Message:20224 In: TECH.WW
From: G8MNYDate: Sat, 04 Apr 26 08:52:00 Z
Newsgroups: TECH.WW
Subject: Actual Antenna SWR
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
>From Peter LZ1LEN
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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:
b + c
SWRa = -------
b - c
where:
SWRa = actual SWR at antenna input (cable far end)
b = coefficient vs.actual cable attenuation in dB
c = coefficient vs.checked SWR (SWRc) @ cable input(measuring point)
Ó
ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ Ú Cable with well-known(in dBs)Loss Ó
³ TRX oÄÄÄÄo SWR oÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄoÄÄÓ Antenna
ÀÄÄÄÄÄÙ ÀÄÄÄÄÄÙ\ checked SWRc / Ó
actual SWRa / Ó
Table #1
--------
Values of "b" vs. actual cable attenuation in dB:
Remark: At right side are shown approx. Cable Length
for 2 typical cables @ 145MHz
----------------------------------------
| dBs | b | RG-213/U | RG-58/U |
----------------------------------------
0.0 1.0 0.0 m 0.0 m
0.5 0.8912 6.25 m 2.12 m
1.0 0.7943 12.5 m 4.25 m
1.5 0.7079 18.75 m 6.38 m
2.0 0.6309 25.0 m 8.5 m
2.5 0.5623 31.25 m 10.64 m
3.0 0.501 37.5 m 12.76 m
3.5 0.4466 43.75 m 14.89 m
4.0 0.3981 50.0 m 17.02 m
5.0 0.3162 62.5 m 21.2 m
6.0 0.2511 75.0 m 25.53 m
7.0 0.1995 87.5 m 29.78 m
8.0 0.1584 100.0 m 34.04 m
9.0 0.1258 112.5 m 38.3 m
10.0 0.1 125.0 m 42.55 m
11.0 0.0794 137.5 m 46.8 m
12.0 0.063 150.0 m 51.0 m
----------------------------------------
Table #2
--------
Values of "c" vs. checked SWR @ cable input (measuring point)
----------------------------------------
| SWRc | c | | SWRc | c |
----------------------------------------
| 1.0 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 dB & SWR at antenna are actual values vs. actual
frequency & all others (bad)influenced factors.
** For determinating of the antenna cable attenuation, you can use both
programms "Antenna Cable Loss & SWR" or "Antenna Cable Loss & SWR Calculator"
on the address: http://www.qsl.net/lz1len/projects.html
Examples:
--------- 1+0
1) dBs=0dB (b=1); SWRc=1 (c=0); SWRa= ----- = 1 (Border case, the Dream)
1-0
0.5623+0.2 0.7623
2) dBs=2.5dB (b=0.5623); SWRc=1.5 (c=0.2); SWRa= ---------- = ------ = 2.1
0.5623-0.2 0.3623
0.501+0.2875 0.7867
3) dBs=3dB(b=0.501); SWRc=1.8(c=0.2857); SWRa= ------------ = ------ = 3.65
0.501-0.2875 0.2153
0.4466+0.2857 0.7323
4) dBs=3.5dB(b=0.4466); SWRc=1.8(c=0.2857); SWRa=------------- = ------ = 4.55
0.4466-0.2857 0.1609
5) Stop! Don't panic! A 5 to 1 SWR does'nt hurt anything :)
Have fun, a small SWR & 73 de Peter, LZ1LEN lz1len@qsl.net
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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