It also depends on the line loss per unit length. A high-loss feedline
will be more forgiving as measured, but less energy is transferred to
the load. If a line is very low-loss per length, you can tune out the
reactance or mismatch (by transformation or direct compensation) without
losing much energy. High loss feedline just burns up the energy in the
feedline and the load can't radiate very much. In this case, a good SWR
"match" won't help get a lot of signal on the air. This is one reason
that source-end SWR isn't a good measure of radiation efficiency (not
considering the actual antenna). I can get 1:1 on a good 50 ohm load,
just like anyone else, and no one can hear my 1kW transmitter.
The SWR varies along the length of all feedlines, but at low SWRs it
doesn't make much difference on energy transferred from source to the
load. In most cases 2:1 is perfectly acceptable to the transmitter's
protection circuitry.
Vic is also correct about diodes; their I-V curve is nonlinear. Ar low
power levels, this can matter a lot on precision of the SWR measurement.
73,
matt W6NIA
On 11/4/2016 9:10 PM, Vic Rosenthal 4X6GP wrote:
I have always understood that the SWR on a line will be almost the
same wherever it is measured (it will increase slightly as you
approach the load due to losses). It is the impedance of the load as
transformed by the line that changes.
One thing that I've noticed is that SWR meters often differ when the
SWR is low, because the amount of reflected power is very small, and
diodes are often nonlinear dealing with small signals. The difference
between an SWR of 1.1:1 and 1.2:1 is a very tiny amount of reflacted
power.
73,
Vic, 4X6GP
Rehovot, Israel
Formerly K2VCO
http://www.qsl.net/k2vco/
On 5 Nov 2016 04:43, Ron D'Eau Claire wrote:
How much coax is between the K3 and P3 monitor? Only a few feet will
create
a growing error as the SWR increases, since the standing waves on the
coaxial line produce different readings at different points along the
line.
If you want to compare the SWR meters in the K3 and P3, be sure the P3
sensor is mounted directly on the K3 without a length of coax in
between.
Even so, the K3 SWR sensor will be "looking" at the P3 sensor through
the
KAT3. Although it is bypassed, the older KAT3 ATUs still routed RF
through
them on its way to the K3 rear-panel ANT connector.
Even so, as Fred pointed out, there will be differences since SWR
meters are
no "precision" devices because they don't have to be. The K3 is quite
happy
at load SWRs up to 2 or 2.5:1 and even above that the amplifier will
protect
itself.
73, Ron AC7AC
On 11/4/2016 6:59 PM, dm...@nexicom.net wrote:
I recently installed the TX monitor in my P3 and have a discrepancy
with the SWR reading between it and the K3
with a dummy load connected the P3 and K3 read 1.12 and 1.0
respectively a level of error that is to me quite acceptable.
however when connected to an antenna I will see the SWR on the K3
often close to 1:1 when the P3 will show up to a 1:2. as the SWR
climbs above 1:2 on the P3 the K3 reading will track it fairly
closely but about a level of 1 below. example
K3 1:1 P3 1:2,
K3 1:1.5 P3 1:2.5
K3 1:2 P3 1:3 and so on
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[Shiraz]
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