[Elecraft] KX3 with BL2 Question

[Don Wilhelm]

Rich,

It is a common misconception that the characteristic impedance of the 
feedline is what you will have to match.  That is only true for resonant 
antennas.  The feedpoint impedance of a dipole in free space is 72 ohms, 
but when mounted at practical heights above ground, the feedpoint 
impedance will drop.  For most HF antennas, 50 ohms is a better choice.

The other point is - yes, a balun is a good thing.  For parallel 
feedline, the balun should be at the junction of the coax and the 
parallel feedline to keep the antenna and feedline currents opposite and 
equal (the required condition for balanced), and to keep common mode 
current to a minimum.

For non-resonant antennas, the feedpoint impedance of the line at the 
transmitter can vary from very low to very high.  It all depends on the 
length of the feedline, the length of the radiator and the frequency.
In many cases, feeding an antenna with 450 ohm feedline, a 1:1 balun 
will work better than a 4:1 balun.
You must match the feedpoint impedance, and that is not the 
characteristic impedance of the line.

BTW, zip cord is quite lossy at RF, and should only be used in 
situations where nothing else is available.
Your example will work, but it is a one band antenna.  The main reason 
for using parallel feedline is to be able to use the antenna on multiple 
bands.

73,
Don W3FPR

On 10/4/2017 10:35 AM, rich hurd WC3T wrote:
> Would not a bnc-to-banana plug adapter hooked to a doublet made from 
> 100' of zip cord be equally useful?   Is there even a reason to have a 
> balun in-circuit?   I've read the characteristic impedance of the zip 
> cord (or speaker wire, etc.) is about 75 ohms.  If you "unzipped" 67 
> feet or so and put on a tie wrap (to avoid further unzipping), you'd 
> have a quite serviceable 40 meter dipole with an SWR of 1.5:1 or so.
>