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Axial Ratio (CBR Antenna)

Axial Ratio (CBR Antenna)

Axial Ratio (CBR Antenna)

(OP)
thread247-93327: Axial Ratio

The following statement was posted on 6 Oct 2004 in the referenced thread: "In FM and TV broadcast transmit antennas, the cavity-backed radiator has about the lowest (true) c-pol axial ratio, as installed."

In support of that statement, the attached gif shows the measured, h-plane axial ratio of such an antenna. The fine ripple shown on the pattern is a measure of the true, c-pol axial ratio of that design, and is produced when the antenna/tower section under test are rotated slowly on its vertical axis while recording the output voltage of the antenna operating in receive mode.  The transmit antenna used as the source during the measurement was a rapidly rotating, linear dipole.

http://i62.photobucket.com/albums/h85/rfry-100/CBR_Axial_Ratio.gif

 

RE: Axial Ratio (CBR Antenna)

A single data point (1.5 dB) fails to prove that it's the lowest. You need to go back and test every other possible type of antenna, including those that haven't yet been invented.

winky smile !!!!

 

RE: Axial Ratio (CBR Antenna)

(OP)
True, even though I wrote "about" the lowest.  However in my 40+ years of broadcast industry experience the statement is true for FM broadcast transmit arrays installed on large cross-section towers.

The most common types of side-mounted, "c-pol" FM broadcast transmit antennas can have axial ratios of 10 dB and more, as shown in paper 6 at http://rfry.org , and proven by their real-world performance.

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