[TowerTalk] 75 or 70 Ohm twinlead or ladderline cable - does it exist?

[Steve Hunt]

I made some further measurements this morning on a length of 10A 
Figure-8 power cable typical of the sort I've seen UK Hams press into 
service as RF feedline; the dominance of the dielectric losses was even 
more apparent!

The cable comprised 28x0.19mm copper strands inside a 3mm diameter PVC 
jacket. Zo was measured as 130 Ohms and Vf as 0.66.

These were the Copper losses, Dielectric losses and Total matched losses 
per 100ft derived from the measurements:

1.8MHz: 0.2dB, 0.15dB, 0.35dB
3.5MHz: 0.3dB, 0.3dB, 0.6dB
7.0MHz: 0.4dB, 0.6dB, 1.0dB
14MHz: 0.6dB, 1.1dB, 1.7dB
28MHz: 0.8dB, 2.2dB, 3.0dB
50MHz: 1dB, 4dB, 5dB

As you can see, the dielectric losses exceeded the copper losses at any 
frequency above 3.5MHz!

Steve G3TXQ