Introduction.To measure the cable loss, you should disconnect the other end of the cable from the antenna - for an open line system condition.
Yet it is necessary to know what is the exact cable length in meters.
Introduction.
To measure the cable loss, you should disconnect the other end of the cable from the antenna - for an open line system condition.
Yet it is necessary to know what is the exact cable length in meters.
1st (ground) case. Cable Loss from SWR
For instance SWR=3 at cable length of 30 meters, we get 10 dB/100m.
In normal case, the typical values for 145 MHz is as follows:
- for RG-58C/U = 23.5 dB/100m ; for RG-213/U = 8.0 dB/100m
2nd case. SWR from the Cable Loss
If you want to know what the checked SWR should be for a length of cable in meters of which you know the loss in decibels per 100 meters (the OEM's data) :
For instance, 30m cable at 145 MHz should give an SWR of :
- for a quality 23,5dB/100m (RG-58C/U); SWR= (10^0.705 + 1) / (10^0.705 - 1) = 6.07 / 4.07 = 1.49
- for a quality 8dB/100m (RG-213/U); SWR= (10^0.24 + 1) / (10^0.24 - 1) = 2.74 / 0.74 = 3.7
( If the cable is open (or short) circuited at the far end, and if the cable is still in order...hi! )
Hint: The summertime is the best season for antennas 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....") :-))
A successful study and 73 de Peter, LZ1LEN
