[TowerTalk] Balun Recommendation

[Ian White GM3SEK]

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 imbalance
> 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 
before.

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
http://www.ifwtech.co.uk/g3sek