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SWR and TRANSMITTERS
Friends or
Foes!
Making Sense of it!
You just finished hooking your
station up to that new whiz bang antenna or made some changes to your
antenna system to squeeze out a bit more signal and hooked the
feed line into your swr meter.
Now it's time to decipher those swr
readings.
Just exactly what do they mean to
you and your signal on the other end at that DX station you are looking to
get into your log?
Hopefully this simple chart, a comparison or
two and some great information links will help you understand
better, the relationship between swr readings and your total station's
efficiency in sending that rf out where it belongs. This article is in no
way meant to be a short course in swr, antennas or antenna systems.
The links below will help do that.
Study them, and save to your
favorites, they are worth your time.
POWER LOSS AT
VARIOUS
SWR READINGS
An example using the chart
below:
Assume your transmitter is producing
exactly 100 watts to the antenna connector and your SWR Meter is reading
1.6 to 1.
(See Green section in chart below)
This chart "assumes" that there are no losses in the
feedline in a "perfect world".!
|
SWR
READING |
% OF
POWER LOSS |
OUTPUT
TO ANTENNA* |
|
1.0:1 |
0.0% |
100.0% |
|
1.1:1 |
0.3% |
99.7% |
|
1.2:1 |
0.8% |
99.2% |
|
1.3:1 |
1.7% |
98.3% |
|
1.4:1 |
2.7% |
97.3% |
|
1.5:1 |
3.0% |
97.0% |
|
1.6:1 |
5.0% |
95.0% |
|
1.7:1 |
6.0% |
94.0% |
|
1.8:1 |
8.0% |
92.0% |
|
2.0:1* |
11.0% |
89.0% |
|
2.2:1 |
14.0% |
86.0% |
|
2.4:1 |
17.0% |
83.0% |
|
2.6:1 |
20.0% |
80.0% |
|
3.0:1 |
25.0% |
75.0% |
|
4.0:1 |
38.0% |
62.0% |
|
5.0:1 |
48.0% |
52.0% |
|
6.0:1 |
55.0% |
45.0% |
|
10.0:1 |
70.0% |
30.0% | *Percentage
of OUTPUT Power with perfect antenna load
and no other losses in the antenna system!
*SWR 2.0:1*
Most transceivers start to reduce power at this SWR
level
Using an
swr reading of 1.6 :1 in the example above, our percentage of reflected
power would be 5 % with 95 % of transmitter power usable or 95 watts to
the antenna assuming no other loses in your feed line
or antenna. Since no feed line is perfectly lossless and no antenna
is perfect in every sense of the word, these numbers should give you an
idea of how your transmitter and antenna
system would be performing into a perfect load with no loss
anywhere in your antenna system.
You have to
assume that your antenna system is not and never will be that 100% perfect
system we all strive for. An "antenna system" is everything between the
transmitter up to and including the antenna.
In the example above we assume that, in
fact, we have a perfectly matched and 100% efficient antenna and
the feed line has NO loss. We also assume that our transmitter's
output stage protection circuit is working properly.
Use the numbers in
the chart as a guide. Just because you have a low swr reading, does not
mean that your antenna system is perfect!
Read on!
Here is
something to think about. Consider a dummy load with a perfect 50 ohm load
inside it attached to your transmitter with an swr meter in between.
You take a reading and get that perfect match or a 1:1 swr reading.
Now I ask you, does that dummy load make a good
antenna? NO.
You can have much the same situation with antenna
systems that show a really great swr but your not "getting out". You will
learn why in the links below. It called line loss which increases the SWR
AT THE ANTENNA and, the SWR meter will never see it! You will learn why in
the links below.
Here is a hint for
your station SWR record keeping!
When taking the final swr
measurement on your new antenna system or after making changes to it,
record them in a permanent record for safekeeping. Check your swr from
time to time on the same frequency and of course using the same antenna
system to see if the swr has changed or is changing gradually. Then
try to find out why before the reading gets too high and your transmiter
starts shutting down power. Recording swr in a permanent record rather
than your memory helps with troubleshooting antennas, feedlines,
etc later.
Keeping good records of your swr from the time changes
were made to your antenna system until a later date can help you see
trends in swr changes which can tell you that "something" is changing over
a period of time. If you do not keep a record of your swr between two
points in time, then you may be in for a surprise some day months
later when your transmitter starts telling you that you should
have....there goes that DX contact you
needed!
Running a station with high SWR!
Does it really matter? It all depends on
how you look at it and understand what is going on in the entire station
system including the antenna and all that is connected to
it.
Now, assuming you have had a bit of time to look at the
chart above, let us consider what difference it would make on the recieve
end, (the DX station), if we compared two
identical stations at your QTH that could be switched back and forth, the first station with a 1:1 swr and the
second station having a 5:1 swr. These readings are at the station end of
the feedline.
Looking at the chart you
will notice under the 5:1 swr section that the station
looses almost
HALF of it's output power due to the transceiver "cutting back"
power for protection, or from 100 watts to about 52 watts out to the
antenna system.....reducing from 100 watts to about 52 watts is about a
3db loss in signal strength....only barely enough to notice on the S meter
with the DX station! The DX station can not hear the difference, he can
only see it on the S meter. In voice communications, it's what you HEAR
that tells the story.
Now don't misunderstand, we
are not telling you it is OK to run your station like that with a high swr
at the station end of the antenna system. This is just to show you that
having a high swr is not all that terrible when it comes to actual signal
comparisons!
The proof is in the db loss if we are assuming
that most S meters increase 1 S unit for a 6db change in signal strength
which is the same as a multiplying by 4 times the power output at the
transmitter. In this high SWR example above, we are cutting the output
from the transmitter to the antenna by about half, from 100 watts to
about 52.
Another example would be to
consider two identical stations sitting side by side. One is running 100
watts with a 6:1 swr and the other station is running 200 watts with a 10:1 swr. By looking at the chart,
there is only 15 watts difference between their output power even
though one station is running 100 watts more than the other....The DX
station could not tell the 15 watts difference between the two! The
higher power station is actually only getting about 60 watts from his
transmitter while the lower power station is getting about 45 watts from
his transmitter. So that's only about 15 watts difference between the two
stations! That's not enough difference to make a
difference! The higher power station would have to run
at about 600 watts out of his
transmitter assuming the same 10:1 swr to get about 1 S unit higher
reading over the lower power station.
So the bottom line here is
that the higher the swr ratio is to (1), the less output to the antenna
system due to transmitter power reduction.
When you add to this fact
what is actually going on AT THE ANTENNA after the signal goes thru that
feedline, you may be horrified!
That perfect 1:1 reading at the
transmitter may translate to a horrible reading where it really
counts...
at the
antenna!
~~~~~~
Learn More about SWR
(You may learn that higher
swr can be better!)
Understanding SWR by Example
Darrin Walraven, K5DVW, ARRL pdf
file
A Mini Primer On SWR
Measurements
John Breckenridge, WB6FRZ
Problem Solvers for Wire Antenna
Installations
Alpha Delta Communications, pdf file
What Does Your SWR Cost
You?
Stan Gibilisco,
W1GV
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Copyright N4UJW/Hamuniverse.com
2007 |
