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Hi Pete,
thank you for your interest in this. My European coax cable type "Aircell 7" is not common in the USA. So it was not listed within SimSmith's coax table. In my case length measuring was easy. I even used and compared two methods: 1. By normal length measuring of the cable. That - in this case - was easy, as the coax cable was mostly accessible(except some house entering feed through) laying on the ground in the garden. 2. For comparison - I also did this: In my shack I had a sample of exactly the same coax from the very roll I had cut the antenna feed cable from. I determined the length of this sample, too, by normal length measuring. Then I measured by TDR measurement both lengths again with my AA-600's TDR function using in TDR an assumed VF = 1. Comparing the length results of the two methods made me find the cable Vf (as well as electrical and mechanical length of my cables). In SimSmith I used Vf and length as determined above, and I entered these in the simple cable model. The attenuation I did not measure (as I actually should have ), but I used the manufacturer's specs instead. This was my first such antenne impedance determination from the rig side end in the shack. The result was satisfactory, but I think, my accuracy could have been improved by exact attenuation measurement instead of just using manufacturer's specs. But in principle, that worked quite nicely. Never before was I able to know my vertical 40m endfed dipole's impedance over frequency. Now I have a good antenna feedpoint .s1p file. There are no exact endfed data available. In my special case the antenna hangs in a birch tree, that drinks many buckets of water each day. Further, as indicated in the video, interrupting the stub match for measurement would have changed the total system and would have made results useless. So the impedance at the dipole end was uncertain, until I did the easy rig end measurement and used SimSmith, as explained in the video. My vertical with it's all new stub match adjustment profile used is further described in the attached powerpoint presentation verticals.ppt . The profile was used for stub length and distance determination by easy adjustment of the two length without any coax cut-and-try over and over again. 73, Hans DJ7BA -----Urspr¨¹ngliche Nachricht----- Von: [email protected] <[email protected]> Im Auftrag von WB2UAQ Gesendet: Donnerstag, 9. Januar 2020 01:43 An: [email protected] Betreff: Re: [nanovna-users] Measuring resonance from coax far end. Hans Your suggestion would work if you know how long the transmission line is in wavelengths. How can you simply measure the impedance at the shack end and know if the impedance you measured is due to the antenna's impedance or the rotation around constant SWR circle? Another way to look at it is, you measure the Z at the rig end and plot this point on the Smith Chart. Now to find the antenna's Z, you rotate around the chart by the electrical length between the rig end and the antenna (towards the load). Another way to do it is to measure the Z at the rig end and use a program such as TLW (comes with the ARRL Ant Book) to calculate the antennas Z BUT again you need to know the electrical length of the transmission line (or if you know transmission line type and it is in TLW's library of cables, you can enter the physical length). 73, Pete, WB2UAQ |
