Great post Bob, patient, thoughtful, and to the POINT. i.e. people do your homework before jumping into the 'fray'. There are lots of 'us' guys out there willing to help, but it is your responsibility to do your 'own' homework FIRST. That advice being, READ the original question and then go from there.
Mike C.
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On 2/19/2022 7:40 AM, Bob Ecclestone VK2ZRE wrote:
As happens so often in these threads, we need to go back to the original post and read the question.
Kevin was puzzled as to why his S11 results did not show a distinct "short" when he tested his 300 Ohm ladderline connected directly to his NanoVNA unbalanced 50 Ohm S11 port.
He posted the sweep results in that original post..
Some folks offered possible reasons, others possible work arounds. But no one explained to him why he got the results he did. No one ANSWERED his original question.
I am not going to dispute any of the subsequent comments made regarding direct connections, common mode chokes or anything else posted here.
I offered one possible solution using a broadband impedance matching transformer because it is quite possible that Kevin is actually building an antenna being fed with 300 Ohm ladderline and that by building the transformer, he could verify his stub and then go on to test the completed antenna. As the system is now impedance matched back to 50 Ohms, his S11 results on the complete antenna system would now better reflect (pun intended) the actual system performance.
If folk are not comfortable winding their own transformers, there are bare transformers and packaged units readily available on the web.
If you are comfortable winding your own, it is almost as easy to wind a 300/50 matching transformer as it is a CM choke.
The difference is, you now have a piece of test kit specific to the task at hand.
The task in this case centres around 300 Ohm ladderline.
I do not know why Kevin got the results he did originally, but I am confident that if he uses a matching transformer, the results he gets next time should be closer to what he expects.
And as a final comment, the ladderline stub should be tested and trimmed well clear of any thing as even moisture on a non conducting pole will be enough to add distributed capacitance across the line and affect the apparent Velocity Factor. Metal, including other cables, in close proximity is a definite no no.
Which brings me to offer a possible REASON for Kevin's results.
He states he had 5.175m of "JSC 1320" cable. A quick check of the specs for 1320 ladderline gives a VF of 0.82
Therefore the ELECTRICAL wavelength of the line is 4(5.175/0.82) = 25.24m. This gives a frequency of 300/25.24 = 11.89MHz
Since Kevin only started his sweep at 12MHz, he did not sweep the actual stub frequency of 11.89MHz.
If the ladderline was lying on the ground or close to any metal, the "apparent" VF would be even lower and the electrical length and thus quarterwave frequency would be even lower again.
Looking at the Smith Chart, we can see it is approaching a zero Z but does not quite make it. Why? Because he should have started the sweep at a lower frequency.
That's my theory anyway.
So it pays to go back and read the entire original post.
73...Bob VK2ZRE
Mike C.