Talk:450 Ohm Ladder Line

calculate antenna size

When creating an HF antenna they say (they being experienced hams) that you should add about 2' extra to compensate for the inaccuracy of the magic number "468." According to the ARRL Antenna Book, the "end effect" due to the attachment of insulators at the ends of the antenna results in the approximately 5% reduction in length from the free-space 492/f to 468/f. The percentage varies slightly with different installation. Height of the antenna above the ground has a significant impact. Start with an estimated value based on a more realistic formula such as 490/f that results in a small amount of extra wire for attaching insulators. During tuning, twist the wire connections together or use clamps, then raise the antenna into position and measure. When it’s right, only then solder and weatherproof the connections.

source: http://www.eham.net/articles/23802

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300 ohm TV lead-in is about 1/2 - 1 dB per 100 ft at 30MHz.

Heavy gauge Window line is about half the loss of 300 ohm line, or .25 to .5dB 100ft at 30MHz.

Real ladder line from 12-14 ga conductors is about half of ladder line.

Most of the loss is in the copper losses.

I don't know why more people don't use 300 ohm line. Probably power handling at high SWR, plus lines with solid dielectrics bridging large areas of the spacing are very prone to showing greatly increased loss when wet.

Window line (commonly miscalled ladder line)and twinlead both have poor wet weather performance, real open wire ladder line hardly changes when wet.

Balance lines (i.e. 300 ohm TV :twin lead"; 450 window/ladder line and true open wire line) generally require some understanding of the principals related to their use. Line lenghts can become an issue and one needs to understand what changes may be necessary to fix "problems".

Popular lengths for 80-10 meter dipoles are 130'(the "McCoy" dipole), 105'(G5RV), or 88'(W4RNL, visit his website for his reasoning on this length).