Topband: more feedline data

[K9AY]

I discovered an error in my calculations of basic resistive feedline loss, and I am not afraid to admit it!  In the process of reviewing various references, I got the units wrong. Some formulas used frequency "f" in Hz, others in MHz. Murphy led to mix them up in my calculations.  Here is a summary of the feedline loss in the recent discussion, checked and double-checked. The pertinent formulas can be found in "Reference Data for Radio Engineers," 6th ed, ITT/Howard W. Sams, 1982, pp 6-4 to 6-8, and 24-13.

At 2 MHz, skin depth on a copper conductor is 0.005 cm, or 2 mils.  Thus, the entire conductor is not being utilized by the RF current flow and its effective resistance is greater than the DC resistance.  The AC resistance is calculated as 0.011 ohms/foot, which must be doubled to include both conductors of the transmission line.

Keeping with the familiar loss per 100 ft. used for most feedline specifications, the 2.2 ohms resistance in 100 feet of 450-ohm feedline results in a loss of 0.02 dB/100 feet at 2 MHz, or 0.028 dB/100 feet at 4 MHz.  1400 feet of such a feedline would have a total loss of 0.28 dB at 2 MHz.  

If the same wire was used at wider spacing in a 600 ohm line, the  2 MHz loss would be reduced to 0.015 dB/100 feet.

On this count, K8LV is correct -- the laws of physics insist that this is the lowest loss achievable.   And Maxwell's work has held up pretty well over the years...

On the issues of feedline radiation and low-loss transmission line transformers, my earlier comments stand as-is.  

With these numbers, the loss for 1400 feet of Tom's open-wire line should be in the range of 0.4 dB -- it would take 3" hardline or Heliax(R) and many $$$ to achieve this with coax.

73, Gary
K9AY