ANTENNA FEEDPOINT IMPEDANCE ?
QUESTION: Where do we measure the antenna's feedpoint impedance?
ANSWER: At the antenna's feedpoint.
PROBLEM:
My arms are too short.
I have been a ham radio operator for 60 years.
I have been interested in antenna technology for 59 years, 364 days.
I have never been able to measure an antenna's true feedpoint impedance.
(Exception: End-Fed Half-Wavelength antennas)
FACT: Unless the antenna's impedance is purely resistive and a perfect match to the impedance of the feedline (i.e., 50 Ohms), the impedance along the coax varies due to transmission line transformation.
In theory, the exact impedance at the feedpoint repeats itself every [electrical] half wavelength along the coax. However, rarely is the length of coax running back to the shack exactly an electrical half wavelength long. And even if it is, it is only true for one frequency.
THEREFORE: Measuring the impedance/SWR at any point along the coax does not show the feedpoint impedance, unless it happens to be exactly a half wavelength away from the feedpoint.
FACT: When measuring SWR we are usually measuring the impedance/SWR of the "Antenna System", not the feedpoint impedance/SWR.
EXAMPLE: My 80m OCFD Antenna
The antenna covers all of the ham bands in the HF spectrum. The question is, how well does it work on each of the bands?
Until now, the best I could do is connect an 80m electrical half wavelength of coax to the feedpoint and measure.
Using that transmission line, I got the SWR curves shown on the right.
Better than nothing, but really only close to being accurate on the even harmonic bands.
