In an effort to finally end discussion of this topic on the reflector,
we conducted an experiment to compare the performance of a matched
antenna to that of a mismatched antenna with tuner.
This post is quite long. I hope this does not bother anyone too much.
This is the basic experiment that we conducted:
We first set up an antenna and connected it directly to our
transceiver via a coaxial cable. The antenna was resonant at 21.2
MHz, with a measured feedpoint impedance of 39.7 - j0.7 (approx.).
This calculates to a VSWR of 1.32:1. The cable we used had a total
matched attenuation of 1.64 dB (approx.). We used an HP8753E network
analyzer to make the attenuation and impedance measurements. We did a
full two port calibration (open, short and load) to calibrate the
setup.
We then placed a bird wattmeter at the output of the transceiver,
keyed up and set the forward power to 100 Watts using the RF output
dial on the transceiver (a 100 W plug-in was used in the meter). The
measured reflected power was less than 1/2 watt. We then used a bird
wattmeter located near the antenna feedpoint to measure a forward
power near 70 watts and a reflected power near 0.7 watts. I am
uncertain how accurate the wattmeter really is, +/- 2 watts? Anyway,
based on a theoretical analysis (using 1.64 cable attenuation) we
should have delivered about 67.2 Watts of power to the antenna. The
69.3 watts we measured indicates a total cable attenuation of about
1.6 dB. This is pretty close to the 1.64 dB attenuation measured and
indicates a good measurement and setup.
Next, we detuned the antenna to have a feedpoint impedance of
approximately 6.2 + j14.4 ohms. This calculates to a VSWR of about
8.74:1. We then measured the VSWR at the input to the coaxial cable.
It was about 3.0:1.
We then placed an Ameritron manual tuner between the antenna and the
transceiver. We keyed up, and tuned out the reflections. We verified
with the bird wattmeter that we had 100 watts forward power delivered
from the transceiver to the tuner. We then measured forward and
reflected power at the output of the tuner. The forward power was
measured to be near 130 watts. (This was a bit tough as we had to go
to the 2500 watt plug-in). The reflected power was measured to be 27
watts (100 W plug-in). Given these measurements, we would all agree
that the steady state power delivered to the transmission line was
near 100 Watts. We did not try to determine the forward loss through
the tuner.
We then measured forward and reflected power at the antenna feedpoint
terminals. The forward power was measured at 98 Watts and the
reflected power was measured at 62 watts. The net steady state power
delivered to the antenna was measured to be 36 Watts. This represents
a relative steady state loss of about 4.44 dB. 100 Watts input, 36
Watts delivered to the antenna.
I next set out to verify if these measurements were consistent with
the analysis and discussion that I have presented over the last week.
I also set out to verify if these measurements were consistent with
the analysis presented by Walt Maxwell (W2DU) in his book
"Reflections". Remember that in my previous discussions I stated that
Walt's and my analysis agreed when we both assumed a lossless
transmission line. I was curious to see how they handled attenuation.
For the analysis, I used an antenna with an 8.74:1 VSWR, a lossless
tuner, and a matched transmission line attenuation of 1.64 dB.
When I used my concepts and analysis techniques (detailed in the
article I sent some of you), I calculated the following power
distribution within the system:
Initial power delivered to the tuner/transmission line: 70.3 watts
Initial power delivered to the antenna: 17.8 watts
Forward power measured at tuner output: 142.2 watts
Reflected power measured at tuner output: 42.2 watts
Steady state power delivered to the antenna: 35.9 Watts. (17.9 Watts of
initial power plus 18.1 Watts of reflection echoes)
These values are certainly close enough to confirm my analysis of the
tuner operation and the voltage interactions (including what I call
echoes) occurring within the system. I agree that, in HF amateur
communications, the presence of reflection echoes most likely does not
disturb signal quality. However, it is still technically correct to
recognize that they exist and that voltages within the system interact
in the manner I outlined in previous posts to the reflector.
Next, I pulled out Walt Maxwell's book and went to section 6.3 on
reflection gain. On page 6-9, Walt presents a formula to determine
absorbed power by the antenna. If we use an antenna with an 8.74:1
VSWR (rho=0.795) and a matched attenuation of 1.64 dB, Walt's formula
calculates the absorbed antenna power to be 35.9 Watts.
Walt and I both agree on the level of steady state power delivered to
the antenna. I believe that only difference in our method is that I
present a more detailed discussion of voltages within the system
while Walt simply jumps to the end result. (Walt's book does have
more details in the appendix) Either approach is valid if it is
recognized that the total power at the antenna is made up of an
initial power plus the power from a number of reflections.
In Walt's book, page 6-7, Walt states that line attenuation is the
key to whether the VSWR has any practical impact on efficiency at
all. This statement is true and has been illustrated in the
experiment. With the 8.74:1 antenna mismatch and tuner, we measure
the total system attenuation to be about 4.4 dB. 36 Watts are
delivered to the antenna with 100 Watts input. From standard
attenuation curves, we can determine that a 1.64 matched cable
attenuation will translate into a 4.4 dB attenuation (approx.) due to
an 8.74:1 VSWR.
I would like to note that some have posted statements indicating that
there will be no measurable difference between a matched antenna and
a mismatched antenna/tuner combo, or that the difference would not be
significant. Even with our somewhat low matched attenuation of 1.64
dB, an antenna with an 8.74:1 VSWR introduces about 2.8 dB more
system loss than a matched antenna due to the increased cable
attenuation.
The practical end to all of this is certainly nothing new. Keep your
feedline loss low and your antenna VSWR minimized.
This ends this thread for me.
73
Steve
VE9SRB
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