Something You Already Knew - Maybe?

"My Feed Line Tunes My Antenna"... This article was written in the March 1956 issue of QST by Byron Goodman, W1DX. It has been reprinted twice since then - once in 1977 and once in 1991 so it has some good insight that has lasted with time.

Here are my excerpts from this ... basically, main points for me to remember. I hope these bits are useful reminders. I have to read this about once a day to keep it fresh in my mind ...

When we say 50 ohm coax or 300 ohm twin-lead, we are talking about the characteristic impedance of the line. "Characteristic impedance of a transmission line is the value of resistance that when used as a termination for the line, makes the input impedance of the line independant of the electrical length of the line."

The simple view of this: A 50 ohm load (antenna or other resistance) on one end of your 50 ohm coax feed line will measure as a 50 ohm load at the other end of the line regardless of the actual length of the line. No tuner needed here because the load matches the characteristic impedance.

SO: The system is resonant and the whole load is resistance only.

But what if your antenna is measured as a 100 ohm load and you are using a 50 ohm feedline? You no longer match the characteristic impedance of the line. The impedance you measure at the end of the line will vary depending on the electrical length of the line.

The electrical length of the feedline is measured in wavelengths. To calculate the electrical length of the feed line, you must know the frequency, the actual length (in feet) and the velocity factor. So your physical feed line might be 75 feet but the electrical length changes as you change frequencies.

When the antenna load is different from the feed line characteristic impedance then the impedance we measure changes as we move down the line.

The Half Wave Rule to remember:
Every 1/2 wavelength down the line, the impedance we measure equals the antenna load and, obviously, this changes with frequency. And this load is resistance only.

The Quarter Wave Rule to remeber:
Every quarter wavelength, the impedance we measure is resistance only but the value is not equal to the antenna load.

Who Cares? Well, I guess I do. The reminder here is that the antenna load is really a reactive load. The antenna load is a combination of resistance plus capacitance (capacitor) OR resistance plus inductance (coil). The tuners we use help to bring the reactance of the load to pure resistance.