We are all told that VSWR doesn't matter when using low losstransmission lines, since the RF energy will travel from thetransmitter up to the mismatched antenna, where a certain amount ofthis RF energy will reflect back towards the transmitter; after whichthe RF will then reflect back up to the antenna -- where the energy iseventually radiated after bouncing back and forth between thetransmitter and antenna. I understand the concept, but what I don'tquite understand is why the reflected RF energy isn't simply absorbedby the 50 ohm output of the transmitter after the first reflection?For the RF to bounce back and forth, wouldn't the transmitter'simpedance have to be very, very high (or low) when the reflected RFenergy hit its output stages? I know I'm missing something vitalhere...Thanks!-Bill

That's assuming you use an antenna tuner. The tuner will transform the transmitter's output impedance* just as it transforms the line. Were the transmitter output impedance actually at 50 ohms, on the other side of the tuner it would have the same VSWR as the line when everything was tuned up.Having said that, the VSWR _does_ matter somewhat when using low loss lines, both because the line loss is low but not zero, and the tuner loss will tend to go up as you correct for higher and higher VSWR.* I am _not_ going to start the Big Transmitter Output Impedance Debate. sed denizens -- just don't comment on what a transmitter's "actual" output impedance may be, lest you start a flame war.-- Tim WescottWescott Design ServicesPosting from Google? See "Applied Control Theory for Embedded Systems" came out in April.See details at

VSWR doesn't matter?

Mar 12, 2007 61 Replies

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Richard Fry 19 years ago

"joseph2k" wrote

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Analog TV ghosts can be produced within the TV transmit antenna system as well as by reflections of the transmitted signal in the propagation environment. I know this from my experience as an RCA Broadcast Field Engineer, because I've evaluated and corrected many transmit antenna systems that had been the source of such ghosts.

For example, a reflection from a mismatch between a 1,000 foot long, air-dielectric transmission line and the TV transmit antenna connected to it produces a ghost with ~ 2 µs delay from the main image. The active scan width of an NTSC TV line is about 53 µs, so 2/53 = ~4% of the width of the screen, or maybe 5% counting overscan. This ghost is easy to see in a typical TV set/viewing setup.

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Michael A. Terrell 19 years ago

Did you ever work on a TTU-1 or TTU-25B?

Service to my country? Been there, Done that, and I\'ve got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida

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