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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