An antenna question--43 ft vertical
Jeff wrote:
On 09/07/2015 17:03, Ralph Mowery wrote:
"Jeff" wrote in message
...
By 'apparent SWR' he means as indicated SWR on the meter, and yes it can
change at various point on the line due to inadequacies in the meter;
the
'real' VSWR will of course remain the same at any point on a lossless
line.
Jeff
That is what I mean Jeff. If there is any SWR, by changing the length of
the line, the voltage/current changes in such a maner that at certain
points
you may get a 50 ohm match at that point.
Absolutely NOT. By changing the length of a transmission you will NEVER
create the situation where you get a 50 ohm match from an initial
mismatch.
This is clearly demonstrable on a Smith chart. Take any starting point
other than a pure 50 ohms and add a length of transmission line. What you
will find is that as you increase the length of line your point will
merely rotate around the chart at a fixed radius (known as a constant VSWR
circle), it will never spiral into the centre which is 50 ohms and where
it must be for a perfect match.
The only time that it will start to spiral inwards is if the line is
lossy, but you will need a very long length, and the spiralling inwards is
due to the loss in the coax NOT any matching characteristics due to the
length of line.
If such an effect as you are talking about is observed it is merely due to
the finite, and often poor, directivity of the SWR meter giving you a
false reading.
Also it is worth noting that achieving 50 ohms as a magnitude |Z| of the
complex impedance (Sqrt(R^2+X^2)) is not the same as getting a good match
with 50 ohms resistive. Even if |Z| = 50 ohms it will have a VSWR greater
than 1 if Z0. Again, plot the point on a Smith chart and you will see
that it can never be in the centre of the chart.
Jeff
That is easy to disprove Jeff.
If I have a 50 ohm load and use a 1/2 wave of any impedance line other than
50 ohms, the swr will be greater than 1:1, except at 1/2 wave multiplies of
the line. At this point there will be a 50 ohm match. The swr of the
line will not actually be 1:1 but some greater value.
That is correct, but not the situation that we are discussing, we are
talking about matching a load to a 50 ohm transmission line. In that
case changing the length of line will NEVER result in a match. Using a
*different impedance* length of coax as a transmission line transformer
is a totally different case, and as you say will result is a standing
wave on the line and associated losses.
Jeff
So you are only interested in special cases?
--
Jim Pennino
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