[TowerTalk] SWR is what SWR meter measures

[Martin, AA6E]

On 5/13/05, Ian White GM3SEK <g3sek at ifwtech.co.uk> wrote:
>That seems like sufficient logical proof that "forward power and
>reflected power" is faulty as a concept. Just because you can calculate
>these quantities, that doesn't mean they have any physical meaning at
>all.

I'd rather say that "impedance at the transmission line" and "forward
/ reverse / circulating power" are just different ways of analyzing
the same physical system.  Sometimes one is more useful that the
other.

Forward and reverse power exist in the sense that you can "catch" the
reverse power and use it to heat things.  Consider the "circulator"
which can be realized as a ferrite based device at microwaves.  It has
the property that all the Tx power you put in on one port goes out to
the antenna port (i.e. perfect match for the Tx) and all the power
received from the antenna port goes into the Rx port, if the Rx is
matched.  If you put one of these between the Tx and the mismatched
transmission line / antenna, you can convince yourself of the reality
of reflected power, because your load on the Rx port gets hot.  It
doesn't much depend on length of transmission line.  The typical SWR
bridge works this way, comparing power flow in the two directions.

You can analyze it all in terms of R+jX as seen at the various ports,
if that's what you prefer.  The numbers should come out the same.  The
complex impedance will depend critically on transmission line length,
while the "SWR" you measure should not.

73 Martin AA6E
-- 
martin.ewing at gmail.com
http://blog.aa6e.net