# Actual Antenna SWR

Hi folks! In Antenna We Trust! All others things are secondary.

Below is one little, easy for calculate formula with two tables, which could be a wake-up call for you. Don't over-trust the goodlooking indication of your precious SWR-meter, which is situated at your worktable. Because the SWR value at the input of antenna cable is not a actual SWR of your favorite "WONDER-antenna".

Description:
You can calculate it yourself what is "the simple truth" with following simplified formula:

where:
SWRa = actual SWR at antenna input (cable far end)
b = coefficient vs. actual cable attenuation (in dB)
c = coefficient vs. checked SWR (SWRc) at cable input (measuring point)

Table #1

Values of "b" vs. actual cable attenuation in dBs:

(At right side of the "b"-columns are shown approx. Cable Length for two typical cables at 145 MHz)

dB
b
RG-213/U
RG-58/U
dB
b
RG-213/U
RG-58/U
0
1
0 m
0 m
4.5
0.3548
56.25 m
19.15 m
0.5
0.8912
6.25 m
2.12 m
5.0
0.3162
62.5 m
21.2 m
1.0
0.7943
12.5 m
4.25 m
6.0
0.2511
75.0 m
25.53 m
1.5
0.7079
18.75 m
6.38 m
7.0
0.1995
87.5 m
29.78 m
2.0
0.6309
25.0 m
8.5 m
8.0
0.1584
100.0 m
34.04 m
2.5
0.5623
31.25 m
10.64 m
9.0
0.1258
112.5 m
38.3 m
3.0
0.501
37.5 m
12.76 m
10.0
0.1
125.0 m
42.55 m
3.5
0.4466
43.75 m
14.89 m
11.0
0.0794
137.5 m
46.8 m
4.0
0.3981
50.0 m
17.02 m
12.0
0.063
150.0 m
51.0 m

Table#2

Values of "c" vs. checked SWR at cable input (measuring point).

SWRc
c
SWRc
c
1.0
0.0
1.8
0.2857
1.1
0.0476
1.9
0.3103
1.2
0.0909
2.0
0.3333
1.3
0.1304
2.5
0.4285
1.4
0.1666
3.0
0.5
1.5
0.2
4.0
0.6
1.6
0.2307
5.0
0.6666
1.7
0.2592
10.0
0.8181

Notes:
* The cable attenuation in dBs and SWR at antenna input are actual values vs. actual frequency and all others (bad)influenced factors.
* For determinating of the antenna cable attenuation, you can use both programms: "Antenna Cable Loss and SWR" or "Antenna Cable Loss and SWR Calculator" on the address: www.qsl.net/lz1len/projects.html

Examples:
1) dBs=0dB (b=1); SWRc=1 (c=0); SWRa= 1+0 / 1-0 = 1 (Border case, the Dream)

2) dBs=2.5dB (b=0.5623); SWRc=1.5 (c=0.2); SWRa= 0.5623+0.2 / 0.5623-0.2 = 0.7623 / 0.3623 = 2.1

3) dBs=3dB (b=0.501); SWRc=1.8 (c=0.2857); SWRa= 0.501+0.2875 / 0.501-0.2875 =0.7867 / 0.2153 = 3.65

4) dBs=3.5dB (b=0.4466); SWRc=1.8 (c=0.2857); SWRa= 0.4466+0.2857 / 0.4466-0.2857 = 0.7323 / 0.1609 = 4.55

5) Stop! Don't panic! A 5 to 1 SWR does'nt hurt anything :)

Have fun, a small SWR and 73 de Peter, LZ1LEN, www.qsl.net/lz1len

 Enter your search terms Submit search form

Ham radio Data Center - free schematics | Free HAM Directory | About me | Acronyms  | CW | Data Sheets | Docs | Download | E-mail | HOME | Ham projects | Hobby circuits | Photo galery | PIC | QTH photos |