SWR readings whats going on
Last updated: Sat, 13 Mar 2010 07:07:43 -0800
View thread
at
FlexRadio: Reflector Users
I am not going to speculate on the quality or functionality of your ATU,
however I would like to point out your measurement techniques.
Understanding the fundamentals of how your radio system works is important
to troubleshooting. I will try to explain some things below, and then
provide a few links to other articles for further reading that may help
clear things up.
When you use an antenna tuner, the antenna tuner splits your radio system
into two parts: The parts before, and the parts after the antenna tuner.
The part before the antenna tuner is your radio (or the output of your
amplifier), and the piece of coax that connects the radio to the antenna
tuner. (in the case of the FLEX 3000, 5000, this is a small piece of
transmission line internal to the radio.) The part after the antenna tuner
is your antenna, and the coax between your antenna and the antenna tuner.
Your radio has been designed to have an output of 50 ohms, non reactive.
Your feedline is 50 ohms, your antenna however, is not 50 ohms. (if it were
50 ohms, you wouldn't need your tuner)
Very few antenna designs are 50 ohms. Many antennas are designed with
matching networks (baluns, gamma matches, etc.) so that the input to the
match is near 50 ohms at the frequency of intended operations.
Most active RF devices, (in your case, the final stage of transistors in the
power amplifier stage of your radio) don't like to be operated with a bad
output match. (transistors rarely have a 50 ohm output impedance, so a
matching network is used in the radio to transform the output impedance to
50 ohms. You then have a 50 ohm piece of coax that goes to your antenna
tuner.
Your antenna tuner 'tunes' until it appears to have an input impedance of 50
ohms. The SWR is measured at the INPUT of your antenna tuner, your system
is happy when it measures 1:1 SWR on the input of the antenna tuner.
But what about the SWR on the output of the antenna tuner?
Your antenna is still not 50 ohms, its probably in the range of 10 - 1000
ohms, with a non zero complex component as well. The feed line that runs
between your antenna tuner and your antenna is not the same impedance as
your antenna, its probably 50 ohms if you used coax (or 75 ohms 300 ohms,
450 ohms, etc. if you use a different type of feedline here, it really
doesn't matter what impedance it is, because its going to be different than
your antennas impedance)
What does this mean? THERE WILL ALWAYS BE A SWR HIGHER THAN 1:1 AFTER THE
ANTENNA TUNER (except in the special case where the antenna impedance is
equal to the transmission line impedance.) This is because the impedance
mismatch between the antenna and the transmission line will reflect some of
the RF energy instead of radiating it in the antenna. This reflected energy
then travels in reverse down the transmission line until it gets to the
antenna tuner, where it will be reflected again (this time nearly 100%) and
the energy will add back to the forward power traveling along the
transmission line, the process will get repeated infinitely, though the
reflected components will be surpressed as they travel through the coax, as
the coax will attenuate the signals the longer they have to travel back and
forth on the feedline.
So what does this all mean? If you are using an SWR meter AFTER your
antenna tuner, you should not expect to see a SWR of 1:1, AND having an SWR
that is higher than 1:1 in this part of the system is sometimes BETTER! If
you do, you probably don't need your antenna tuner in the first place.
Take a look at the block diagram for the FLEX-3000 : http://kc.flex-radio.com/KnowledgebaseArticle50406.aspx?Keyw[..]
am
Notice the SWR Bridge comes BEFORE the ATU. All auto tuners will be built
this way.
For further reading, please examine these articles, the provide examples and
thought experiments that will help you gain understanding
SWR Meters Make You Stupid [ http://www.eham.net/articles/23317 ]
The Real SWR Page! [ http://www.hamuniverse.com/wc7iswr.html ]
The SWR Obsession [ http://www.arrl.org/tis/info/pdf/49470.pdf ]
Understanding SWR by Example [ http://www.arrl.org/tis/info/pdf/q1106037.pdf
]
A Mini Primer on SWR Measurements [ http://users.cwnet.com/ysarc/swr.htm ]
What Does SWR Cost You [ http://www.arrl.org/tis/info/pdf/7901019.pdf ]
Good luck, and I hope this helps!
