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
Back to my projects page