Coax loss calculator: SWR at the radio vs antenna

How much of your power reaches the antenna, and why the SWR on your radio looks better than the SWR at the antenna.

Matched loss from the maker's data

Every cable here comes from its maker's datasheet, linked below. Times Microwave publishes a formula for its LMR cables, A = k₁√f + k₂f in dB per 100 ft with f in MHz, and the calculator uses it from 1 MHz to the top of the sheet. Belden, Messi & Paoloni, SSB-Electronic and Draka publish tables; the calculator goes straight through each printed point and follows a power law between them. A two-term fit was tried and dropped: on several sheets it missed the low-HF points by 20 to 30 %.

Below a maker's first printed point the loss is extended from the table and marked. Outside the range a maker gives, you get no number at all.

Matched loss in dB per 100 ft, from the maker's data
Cable	3.6 MHz	14.2 MHz	28.4 MHz	50 MHz	146 MHz	440 MHz	1,296 MHz
LMR-195	0.68	1.35	1.92	2.55	4.38	7.69	13.46
LMR-240	0.46	0.92	1.30	1.73	2.97	5.22	9.14
LMR-400	0.23	0.46	0.66	0.88	1.52	2.68	4.74
LMR-600	0.14	0.29	0.41	0.55	0.95	1.70	3.06
RG-58/U (Belden 8240)	0.62	1.32	1.87	2.50	4.70	8.89	No data from the maker at this frequency
RG-58A/U (Belden 8259)	0.83	1.83	2.69	3.70	6.74	13.17	No data from the maker at this frequency
RG-8X (Belden 9258)	0.55	1.08	1.56	2.10	3.80	6.97	No data from the maker at this frequency
RG-213/U (Belden 8267)	0.33	0.66	0.96	1.30	2.30	4.43	9.62
9913 (Belden, RG-8 type)	0.36*	0.58	0.78	1.00	1.61	2.75	No data from the maker at this frequency
Hyperflex 10	0.31	0.46	0.61	0.82	1.44	2.65	4.69
Hyperflex 5	0.58	0.92	1.26	1.68	2.95	5.24	9.30
Airborne 10	0.25	0.40	0.58	0.73	1.29	2.34	4.15
Aircell 7	0.40*	0.75	1.02	1.31	2.22	3.98	7.19
Ecoflex 10	0.19*	0.42	0.60	0.81	1.43	2.60	4.78
Ecoflex 15	0.15*	0.31	0.45	0.60	1.04	1.88	3.47
H155	0.56*	1.08	1.51	1.98	3.44	6.09	10.85

* Below the maker's first printed point, extended from the table. A dot: no data from the maker at this frequency.

Why the radio shows a lower SWR than the antenna

Power reflected at the antenna travels back down the coax and loses on the way, just like the forward power. The radio sees a smaller reflection than the antenna produces:

ρ at the radio = ρ at the antenna / a, with a = 10^(matched loss / 10)

A lossy run can make a poor antenna look fine. If your meter is at the radio, choose Radio above to see the SWR the antenna really has. When a reading at the radio is higher than the line could ever show, the calculator says so: the reading or the cable data is off.

SWR 3.0 at the antenna, 100′ 0″ of cable at 28.4 MHz
Cable	Matched loss, dB	Total loss, dB	SWR at the radio
RG-58A/U (Belden 8259)	2.69	3.62	1.74
RG-8X (Belden 9258)	1.56	2.24	2.07
RG-213/U (Belden 8267)	0.96	1.45	2.34
LMR-400	0.66	1.02	2.51
Ecoflex 15	0.45	0.71	2.64

What the SWR costs

Total loss with a mismatch is

TL = 10 log[(a² − ρ²) / (a (1 − ρ²))]

On a short, low-loss line a high SWR costs little: the reflected power goes back to the antenna and most of it is radiated. On a long or thin line at VHF the same SWR can cost decibels. A line with no loss loses nothing, whatever the SWR.

Sources

Calculations are aids; go by the rules in force and your own measurement.

Checked against the sources on November 2, 2026.

Coax loss and SWR calculator

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