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home built 300 or 450 ohm

Mar 3rd 2015, 17:41

0001582470H80

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
We stumbled onto several rolls of common CATV coax with wire shield and want to build 300 ohm and 450 ohm line with it. This feed line will be inside a steel building with 48 feet to the exit of the feed line .With velocity factor and db losses as common coax use , can we convert this stuff to shielded 300 or 450ohm and expect the same performance as what is sold on the shelves ? We are using it on 160m , 80m , and 40m sky loops.
Mar 3rd 2015, 18:20

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
No, shielded twin lead will have the same loss as the coax it is made up. The impedance will double--this may allow lower losses if this reduces the SWR on the line--or make it higher if the SWR increases.

Zack W1VT
ARRL Senior Lab Engineer
Mar 5th 2015, 16:51

0001582470H80

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thanks, we will strip the shield and use the center conductor for other antenna projects.Since, it was bound for a landfill, I will put it to use.
May 11th 2016, 23:51

W7SX

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The loss is primarily due to dielectric loss. When using two coaxial lines, and the shields are grounded, then only 1/2 of the voltage will appear in each line (assuming same VSWR and power), lowering the losses considerably. Thus you must compute the maximum voltages for both situations by calculating the VSWR's with both configurations. If the loss is too high, you can still eliminate the ground completely and use the braiding for conductors and build your open wire line accordingly...this will be an excellent low loss line.

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