I had just posted a comment when I received you email Ian. I think you
have summed things up pretty effectively.
The discussion has proceeded without the benefit of a set of equations
that would have be a reference point for what was being changed between
the ideas of one writer and then next.
Still, there is some opportunity to get solutions that apply to most cases
[not the pathological ones of course]. We just have to be sure we are
using the same parameters and weights for them.
Tod, K0TO
On 4/20/12 10:55 AM, "Ian White GM3SEK" <gm3sek@ifwtech.co.uk> wrote:
>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
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