Martin, AA6E wrote:
> On 5/13/05, Ian White GM3SEK <g3sek@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
Here is another way to look at it: With a circulator at the transmitter
output and a load on the return port of the circulator, disconnect the
antenna line from the circulator. Now all the power out of the
transmitter goes into the load on the circulator and the transmitter
still sees a flat 50 ohms.
Is there really reflected power involved or does the power get dumped
into the load because of an impedance mismatch at the antenna port on
the circulator?
We could look at reflected power not even existing on transmission lines
at all. In fact we could look at it as once the transmission line is
initially charged at transmitter turn on there are no more reflections.
What we see are only impedance mismatches that our meters are showing
us. As others have pointed out the swr bridge or watt meter is not
really showing power but the indication is the result of an impedance
mismatch from the value that the meter was calibrated for.
73
Gary K4FMX
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|