Name: SWR Lab — SWR Calculator, Impedance Matching & Balun Designer
Author: nrtechstudio.com

Question 1

How is SWR calculated?

Accepted Answer

SWR (standing wave ratio) is calculated from the reflection coefficient Γ (gamma) with SWR = (1 + Γ) / (1 − Γ). Γ itself comes from the load and line impedances: Γ = |Z_load − Z0| / |Z_load + Z0|. For example, a 100 Ω load on a 50 Ω line gives Γ = 50/150 = 0.333, so SWR = 1.333/0.667 = 2.0 : 1. Enter any one value — SWR, return loss, or Γ — in the converter above and the other two are computed instantly.

Question 2

What is the difference between SWR and VSWR?

Accepted Answer

There is no practical difference: VSWR (voltage standing wave ratio) and SWR name the same quantity — the ratio of maximum to minimum RF voltage along the line. SWR is the shorter, common name; VSWR is the formal one. Both are computed with SWR = (1 + Γ) / (1 − Γ), and both are written as a ratio like 1.5 : 1.

Question 3

What is return loss and how does it relate to SWR?

Accepted Answer

Return loss (RL) is another way to express the same mismatch, in decibels: RL = −20 × log10(Γ). A perfect 1 : 1 match has infinite return loss; SWR 1.5 : 1 equals 13.98 dB return loss; SWR 2 : 1 equals 9.54 dB. Larger return loss means a better match. Note the sign convention: return loss is quoted as a positive number of dB.

Question 4

What is the reflection coefficient (Γ)?

Accepted Answer

The reflection coefficient Γ is the fraction of the forward wave's voltage reflected by a mismatch: Γ = (SWR − 1) / (SWR + 1), ranging from 0 (perfect match) to 1 (total reflection). Γ² gives the fraction of power reflected — at SWR 2 : 1, Γ = 0.333, so 11.1% of the power is reflected back toward the transmitter.

Question 5

What SWR is considered good or acceptable?

Accepted Answer

SWR 1.0–1.5 : 1 is excellent (under 4% reflected power). SWR 1.5–2 : 1 is good and fine for most operation (4–11% reflected). SWR 2–3 : 1 is usable but worth investigating, especially at high power. Above 3 : 1, reduce power or fix the antenna before transmitting — 25% of your power is reflected and some transmitters will fold back or overheat. The mismatch loss between 1 : 1 and 2 : 1 is only about 0.5 dB, so chasing perfection below 1.5 : 1 rarely matters.

Question 6

What is mismatch loss?

Accepted Answer

Mismatch loss is the power lost purely because of the SWR, expressed in dB: mismatch loss = −10 × log10(1 − Γ²). At SWR 1.5 : 1 it is 0.18 dB; at 2 : 1 it is 0.51 dB; at 3 : 1 it is 1.25 dB. It is the unavoidable part of the loss — on top of it you still pay the coax's own attenuation, which also grows because the reflected wave travels the line twice.

Question 7

How does SWR affect coax cable loss?

Accepted Answer

A mismatched line loses more than its matched rating. The reflected wave travels down and back, suffering the cable's attenuation twice. The exact result comes from the transmission-line equation: delivered fraction = a × (1 − Γ²) / (1 − Γ² × a²), where a is the cable's one-way power gain. For example, 100 ft of LMR-400 at 144 MHz (1.5 dB matched) delivers 70.8 W of 100 W at 1 : 1, but only 66.6 W with SWR 2 : 1 at the antenna. Use mode 2 above to compute your exact case.

Question 8

What is an L-network and when should I use one?

Accepted Answer

An L-network is the simplest impedance matcher: two reactive components (one series, one shunt) arranged in an 'L'. Use it when you need to match two resistive impedances at a single frequency — for example 50 Ω coax to a 200 Ω antenna feedpoint. It cannot match over a wide band (bandwidth ≈ f / Q, where Q = √(Rmax/Rmin − 1)), and it assumes the load reactance is cancelled first, which the calculator above does automatically.

Question 9

Series-first or shunt-first in an L-network?

Accepted Answer

It depends on which resistance is larger. If the load resistance is higher than the source (e.g., 200 Ω load, 50 Ω source), the series element goes on the source side and the shunt element on the load side. If the source resistance is higher, the arrangement mirrors: series element adjacent to the load, shunt adjacent to the source. Each arrangement has a low-pass version (series L + shunt C, which also filters harmonics) and a high-pass version (series C + shunt L).

