SWR and TRANSMITTERS
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
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.
POWER LOSS AT VARIOUS
Assume your transmitter is producing exactly 100 watts to
the antenna connector and your SWR Meter is reading 1.6 to
*Percentage of OUTPUT Power with
perfect antenna load
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
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.
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.
Looking at the chart you
will notice under the 5:1 swr section that the station
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
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.
When you add to this fact what is actually going on AT THE
ANTENNA after the signal goes thru that feedline, you may be
Learn More about SWR
(You may learn that higher swr can be better!)
Understanding SWR by Example
A Mini Primer On SWR Measurements
Problem Solvers for Wire Antenna
What Does Your SWR Cost
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