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Splitting a cophased feedline.

Apr 3rd 2016, 06:56

AF5SX

Joined: Mar 7th 2013, 06:18
Total Topics: 0
Total Posts: 0
First, the set-up I am about to describe and question is in a Class 8 truck (18 wheeler).

I have always been told that if I start with a 50 ohm cophased feedline and split it at the junction I will end up with two 75 ohm lines.

My question (I just can't seem to wrap my head around the fact that, IF the feed line is constructed of two sections of 50 ohm line, when I split it, each side becomes 75 ohm), how and why does each side become 75 ohm instead of two, shorter, 50 ohm lines?
Apr 3rd 2016, 13:01

AA6E

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The simple analysis is that if you have two 75 ohm lines (resistively terminated -- not coupled to each other) and you connect them in parallel, your radio "sees" 37.5 ohms. In reality, the two antennas do couple to each other, so it's hard to predict exactly what the net impedance will be. Somebody is probably just giving you their rule of thumb. In the ham world, equipment wants to connect to a 50 ohm load, but any impedance between 25 and 100 ohms can give you under 2:1 SWR, which is often good enough.

I wonder why truckers go to the trouble of dual whips. Is it for more gain (which you'd only get in certain directions) or for a more symmetric antenna pattern? Or maybe the style? :-)

73 Martin AA6E

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