[Elecraft] Antenna Feedline Question

[N2EY at aol.com]

In a message dated 5/14/06 1:08:55 PM Eastern Daylight Time, 
rondec at easystreet.com writes:


> 40 meter 
> dipole that I will feed with 450 ohm 'window' line.  
> 
> 
> I am planning to run the 'window' line all the way into the shack to the K2
> 
> (which is about 5 or ten feet from the outside wall).  My question is should
> 
> I also use the BL1 at the K2 and will I have any trouble tuning to the other
> 
> bands. 

There's no magic answer to that question. It all depends on a bunch of 
factors, such as the length of the window line.


 I guess I don't understand what 4:1 means, in this case as I'll be 
> 
> using 450 ohm feedline to the K2.

In the case of the BL1, 4:1 means that the BK1 not only converts from a 
balanced line to an unbalanced line, but also changes the impedances by a factor of 
4. A 200 ohm impedance on the balanced side will be transformed to a 50 ohm 
impedance on the unbalanced side, etc.

However it should be remembered that baluns, even the BL1, are not without 
their limitations. If the impedance appearing at the shack end of the line is 
very high (thousands of ohms) or very low (a few ohms), or very reactive, the 
BL1 may not be   able to do the job very well.

A useful tool for situations like these is antenna modeling software. A good 
modeling package will give you an idea of how the antenna will work *before* 
you put it up. Note that no modeling package is perfect, and none can be any 
better than the information you give them. But they're a starting point.

For this specific application, I suggest downloading the program DIPOLE3 from 
here:

http://www.btinternet.com/~g4fgq.regp/page3.html#S301"

DIPOLE3 is a freeware DOS executable that will give you some info on how your 
proposed dipole will work. It won't tell you patterns or elevation angles, 
and won't store the data, but it will give you an idea of what the feedpoint 
impedances will be, SWR, and approximate system efficiency.

The only catch is that you have to enter the data in metric units. That's not 
really hard once you know the conversions, and you really only do it once for 
each antenna configuration.

Fool around with the values and get an idea of what goes on. 

73 de Jim, N2EY