[Elecraft] Balun with a G5RV?

[Bill Coleman]

On 4/24/02 10:54 AM, Ron D' Eau Claire at rondec@easystreet.com wrote:

>> Most of the losses in coax (or open wire, for that matter) are resistive.
>> Increased losses at higher SWR are largely due to reflections making
>> multiple trips across the resistive loss.
>>
>> Bill Coleman, AA4LR,
>
>If so, then:
>
>Why does open wire line at high SWR's have only a tiny fraction of the loss
>of coax even though it may use the same or smaller conductor sizes as
>coaxial line?

Look carefully at the tables for transmission line losses in the ARRL 
handbook. Note the losses in the table are for the MATCHED condition. A 
450 ohm transmission line matches to a 450 ohm load. Because of this, 
significantly less current flows in a 450 ohm transmission line, leading 
to lessor I^2R losses. 

For coax, transmission line impedances are lower. Plus, the center 
conductor of most coax is generally very small compared to the conductors 
used in open wire. So, the I^2R losses are higher.

Dielectric losses aren't non-existant, they just don't play a big role at 
HF.

So much for the matched condition. There's another chart in the ARRL 
handbook that shows the losses induced by SWR. These losses relate to the 
losses in the matched condition. Open wire gets a head start since it has 
lower loss when matched. If the load impedance is much higher than 50 
ohms, open wire will likely show a more favorable SWR than coax, which 
leads to lower overall losses. If the load impedance is 50 ohms, or 
lower, than the increased losses due to reflections may make coax a 
better transmission line than open wire, in this scenario.

>Why does the loss in various types of parallel conductor lines (open wire,
>ladder line, twinlead, etc.) increase in direct proportion to the amount of
>insulation between the wires?

Three reasons: a) smaller conductors (more R), b) smaller impedances 
(more I), c) more dielectric losses. The change in impedance has probably 
more to do with the increase in losses than the conductor size or the 
dielectric losses (although the presence of a dielectric does plays a 
role in lowering the impednace).



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