1---
2version: 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# draftply — 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. These are typical datasheet
14# figures, rounded — your cable's data sheet is the better source, and
15# batches vary by a few percent.
16# 1 RG-58 C/U 2 RG-8X 3 RG-213
17# 4 LMR-400 5 Aircom Plus 6 LMR-600
18att_100mhz = [16.0, 12.0, 7.0, 5.9, 4.8, 3.8]
19
20cable = 3 # ← pick a cable (index above)
21length_m = 30 # m of coax between rig and antenna
22freq = 14.2 # MHz
23p_in = 100 # W at the transmitter
24swr = 1.5 # SWR measured at the antenna feed point
25
26att_ref = att_100mhz[cable - 1]
27
28# ── Attenuation at your frequency ──────────────
29# Below ~500 MHz the loss of a coax is dominated by conductor (skin
30# effect) loss, which grows with the square root of the frequency.
31# Above that the dielectric adds a term proportional to f — for UHF and
32# up, read the value straight off the data sheet instead.
33att = att_ref * sqrt(freq / 100) # dB per 100 m
34loss_matched = att * length_m / 100 # dB, perfectly matched line
35
36"Cable attenuation (dB/100 m)": att
37"Matched line loss (dB)": loss_matched
38
39# ── What the SWR adds ──────────────────────────
40# Power reflected at the antenna travels back down the line and is
41# attenuated a second time. With the matched loss as a power ratio a,
42# total loss = 10·log((a² − Γ²) / (a·(1 − Γ²))).
43gamma = (swr - 1) / (swr + 1) # reflection coefficient
44a_ratio = 10^(loss_matched / 10)
45
46loss_total = 10 * log10((a_ratio^2 - gamma^2) / (a_ratio * (1 - gamma^2)))
47loss_extra = loss_total - loss_matched
48
49"Reflection coefficient Γ": gamma
50"Extra loss caused by the SWR (dB)": loss_extra
51"Total line loss (dB)": loss_total
52
53# ── Power ──────────────────────────────────────
54p_out = p_in * 10^(-loss_total / 10)
55p_lost = p_in - p_out
56efficiency = p_out / p_in * 100
57
58"Power reaching the antenna (W)": p_out
59"Power heating the coax (W)": p_lost
60"Efficiency (%)": efficiency
61
62# ── Rule of thumb ──────────────────────────────
63# 3 dB means half your power is gone in the feed line.
64len_3db = 300 / att
65"Length that costs 3 dB (m)": len_3db
66