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Re: [TowerTalk] Balun Recommendation

Subject: Re: [TowerTalk] Balun Recommendation
Date: Fri, 20 Apr 2012 21:56:32 -0400 (EDT)
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Hi Ian and others:
Is common mode current the same as rf current flowing on the coax shield  
because it is connected to one half of the antenna?
If so, no wonder the feed point is unbalanced; there are two return current 
 paths for any voltage present on the shield side of the antenna. The "hot" 
side  of the antenna reflection sees the only the coax center conductor and 
associated  impedance. The other side of the antenna reflections sees the 
inside of the coax  and associated impedance in parallel with the outside of 
the coax and it's  associated impedance. The total impedance then has to be 
less than the "hot"  side. 
So, there is a balanced antenna feedpoint with say 50 ohms referenced to  
ground on one side and maybe 35 ohms on the other side reference to ground. 
The  voltage would have to divide 35/50 in that case.
Providing a means to reduce the current on the shield has to improve the  
voltage ratio on the two halves of the antenna as that makes the impedance  
referenced to ground more nearly equal. Series choke losses also have a 
parallel  equivalent that would tend to degrade the balance. So, it seems the 
choke can  help balance to a large degree but never completely restore balance.
Any corrections to this understanding are cheerfully accepted.
73, Gerald K5GW
In a message dated 4/20/2012 11:57:10 A.M. Central Daylight Time, writes:

Gerald  wrote:
>Hi All, at the risk of really showing  some ignorance, a question:
>If the choke is doing a good job  and located at the feedpoint, isn't the
>common mode circuit quite  short?
>Seems that the common mode current is the cause of the  feed point 
> rather than the imbalance causing the common mode  current. Chicken first
>or egg  first?

Neither; or  both.

The only parameters that can truly be labeled "first causes" of  antenna 
behavior are the physical dimensions of the antenna elements and  
feedline, their location relative to ground and other nearby objects,  
and the frequency and power level of the applied signal.

These are  the parameters that constrain the antenna's behavior according 
to the laws  of EM physics. It is no coincidence that these are also the 
inputs  required by an antenna modeling program, which then computes the  
one-and-only solution that will fit all the facts supplied.

Antenna  currents, feedline currents (both CM and differential) and all 
the related  voltages and impedances are all related parts of the 
antenna's behavior -  but they are all *consequences*, not causes.

Because these are all  *related* consequences, a change in one will be 
accompanied by a change in  all the others. But "chicken or egg" 
arguments are futile because none of  them has been the root cause of the 
changes we're talking  about.

So here we are, with an antenna system in which some  unwanted CM current 
is flowing on the feedline. If we now install a CM  choke, we are adding 
completely new constraint upon the system's behavior,  in the form of a 
high impedance at that particular location (which  modeling software 
would call a "load").

The key concept is that we  are deliberately forcing the ENTIRE 
antenna-feedline system to behave  differently from the way it did 
without the choke. Everything changes to  accommodate this new 
constraint, so NONE of the RF currents and voltages  will be the same as 

Many of the "chicken or egg" arguments  that we've seen in the past few 
days contain a hidden assumption that CM  voltages or currents will be 
the same both before and after the choke is  inserted. They won't! The 
only valid solution is to recompute the behavior  of the entire 
antenna-feedline system. Unfortunately that will only  produce an answer 
which is valid for a particular situation; but it also  explains why 
anecdotal reports can be so variable.


73 from Ian  GM3SEK

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