# 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) - from table #1
c = coefficient vs. checked SWR (SWRc) at cable input (measuring point) - from table #2
Important hint: Don't use arbitrary(wrong) values for SWR and Cable Attenuation. In real life "c" always is lower or equal too to "b". When c = b, SWR = infinity. If "c" is greater than "b", the result is negative (imaginary) value.

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 programs: "Antenna Cable Loss and SWR" or "Antenna Cable Loss and SWR Calculator" on the address: www.qsl.net/lz1len/projects.html
*** QTC: For an automatic calculation of the actual antenna SWR you can use too the our new program
"Actual Antenna SWR Calculator" on the same address.

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 :)

! Last of all, one reminder:
The summertime is the best season for antenna and feeder lines maintenance!
(A suitable quatation for this case from the "QUA UARL"-Bulletin, OnAir's "pearls" by UY5XE: "...this, that the winter already has arrived, I understood, when drove on my ass from the roof..." )
(In Russian: Эфирные "перлы": "...то, что зима уже настала, я понял, когда съехал на заду с крыши...") :-))

Have fun, a small SWR and 73 de Peter, LZ1LEN

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