[Elecraft] Baluns and 450 ohm line

[Bill Coleman]

On 6/4/02 11:59 AM, Ron D'Eau Claire at rondec@easystreet.com wrote:

>The second approach to minimizing feeder losses is to use an inherently low
>loss feedline such as true open wire feedline. True open wire feedline has
>almost all "air" insulation. "Air" is the lowest loss dielectric/insulator
>available. The minimal insulation that does exist to hold the wires the
>proper distance apart is of very high quality in good line.

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. 

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.

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.

>With this sort
>of feedline you do not need to care about the SWR. The highest SWR an
>antenna can produce in a line hundreds of feet long on, say, 10 meters, will
>not have signfiicant losses.

Actually, SWR does matter in open wire lines. Particularly if the SWR is 
quite high, the losses due to reflections can mount up considerably. 
There are cases where an antenna that would closely match a quality 50 
ohm coaxial cable will show more loss when connected directly to open 
wire line, due to the high SWR.

Traversing a 12 guage conductor ONCE is going to have less loss than 
traversing it 20 or more times.

>So-called "window line" and even "twinlead" are compromise "open wire"
>lines. They have a lot of insulation, so their losses are much higher than
>true open wire line. Still, in most high-swr applications they will have far
>lower losses than coax.

These feedlines have higher losses usually because a) they use smaller 
conductors than open wire, and b) they have lower characteristic 
impedances, which tends to increase the I^2R losses for the matched 
condition.

>The r-f field around each wire is still
>trapped in the dielectric between the center conductor and the shield in
>each coaxal line. That's where the losses occur, but it also makes the
>balance less critical since there won't be any radiation or pickup from the
>line because of the shielding.

Again, most of the losses in HF are resistive. Coax is more lossy in part 
because the center conductor is rather small.

>With true open wire line, these concerns about keeping a moderate SWR  to
>avoid excessive losses do not exist. 

Actually, they do exist. However, the very low matched loss of open wire 
lines tends to keep the losses due to reflections down until the SWR 
reaches very high levels.


Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901