On Wed, May 14, 2025 at 06:50 AM, Team-SIM SIM-Mode wrote:
Here My message about RG213 coax measurements posted here on Avril 5 2025:
for My RG213 cable (25m length) loaded by a 50.3 Ohm resistor ,
I used the centered impedances circle methode on smith graph with the
renormalized Z0 impedance ( option added by DiSlord) for different ferquency's
band (span always fixed to 4 Mhz) :
2Mhz ---> Zc = 52.6 Ohm
3Mhz ---> Zc = 52.5 Ohm
7Mhz ---> Zc = 52.0 Ohm
14Mhz ---> Zc = 53.0 Ohm
18Mhz ---> Zc = 53.0 Ohm
21Mhz ---> Zc = 54.0 Ohm
24Mhz ---> Zc = 54.0 Ohm
29Mhz ---> Zc = 52.0 Ohm
50Mhz ---> Zc = 49.0 Ohm
100Mhz ---> Zc = 43.5 Ohm
Your measurements do not agree with the calculated values for typical RG213. The characteristic impedance of Belden coax is around 50.6 ohms at 1 MHz and slowly decreases to about 50 ohms at 100 MHz. You can see that in the attached graph which is based on the parameters for this type of cable. It decreases because the inductance is decreasing with frequency.
The problem with your test method is that you will not have a pure resistance at the end of the cable as the frequency is increased. Any resistor (including SMD) will have some series inductance and there will also be capacitance across the resistance. The reactance associated with these components will be small at low frequencies but will be considerable at your highest measured frequency of 100 MHz. This is particularly true if you try to use any kind of small adjustable potentiometer for your test.
The end result is that your measurements will not be correct because you don't have a pure resistance load to base your measurements on.
Roger Need