Greg Zenger, N2GZ
however I would like to point out your measurement techniques.
Understanding the fundamentals of how your radio system works is important
to troubleshooting. I will try to explain some things below, and then
provide a few links to other articles for further reading that may help
clear things up.
When you use an antenna tuner, the antenna tuner splits your radio system
into two parts: The parts before, and the parts after the antenna tuner.
The part before the antenna tuner is your radio (or the output of your
amplifier), and the piece of coax that connects the radio to the antenna
tuner. (in the case of the FLEX 3000, 5000, this is a small piece of
transmission line internal to the radio.) The part after the antenna tuner
is your antenna, and the coax between your antenna and the antenna tuner.
Your radio has been designed to have an output of 50 ohms, non reactive.
Your feedline is 50 ohms, your antenna however, is not 50 ohms. (if it were
50 ohms, you wouldn't need your tuner)
Very few antenna designs are 50 ohms. Many antennas are designed with
matching networks (baluns, gamma matches, etc.) so that the input to the
match is near 50 ohms at the frequency of intended operations.
Most active RF devices, (in your case, the final stage of transistors in the
power amplifier stage of your radio) don't like to be operated with a bad
output match. (transistors rarely have a 50 ohm output impedance, so a
matching network is used in the radio to transform the output impedance to
50 ohms. You then have a 50 ohm piece of coax that goes to your antenna
tuner.
Your antenna tuner 'tunes' until it appears to have an input impedance of 50
ohms. The SWR is measured at the INPUT of your antenna tuner, your system
is happy when it measures 1:1 SWR on the input of the antenna tuner.
But what about the SWR on the output of the antenna tuner?
Your antenna is still not 50 ohms, its probably in the range of 10 - 1000
ohms, with a non zero complex component as well. The feed line that runs
between your antenna tuner and your antenna is not the same impedance as
your antenna, its probably 50 ohms if you used coax (or 75 ohms 300 ohms,
450 ohms, etc. if you use a different type of feedline here, it really
doesn't matter what impedance it is, because its going to be different than
your antennas impedance)
What does this mean? THERE WILL ALWAYS BE A SWR HIGHER THAN 1:1 AFTER THE
ANTENNA TUNER (except in the special case where the antenna impedance is
equal to the transmission line impedance.) This is because the impedance
mismatch between the antenna and the transmission line will reflect some of
the RF energy instead of radiating it in the antenna. This reflected energy
then travels in reverse down the transmission line until it gets to the
antenna tuner, where it will be reflected again (this time nearly 100%) and
the energy will add back to the forward power traveling along the
transmission line, the process will get repeated infinitely, though the
reflected components will be surpressed as they travel through the coax, as
the coax will attenuate the signals the longer they have to travel back and
forth on the feedline.
So what does this all mean? If you are using an SWR meter AFTER your
antenna tuner, you should not expect to see a SWR of 1:1, AND having an SWR
that is higher than 1:1 in this part of the system is sometimes BETTER! If
you do, you probably don't need your antenna tuner in the first place.
Take a look at the block diagram for the FLEX-3000 : http://kc.flex-radio.com/KnowledgebaseArticle50406.aspx?Keyw[..]
am
Notice the SWR Bridge comes BEFORE the ATU. All auto tuners will be built
this way.
For further reading, please examine these articles, the provide examples and
thought experiments that will help you gain understanding
SWR Meters Make You Stupid [ http://www.eham.net/articles/23317 ]
The Real SWR Page! [ http://www.hamuniverse.com/wc7iswr.html ]
The SWR Obsession [ http://www.arrl.org/tis/info/pdf/49470.pdf ]
Understanding SWR by Example [ http://www.arrl.org/tis/info/pdf/q1106037.pdf
]
A Mini Primer on SWR Measurements [ http://users.cwnet.com/ysarc/swr.htm ]
What Does SWR Cost You [ http://www.arrl.org/tis/info/pdf/7901019.pdf ]
Good luck, and I hope this helps!
Greg Zenger, N2GZ
