Very true, Carl . . .
I think that it helps to recall that the "SWR bridge" in any rig is
actually serving only to give an indication of when the transmitter is
loaded to its design load, usually 50 ohms resistive. Only its "reflected
power" indication is of much use in making external tuning adjustments.
When all the external tuning, etc. has been done to present the transmitter
with its required 50 ohm resistive load *at the point where the internal
SWR bridge is making its mesurement* the "reflected" power will be zero and
hence the SWR reading will be 1:1 or zero on the meter. Rigs with internal
tuners will have the SWR bridge on the input side of the internal tuner, so
even though "off" it will have an effect on bridge readings.
When you think of it, the internal SWR bridge is not actually responding to
currents and voltages on an actual transmission line, such as the coax
going from the transmitter to the tuner or antenna. Instead, it is buried
in the midst of a lot of complex circuitry that is composed of PCB traces,
some wires, some discrete components, a lot of distributed capacitance and
inducance, and maybe even a few inches of coax.
So, it helps to alleviate some of the confusion if we don't regard its
reading as an SWR value but rather only as an indication as to whether or
not we have delivered the proper load to the transmitter. If the meter
reads zero or 1:1, we have. If it doesn't, we have not.
Occasionally, the SWR bridges in my MFJ 989C or my Drake MN-4 tuners will
read an SWR of 1:1 when the IC-756PRO internal bridge reads 1:1, but this
rarely happens. There is almost always a small difference. So, in my case,
the final arbiter is the internal meter and all adjustments are made
externally to cause it to read zero or 1:1 on the SWR scale.
The presence of any in-line instrumentation will almost always introduce
minor frequency-sensitive impedance variations in the line which can affect
upstream readings.
72/73, George W5YR - the Yellow Rose of Texas QRP-L 1373 NETXQRP 6
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe SOC 262 COG 8
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
Icom IC-756PRO #02121 Kachina #91900556 IC-765 #02437
All incoming and outgoing email virus-checked by Norton Anti-Virus 2002
Carl Moreschi wrote:
>
> Mark,
>
> I am a little skeptical about the test you did. Don't forget that even with
> the pegasus
> internal tuner in bypass, it still creates a little SWR discontinuity. This
> discontinuity
> would be more of a problem the higher in frequency you go. Even when
> you have two identical SWR meters in series, you will rarely read the same
> SWR.
> The first SWR meter will cause a discontinuity for the second one. I always
> felt
> trying to get SWR to less than 1.5 was silly because even the SWR meter
> itself
> messes up the line it is on. There's no way to measure SWR without changing
> it!
>
> Carl Moreschi N4PY
> Franklinton, NC
|