66.
- A margin of 20–30 dB in free space often shrinks to a few dB indoors. Put realistic losses under Other losses instead of trusting the free-space range.
FAQ
Why is the real range much shorter than the calculated range?
The calculation assumes free space with a clear line of sight. Ground reflection, obstacles in the Fresnel zone, fading and noise all reduce the range.
What end-effect factor should I use?
0.95 is the usual value for thin wire and matches the 468/f(MHz) feet rule. Thick tubing needs a smaller factor; trim on the bench in any case.
Does a VSWR of 2:1 lose half the power?
No. At 2:1 about 11 % of the power is reflected, a mismatch loss of 0.51 dB.
Can I calculate the mismatch of a complex antenna impedance?
Yes. Choose Load impedance R + jX and enter R, X and Z0; the tool shows the complex reflection coefficient too.
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