Monday, July 1, 2019
Versatile 300 ohm slotted ribbon
300 ohm slotted TV ribbon feedline is versatile stuff for the antenna experimenter.
Some uses include:
* Folded dipoles. Instead of having two wires and making up spacers, TV ribbon makes the job easy. Folded dipoles are used in some beam antenna designs. And some dipole users like them for their continuous loop configuration. Because the impedance is four times that of a regular dipole (288 vs 72 ohm) they can be connected straight to another piece of 300 ohm slotted ribbon as the feedline.
* Feedlines. Slotted ribbon is lighter than coaxial cable. Its loss is lower. Its impedance is higher. That can make it suitable for feeding tuned feeder doublets in conjunction with a suitable balanced antenna coupling unit. That allows you to get on multiple bands without lossy traps or messy multiple dipole elements.
* Phase shift. Some antenna designs that drive multiple elements (eg ZL Special) use certain lengths of twisted 300 ohm slotted ribbon to provide a phase shift and enable gain and directivity.
* Stubs. Certain antennas like collinears use 300 ohm ribbon feeder as stubs. Even more well-known is the G5RV, variants of which use about ten metres of ribbon feedline before connecting with coaxial cable via a balun.
* Loading. I have seen certain types of top loaded vertical antenna use 300 ohm slotted feedline at the top. This is mechanically easier than using separate wires and spacers.
The big question then is where do you get it. Coaxial cable replaced ribbon from about the 1970s on TV antenna installations. Ribbon became an indoors feedline only. It went from black to pale (nicer indoors, apparently) and lost its slots. That's bad for loss. Although you could cut slots out with a hobby knife.
Dedicated ham shops sometimes sell 450 ohm ladder type. That's bigger, heavier and often stiffer than ribbon. But it might still be useful in applications where not being 300 ohm is not a problem.
The main reason for this post then is twofold: 1. Alert you to the versatility of 300 ohm feedline and 2. To give you a hint - if you see any for sale - eg maybe a house demolition sale or a hamfest - then snap it up!
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