RadioBanter - View Single Post - An antenna question--43 ft vertical

Ian Jackson[_2_] · July 10th 15, 07:37 PM posted to rec.radio.amateur.antenna

In message ,
writes
Jeff wrote:
On 09/07/2015 18:35, wrote:
Jeff wrote:

Can you measure VSWR on a 1 meter long Lecher line at 1 MHz?

VSWR is not meaningful in such a situation, however, you can measure
return loss and Reflection Coefficient etc.. Of course that in not to
say that VSWR is not used in situations where it is not appropriate in
order to indicate how good a match is, when RL or Reflection Coefficient
would be more appropriate.

Jeff

Jeff

Are you trying to say that VSWR is not meaningfull at 160M (to put it
in an Amateur context)?

For those that don't know, a Lecher wire is just a carefully contructed,
rigid parallel transmission line upon which one would slide a high
impedance sensor to find voltage minimum, maximum, and where they
occured. That and a Smith chart were used to solve transmission line
and impedance matching problems and were often home built by Amateurs
in the early VHF days.

Today you would use a VNA (Vector Network Analyzer).

Unless you have a very long feeder at 160m you cannot have a complete
voltage maxima and minima from the standing wave on the line so VSWR is
meaningless. That is not to say that you cannot calculate an 'effective'
VSWR from other quantities such as return loss, S11, by measuring the
forward and reflected signals as you would with a Network Analyser or
SWR bridge.

Jeff

Nope, VSWR is always meaningful and you have the cart before the horse.

VSWR is a consequence of an impedance match and standing waves are
a consequence of a VSWR greater than 1:1 on a transmission line.

Attach a SWR meter directly to the output of YOUR transmitter and a
1 Ohm resistor directly to the other end of the SWR meter.

The meter reading will be the same as the calculated value, there will
be no standing waves as there is no transmission line, but the results
WILL be meaninful to your transmitter.

Even when the only transmission line consists the output connector of
the SWR meter, and maybe an inch of internal coax, there will still BE a
standing wave - but it will only be a tiny portion of longer one.

However, surely an SWR meter is really measuring the ratio of the go and
return signals - ie the RLR? If so, would it not end all this
essentially esoteric argument if we called it an RLR meter?

--
Ian