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.

An SWR Meter is a Dangerous Thing!

The Feb 2009 issue of QST has an article titled "Keeping Current with Antenna Performance". This article gives a whole new perspective on using an RF Ammeter to determine if your system is resonant. This article states "An SWR meter is a dangerous thing in the wrong hands." It continues, "Many assumptions have to be made (not always valid ones) when one uses low SWR readings as an indication of maximum radiated antenna power".

That sort of catches your attention, doesn't it? It sure did mine. Now bear in mind, I am intrigued by this article but I'll need to do some pondering to absorb what is being said here. It seems simple enough and I'm trying to capture the main things that struck me in the article for this blog. Hopefully, you'll follow up and read the whole thing!

The author Eric Nichols, KL7AJ, points out "That is why in the broadcast industry the FCC has always required the direct power measurement technique for determining transmitted power. This is done by measuring radio frequency current at the antenna feed point, where the feed point resistance is known."

The key quote, for me anyway, is:
"For any given antenna -- that's an antenna with a fixed radiation resistance -- maximum current always results in maximum radiated power."

In the article, two homebrew devices are outlined to measure relative current and assist us with getting maximum current indication for our antenna / transmitter / feedline. (The RF ammeter must be placed somewhere along the feedline, i.e. after the tuner in the system.)

This article is too good to miss. More to come as I dig it out ... AR