Post by ad5vs on Jul 21, 2014 22:41:37 GMT -5
I have a short length of RG58 coax from my rig to an Amidon 4:1 balun. I have ladder line from the balun out to my OCF multiband dipole. I wanted to determine the Z0 of the ladder line so I performed a custom cal at the balun to ladder line terminals with the ladder line disconnected of course. When I performed the short circuit step I got a warning that the impedance was about 2400 ohms but was allowed to continue. I got this warning again when I terminated across the terminals with a 200 ohm resistor. After the cal I ran a scan (50 ohm Zref) with the 200 ohm resistor still connected. SWR was 4 across the ham bands as expected and reactance was very near zero. As far as I could tell the cal looked good at this point.
I then installed a 200 ohm resistor on the far end of the ladder line which is about 60 to 65 feet long, and reterminated it to the balun output terminals. The resulting scan was perplexing. There were numerous resonances within my 3 to 30 Mhz scan range (sine wave phase angle plot with large peak to peak values). Zmag was off scale (greater than 2000 ohms) at many points on the plot with more reasonable values as well at some frequencies. SWR referenced to 50 ohms also varied widely from values as low as 2 to values greater than 10. I expected much flatter plots and finally decided the scan data was suspect. I also tried a TDR and Zmag was again off scale high except for a few "valleys" here and there. I was careful to keep the ladder line clear of the ground and away from any conductors. Do the warnings I got during the custom cal raise any red flags? Please advise on anything I may be overlooking. Is grounding on the balun important?
I then installed a 200 ohm resistor on the far end of the ladder line which is about 60 to 65 feet long, and reterminated it to the balun output terminals. The resulting scan was perplexing. There were numerous resonances within my 3 to 30 Mhz scan range (sine wave phase angle plot with large peak to peak values). Zmag was off scale (greater than 2000 ohms) at many points on the plot with more reasonable values as well at some frequencies. SWR referenced to 50 ohms also varied widely from values as low as 2 to values greater than 10. I expected much flatter plots and finally decided the scan data was suspect. I also tried a TDR and Zmag was again off scale high except for a few "valleys" here and there. I was careful to keep the ladder line clear of the ground and away from any conductors. Do the warnings I got during the custom cal raise any red flags? Please advise on anything I may be overlooking. Is grounding on the balun important?