Question 10

How do you calculate the number of turns on a toroid?

Accepted Answer

Use the core's AL value in nH/N² (nH per turn², straight from the manufacturer's datasheet): turns N = √(L_target(µH) × 1000 / AL), or equivalently L(µH) = AL × N² / 1000. For example, 100 µH on an FT-140-43 (AL = 885) needs √(100 × 1000 / 885) ≈ 10.6 → 11 turns. AL values vary ±20% between batches, so enter your datasheet's value in the designer above and verify the finished coil with an analyzer.

Question 11

What is the difference between an ugly balun and a ferrite choke balun?

Accepted Answer

An 'ugly balun' is an air-wound coil of coax (typically 6–12 turns on a 4–6.5 inch form) that chokes common-mode current with pure inductance. It works well on a single band but, as G3TXQ's measurements showed, one air-wound coil cannot cover all of HF effectively. A ferrite choke balun winds the coax through a ferrite toroid (mix 31/43/61), which adds resistive absorption that works over multiple bands — the better choice for multiband antennas.

Question 12

How many turns do I need on an FT-240-31 choke?

Accepted Answer

For HF (160–10 m), wind 12–14 turns of coax through an FT-240-31 core; for mix 43 use 10–12 turns, and for mix 61 use 8–10 turns. An FT-240 core handles up to about 1 kW SSB, an FT-140 about 200 W SSB — use one FT-240 per 500 W as a rule of thumb. These figures follow the guidance published with the 73qrz choke calculator; derate by 50% for digital modes because their 100% duty cycle heats ferrite far more than SSB.

Question 13

What is the difference between mix 31, 43 and 61 ferrite?

Accepted Answer

Mix 31 (MnZn, µi ≈ 1500) is the best choice for low-band choking (160–40 m) and handles heat well. Mix 43 (NiZn, µi ≈ 850) is the all-round choice for 80–10 m and is best above ~30 MHz. Mix 61 (NiZn, µi ≈ 125) has the lowest loss and is preferred for broadband transformers up to 30 MHz. For a common-mode choke, the material should be more resistive than reactive at your operating frequency — that is why 31 beats 43 on the low bands.

Question 14

How do I wind a 49:1 transformer for an end-fed half-wave antenna?

Accepted Answer

A 49:1 impedance transformer matches a ~2450 Ω end-fed half-wave feedpoint to 50 Ω coax. Wind the primary with enough turns that its magnetizing reactance is at least 4 × 50 Ω at your lowest band (use the toroid designer above), then wind the secondary with 7× the primary turns (turns ratio √49 = 7). Use mix 43 or 61 on an FT-140 (≤200 W) or FT-240 (≤1 kW) core, keep leads short, and always add a 1:1 common-mode choke on the coax — the coax shield otherwise becomes the antenna's counterpoise.

Question 15

Is it safe to transmit into a high SWR?

Accepted Answer

Treat high SWR as a warning, not a catastrophe — but respect it. Most modern transceivers fold back power above ~2–3 : 1 to protect their finals; older or amplifier stages can overheat or arc. Never touch or adjust an antenna while transmitting, stay within your license's power limits, and observe RF exposure rules for your country. Ferrite core power ratings are approximate: halve them for digital modes (FT8/RTTY run 100% duty cycle). This tool gives planning estimates — always verify with an SWR meter or antenna analyzer before operating.

Question 16

Does this tool send my data anywhere?

Accepted Answer

No. The entire SWR Lab runs in your browser with JavaScript — no account, no uploads, no tracking requests. The only network requests the page ever makes are loading the page itself from nrtechstudio.com (and the schema.org context identifier inside the structured data, which is not a fetch). Your inputs never leave your device.

SWR ↔ return loss ↔ reflection coefficient converter

SWR

Return loss

Γ

Reflected power

Delivered power

Mismatch loss

Coax loss calculator — SWR adjusted

Matched loss

Total loss (with SWR)

Power at antenna

SWR at the radio

L-network impedance matching calculator

Network Q

≈ Bandwidth

Reactance cancel

Toroid turns designer

Primary turns

Inductance

Secondary turns

Magnetizing check

Choke / ugly-balun advisor

Inductance

Choking X

Verdict

Classic ugly-balun table (RG-213)

Turns

Bands

Power

Mismatch loss calculator

Mismatch loss

Reflected

Delivered

Return loss

Γ

Formulas & references

Frequently asked questions

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