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
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The article is right in that an SWR meter near the radio lumps the feedline and antenna into the same boat.
ReplyDeleteBad feedline attenuates both ways. That is, it eats up your transmitter's power and eats up the reflected power that is supposed to be telling you that you have issues.
On a different note, multiple SWR meters (or at least multiple power meters to save money) would be a good thing. I'd be real curious how much power comes out of my rig and tuner. I kinda wonder about these "autotuners" everyone has these days.
RADIO --> meter --> tuner --> meter --> antenna.
This seems like a good idea to me.
Good question. Almost everyone has a "tuned antenna system" and relies on a tuner to make a max power transfer. Without some "dials" to let you know what is going on, it is a "trust" issue, I guess. I use my power meter on my radio to tell me what the input to the tuner should be. Then I use the tuner to tell me what I think is going into the feedline.
ReplyDelete