Question on Article

[ptheis]

In the article entitled "Tuning a 160M full sized vertical..." I have a question.
On page 5 in the paragraph under Fig 5 it says "so instead of resonating at 1830 KHz we should move it up to around 1900 KHz or above. Since I don't care about working over 1875 KHz, I used 1900 KHz as my target frequency to create a capacitive X at 1830 KHz."
Why should you move it up?
Tell me more about why you would "create a capacitive X"?
thanks for any assistance in figuring this all out.
I am enjoying the 4170 analyzer, just need to fill my head with more understanding.
Thanks,
Phil K3TUF

[wx0b]

Phil,
Thanks for your post and question. The reason we make the Inverted L or any vertical antenna short, resonating higher in frequency than the real desired frequency is to make it "capacitive" so it will have a reactance of -X at the desired frequency. So in my article I moved the resonance of the vertical to 1900KHz, it had a R of maybe 25 ohms and and X of 0 ohms at 1900 Khz let's say. Measuring it at the destination freq of 1830 KHz it may be more like 20 ohms and -25 ohms of reactance lets say. So now we have created a "phantom" capacitor at the feedpoint. This is a good thing since it is easy to just use a single inductive shunt inductor to match this impedance to R 50 ohms and X 0 ohms. In your case to match the antenna to the coax feed line of 50 ohms, you're going to have to employ a series capacitor and an inductor. Series capacitors in big antennas are always bad news, since you will need a large (read expensive) one to handle 1500 + watts if you use an amp, and they are fragile if you get a nearby lightning hit. Aways try to avoid series capacitors if possible in antenna systems for these reasons. And just by shortening your antenna wire a little you can enjoy a much easier matching system that also doubles as a static bleed discharging system.
Makes sense?
Jay