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Old 1-Sep-2012, 5:21 PM   #14
GroundUrMast's Avatar
Join Date: Oct 2010
Location: Greater Seattle Area
Posts: 4,773
Originally Posted by thom View Post
No kidding, a TV-2900 would do the job?

By 'matching impedances' do you mean between the two antennas? How does one go about that?
I attempted to describe that in post #10 of this thread...

An important concept to read up on... develop an understanding of is: A section of transmission line (coax, twin-lead, etc.) that's 1/4 of a wave length long can act as an impedance transformer.

Knowing the actual impedance of the antenna element is critical to the design process. However, the nominal impedance of a folded dipole in free space is about 277Ω... pretty close to 300Ω. If I connected two of those elements in parallel with no transformation of impedance, the combined impedance would be roughly 150Ω. But, if I use a 1/4 wave long section of transmission line with a characteristic impedance of about 450Ω, a 300Ω antenna element can be made to look like a 600Ω source/load. (Two 600Ω impedance's in parallel have a net impedance of 300Ω.)

Antennas that have balanced two terminal outputs (ex. Antennacraft Y5713) have a nominal impedance of 300Ω. 4:1 balun/matching transformers (ex. TV-2900) transform the impedance down to 75Ω (300/4=75).

Forgive me if I fail to condense a year and a half of college into a few paragraphs.
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