[Elecraft] Baluns and 450 ohm line

[Ron D'Eau Claire]

Bill wrote:

> Uh, Ron, I'd like to point out that at HF, the vast majority of the loss
> in open wire or coaxial lines is resistive in nature.

I disagree, based on what I read in many books on  antenna and transmission
line design. Resistive losses are part of the loss, but not the "majority"
in coaxial line especially. To quote just one reference most Hams have, the
ARRL Antenna Book, "..the loss characteristics of coaxial cable depend
largely on the dielectric material between the conductors..."

> All feedlines exhibit higher losses when feeding a load that does not
> match their characteristic impedance. These losses are due to reflections
> traversing the line.

Right. But the increase in losses is very small when the line has low
inherent losses. Because the losses of good low-loss open wire line increase
so slowly as the SWR goes up, the total loss of good open wire line at high
SWR's is typically less than good coax line operated at an SWR of 1:1

For example, the SWR of 100 feet of open wire line at an SWR of 20:1 at 30
MHz will be 0.9 dB total.  Good quality RG-8 looking into that same 20:1 SWR
will show a loss of over 5 dB. Even if the RG-8 is perfectly matched for
minimum loss, 100 feet of it will show a greater loss than the open wire
line at about 1.0 dB at 30 MHz..

> The reason that open wire has such low loss is that a) its conductors are
> large, usually 14 guage or larger, and b) its impedance is high, which
> leads to very small I^2R losses for the matched condition.

The design information for open wire lines that I have (again, one example
is the ARRL Antenna Book) do not even factor in the size of the wire used in
evaluating line loss - only the dielectric qualities of the insulation and
the amount of insulation used to hold the wires.

The conductor sizes do affect the "characteristic impedance" of the line.
That is, the impedance of the circuit it must be used in if the SWR is to be
1:1. When we speak of "450-ohm ladder line" or "300 ohm twinlead" we are
speaking of the "characteristic impedance" of the line. That has little real
relationship to the impedance of the line in a real antenna. The impedance
on either of those lines can vary from a fraction of an ohm to over a
thousand ohms when connected to an antenna where the impedance of the
antenna does not match the "characteristic impedance" of the feed line. It
is the actual impedance extremes found in the system in which the feed line
is used, not the characteristic impedance of the feed line, that will
determine the resistive losses.

Ron AC7AC
K2 # 1289