I do not think there is a "true" SWR meter which can work by measuring
things at only one point on a transmission line.
SWR is a ratio and can only be indicated by making a set up which responds
to it and even then only by empirical calibration - I think.
The average meter samples the voltage across the line,applies it to a
variable resistor in series with a switch.
It also indicates the current in the centre conductor by a parallel
conductor which has current by induction.
The switch can put these things in series addition or in series subtraction.
At that stage the meter indicates nothing meaningful.
To calibrate for say 50 ohms needs connecting a pure 50 ohm resistor as a
load and then adjusting the variable resistor so that the meter reads zero
when in the subtracting mode "reverse". .The voltage sample and the current
sample are obviously equal but opposite in phase. The meter zero needs
marking as SWR =1 .and meter fullscale marked as infinity. The meter itself
needs a potentiometer so that when in addition mode the needle can be set to
/full-scale and marked 'infinity'
Only now can the meter scale be marked. without any other calibrating
resistors. Because if i is the value of the equal currents at SWR 1 then
full scale = 2i=infinity
SWR 2 will be (2i-i)/(2i+i)= 1/3 scale
SWR5 will be (5-1)/(5+1) = 2/3 scale
SWR 1.5 will be 1/4 full-scale etc.
SWR 7 is at 3/4 scale
This is only true for a 50ohm line becausea 50 ohm resistor was used to set
it up.
The meter can be set up for a 72 ohm line by using a 72ohm dummy load and
adjusting the variable resitor across the line. The SWR reading at 3/4 scale
will always be correct at 7.
So the meter is only an indicator which starts by saying "when we know SWR
is 1 we set it to read 1 at meter-reading-zero and all other values can be
predicted and premarked on the scale.
This list always knows more than I do so if I have misunderstood the
question and taught you what you already knew --then ---Sorry!
G3GVB
----- Original Message -----
From: g3ldo <[email protected]>
To: <[email protected]>
Sent: Saturday, January 15, 2005 3:04 PM
Subject: LF: SWR-off topic
I am currently updating some of the antenna material in the RSGB Radio
Communications Handbook.
I have a question about SWR meters.
In my early days of amateur radio I didn't have an SWR meter. I obtained
some idea of antenna/feeder mismatch by using a multiple of half
wavelength
of coax and measuring the antenna feed Z using the Pi output capacitor
(suitably calibrated) of the transmitter.
At about the same time I was a RADAR technition in the RAF. When a
magnetron
was replaced on 3cm equipment (H2S) the SWR had to be measured using SWR
meter. This comprised a small red neon tube, the sides of which were
calibrated, like a thermometer, in SWR. As the tube was moved in a slot
along the waveguide the maximum and minimum voltage could be seen (VSWR).
The coax SWR meter can only detect voltage, current and phase. If you
replace the antenna with a 200ohm resistor it will register an SWR of 2:1
even though there is no transmission line. From this only conclusion I can
come to is that the standard SWR meter measures voltage, current and phase
from which SWR is implied.
So of all the radial scaled parameters to be found on the cursor of a
Smith
Chart why did someone settle for SWR?. Was a true SWR meter used in the
days
when radio engineers started feeding transmitters via long lengths of open
wire feeder and became the standard measurement of antenna/feeder
mismatch?
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
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