Re: Help about R/X of a coax

by Andrew Tue Dec 28, 2021 7:04 pm

THANK YOU !!!

by **Ivan** Tue Dec 28, 2021 7:02 pm

Hi Andrew,
I got R' = 39,2 and X' = 18,2 as a result.

VBR from Ivan

by Andrew Tue Dec 28, 2021 4:11 pm

Ivan wrote:Hi Andrew,
yes, it is possible! You can use either quite complex formulae, or better a Smith chart or associated calculator.
You need to know the physical length of the cable and its velocity factor - therefore its electrical length. The losses of the cable run must be negligible, which is usually true on HF. Every half of wavelength the impedance is replicated, so the rest of the length is taken into the calculation if the cable is longer.
If I did not make a mistake, input the measured impedance into a Smith calculator or diagram and search the result for shift of 0,1818 wavelength towards the load.

VBR from Ivan

Hi Ivan, first of all, thank you for the infos, then... I'm no EE (I'm an IT engineer with a passion for electronics and radio) so, while I can understand and to some extent work with Smith charts, as I wrote they aren't my "cup of tea", in the case in exam, the cable lenght is 10 meters and the cable is a Belden 8267 (RG213/U) with a VF of 0.66, then the measured parameters at the shack end of those 10 meters of cable are R=76 and X=14 at a frequency of 3.6 MHz

Thank you again !

by **Ivan** Tue Dec 28, 2021 4:04 pm

Hi Andrew,
yes, it is possible! You can use either quite complex formulae, or better a Smith chart or associated calculator.
You need to know the physical length of the cable and its velocity factor - therefore its electrical length. The losses of the cable run must be negligible, which is usually true on HF. Every half of wavelength the impedance is replicated, so the rest of the length is taken into the calculation if the cable is longer.
If I did not make a mistake, input the measured impedance into a Smith calculator or diagram and search the result for shift of 0,1818 wavelength towards the load.

VBR from Ivan

by Andrew Tue Dec 28, 2021 10:05 am

Asking for help since this isn't my "cup of tea", here's the "problem"

Let's say we have a run of 10m of Belden 8267 (RG213/U) coaxial cable, connecting an analyzer at the shack end of the coax we obtain R=76 and X=14 at 3.6MHz of frequency

Now... being impossible (at the moment) to go up the roof and measure the values at the antenna, could we calculate the R and X at the antenna having the above infos in our hands ?

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