Topband: Chokes-Ferrite vs. air

[Tom Rauch]

Hi Barry and all,

Interesting topic..

> Guessing at the number of turns in an air wound choke isn't necessary.
> Calculating the size is easy. Whether a choke is wire-wound (cored or
> air), coaxial wound, or "beaded", it should have a reactance of about 4
> times the resistance (or impedance) of the load, at the lowest frequency
> of use. So, for 50 ohms we need a choke reactance of about 200 ohms. 

The choke impedance required actually depends on the common 
mode impedance of the system at the point where the choke is 
installed, and how sensitive the system is to problems. 
Unfortunately this has no relationship at all to the differential mode 
impedance (normal transmission line mode) of the line.

One interesting thing is moving the balun to the input of a floating 
tuner does not make the job of balancing the system any easier, 
unless the tuner is a ground referenced split-voltage output tuner! 
Moving the balun on "floating networks", like a simple T, H, or Pi 
(that are floated from ground) only changes the differential mode 
stress on the balun, which usually isn't a problem anyway with 
choke baluns! Many articles miss that point, so obviously this is a 
commonly misunderstood topic.

There are cases where a 50 ohm feedpoint can require a balun of 
many k-ohm impedance, and other cases where a hundred ohms 
are more than enough. There are even situations where adding a 
balun can make things worse. 

We can model the antenna and hang a wire from the shield 
connection point, and look at the current in that wire. If we place 
put a load on that wire, we can observe the effects of adding an 
impedance.
 
For example a dipole at 290 feet with a vertical feedline grounded at 
the bottom, modelled that way with one amp of source current has 
0.4 ampere feedline common mode current. A 500 ohm choke 
reduces that to .07A..which might be OK. 

Now if I simply lower the very same antenna to 230 feet and leave 
the 500 ohm "choke balun" in line, the peak feedline current goes 
up to .26 A. If I REMOVE the "balun" feedline current drops under  
0.1 ampere!

We have to be very careful what we do. There are no simple 
answers.  

73, Tom W8JI


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