1---
2app_version: 0.1.0
3precision: 3
4form_title: Coax Cable Loss
5form_description: How much of your power actually reaches the antenna? Pick a cable, enter length, frequency and SWR — get matched loss, the extra loss the SWR costs you, and the power left at the feed point.
6form_inputs: Cable|cable[RG-58 C/U:1,RG-8X:2,RG-213:3,LMR-400:4,Aircom Plus:5,LMR-600:6], Length (m)|length_m, Frequency (MHz)|freq, Power in (W)|p_in, SWR at the antenna|swr
7form_outputs: Loss matched (dB)|loss_matched, Extra loss from SWR (dB)|loss_extra, Total loss (dB)|loss_total, Power at antenna (W)|p_out, Efficiency (%)|efficiency, Max length for 3 dB (m)|len_3db
8---
9# Coax Cable Loss
10// "How much of my 100 W actually reaches the antenna?"
11
12# Cable
13// Attenuation in dB per 100 m at 100 MHz, typical datasheet figures, rounded
14// — your cable's data sheet is the better source, and batches vary by a few
15// percent. LMR-400 and LMR-600 come from the Times Microwave formulas
16// (0.1223·√f + 0.00026·f and 0.0756·√f + 0.00026·f, dB per 100 ft, converted);
17// Aircom Plus is scaled from its 144 MHz value (≈ 4.5 dB/100 m).
18// 1 RG-58 C/U 2 RG-8X 3 RG-213
19// 4 LMR-400 5 Aircom Plus 6 LMR-600
20att_100mhz = [16.0, 12.0, 7.0, 4.1, 3.8, 2.6]
21
22cable = 3 // ← pick a cable (index above)
23length_m = 30 // m of coax between rig and antenna
24freq = 14.2 // MHz
25p_in = 100 // W at the transmitter
26swr = 1.5 // SWR measured at the antenna feed point
27
28att_ref = att_100mhz[cable - 1]
29
30# Attenuation at your frequency
31// Below ~500 MHz the loss of a coax is dominated by conductor (skin
32// effect) loss, which grows with the square root of the frequency.
33// Above that the dielectric adds a term proportional to f — for UHF and
34// up, read the value straight off the data sheet instead.
35att = att_ref * sqrt(freq / 100) // dB per 100 m
36loss_matched = att * length_m / 100 // dB, perfectly matched line
37
38"Cable attenuation (dB/100 m)": att
39"Matched line loss (dB)": loss_matched
40
41# What the SWR adds
42// Power reflected at the antenna travels back down the line and is
43// attenuated a second time. With the matched loss as a power ratio a,
44// total loss = 10·log((a² − Γ²) / (a·(1 − Γ²))).
45gamma = (swr - 1) / (swr + 1) // reflection coefficient
46a_ratio = 10^(loss_matched / 10)
47
48loss_total = 10 * log10((a_ratio^2 - gamma^2) / (a_ratio * (1 - gamma^2)))
49loss_extra = loss_total - loss_matched
50
51"Reflection coefficient Γ": gamma
52"Extra loss caused by the SWR (dB)": loss_extra
53"Total line loss (dB)": loss_total
54
55# Power
56p_out = p_in * 10^(-loss_total / 10)
57p_lost = p_in - p_out
58efficiency = p_out / p_in * 100
59
60"Power reaching the antenna (W)": p_out
61"Power heating the coax (W)": p_lost
62"Efficiency (%)": efficiency
63
64# Rule of thumb
65// 3 dB means half your power is gone in the feed line.
66len_3db = 300 / att
67"Length that costs 3 dB (m)": len_3db
68