So much good discussion about SWR (Standing Wave Ratio) lately on the
Repeater, and to get some new Hams off to a good start on the right foot,
here are some facts your parents should have told you (presuming they were
Hams and knew better themselves when they were kids....).
SWR is one of those mythological creatures that has taken on a Medusa
and Pandora quality of its own. So much mis-information, innuendo, facts
that just aren't so, and multiple wrong-headed ideas are out there in
Hamdom, and so much of it is half truths, or outright wrong!
Unfortunately, we can't put all that back in Pandora's box. The link below
is a great article from the ARRL in QST that I wanted to get everyone new
off to a great start on reading the facts of RF Life and Line Reflections
Below is a snippet, particularly the boldened part by me. Most people
will know that a non 1.0:1 SWR will mean some power is being reflected from
the antenna (not radiating all the power out because of an impedance
mismatch) back down the transmission line. But few seem to know that the
antenna-reflected power arriving back at the transmitter is re-reflected
back up the coax again, most of which (% by SWR at the antenna) will be
radiated when it returns to the antenna.
That is a little-acknowledged fact. From there, things only get
deeper and more algebra/ trigonometry/ calculus driven, such as the phase
angle of the reflected voltage, etc. (If you want a good dose of headache,
read the part about Smith Charts in the article. "The concept behind the
Smith Chart is simple.") But, if you will remember that "*all*" (wink,
wink, nod, nod) the power is radiated, regardless of the SWR, then you will
be one step ahead of the less-knowledgeable crowd. But wait, there is
Does Higher SWR Lead to Lower
Power Being Transmitted?
Not always so dramatically. Believe it or
not, 100 percent of the power is actually transmitted
in both of the previous examples. In the
first case, with a 50 Ω antenna, it’s easy to see
how all the power is transferred to the antenna
to be radiated since there are no reflections.
In the second case, the 33 percent voltage
reflection travels back down to the transmitter
where it doesn’t stop but is re-reflected from
the transmitter back toward the antenna along
with the forward wave. The energy bounces
back and forth inside the cable until it’s all
radiated by the antenna for a lossless transmission
line. An important point to realize
is that with extremely low loss transmission
line, no matter what the SWR, most of the
power can get delivered to the antenna. A later
example will show how this can happen.
*OK, the all-important caveat. I said "*all*" the power is radiated
regardless of the SWR. The * fine print * is, "all the RF power that is
not lost in the transmission line." As the article points out, that is an
important caveat. I have three cases of "loss in the transmission line" to
consider. Please DO read the article. These three cases are my own
First, unless you 1) either are measuring SWR right at the antenna
feed point, or 2) NEED to measure it accurately for design reasons, SWR is
an indicator of efficiency and approximate "quality factor" of your antenna
system, but it is hardly absolute without extreme measures taken to make
1) Typical coax has loss, no matter what frequency, no matter how long, no
matter what size. It is in every factory spec. It is that loss for your
length of coax, type of coax, and band you are operating on that compounds
the Reflected Power Loss = SWR issue. Fine, your HF wire antenna used on 6
m shows you in the shack at the transmitter end that you have a 2.0:1 SWR
(11% Reflected Power from article Table 1). ... BUT, that could be 5.0:1
AT THE ANTENNA if your cable has 3.0 dB (1/2) power loss at VHF (compared
to the < 2.0 dB of loss at HF you bought it for)!
Of your 100 W Shack-end SWR meter-measured output power, for 3 dB coax
loss (1/[10^[3 dB/10]]), only 50 W is reaching the antenna through the 3 dB
(half power loss at VHF, not at all uncommon, and more common by UHF) coax
loss. Of the 50 Watts reaching the antenna, at a 5.0:1 SWR, 44.4% is being
reflected (Table 1) = only 55.6% (only 27.8 W) is being radiated. [50
Watts of RF lost in the cable (its 3.0 dB loss = 1/2 =) 50 Watts of heating
the coax! No different than putting it an oven...]
For a 5.0:1 SWR AT THE ANTENNA (e.g., bad antenna, broken balun, not
meant for that band, operating way outside of its tuned "2.0:1 SWR" points,
etc.) (Table 1 says) 44.4% of the power arriving is being reflected back
down the coax, = 22.2 Watts. On its way back down that same 3 dB loss coax
cable, the REFLECTED power also suffers a 3 dB loss in the coax on the
Return trip, such that, 11.1 more Watts heats the coax (now up to 61 Watts
lost in the coax!!!), AND only 11.1 Watts reaches the SWR meter to be
measured as Reflected Power for SWR display in the Shack! That "11%
Reflected Power" (from Table 1) of the direct value measured going into the
coax from the transmitter means that your Shack SWR meter will show you a
Hence, not only does better coax (and good connectors) get more power
(from less loss) TO your antenna, it also helps you see 'closer to the
truth' of what RF power is being reflected from the antenna = coming back
down the coax with less loss to the reflected power for a more accurate,
but never-really-quite-correct, SWR value in the Shack.
Rerun the above example with 1.5 dB (1 - 0.708 factor) of coax loss
(e.g., larger diameter cable, fix/replace a bad connector (PL-259 @ UHF)
with 2.0 dB of loss at UHF!! with an N connector), and the 5.0:1 SWR at the
antenna (no change to how the antenna was really working!) gets displayed
as 3.0:1 SWR, but, with even MORE POWER OUT THE ANTENNA (70 Watts to the
antenna x [1- 44.4% reflected]) = 39 Watts radiated), in spite of a higher,
but more accurately reported/meter reading SWR of 3.0:1 !!!! !
And that folks is how SWR is a double-ended, twice-twisted, inside
out, enigma wrapped inside a riddle.
2) And now you can understand why some blue-blooded Hams (and commercial
circuits, military) go to using (make their own) air dielectric, open bay,
ladder line. Losses of 0.2 dB per 100 feet at HF are possible! ... But
that stuff has lots of mechanical issues and precautions to overcome that
coax does not have.
So now, with ladder line, even if the SWR at the antenna feed point is
5.0:1 or higher, who cares if the reflected power suffers a 0.2 dB loss
(<5%) loss, coming back down, it only suffers that same loss going back up
to the antenna again, where 55.6% of it is radiated again, .... So, with
very low loss feed lines (not just coax), SWR plays even less importance to
antenna system efficiency. However, there are real impacts of a 10:1
VOLTAGE RATIO (VSWR) on the line for a 12 V transistor made to handle only
50 V! You get the idea, ..... For 100 W into 50 ohms, it is 70.7 V RMS =
~ 100 peak voltage x 10:1 ratio = 1000 V on the line = trouble = arcing and
sparking for many circuits designed with 12V (to 50 V) in mind, not 1000
V!!! And you thought 500 V capacitors were over-kill!! THIS is one good,
solid, reason to keep SWR down. They didn't (know or) care about SWR in
the early days of tubes!
3) What about loss-less feed line lengths? How long is the "feed line"
inside your handheld radio to the rubber duck ON your handheld??
Essentially, the short length of connector wire to the antenna socket
inside a handheld radio is part of the RF power transistor + antenna filter
+ impedance matching circuit. You "tune" the (handheld) transmitter for
the (correct) load attached to it, typically 50 ohms, so the test meter
readings are reasonably accurate. Even if the rubber duck is 5 or 500
ohms, with essentially "no feed line" to speak of, "all the power gets
radiated." While not entirely correct to the design engineer (which I was,
so I get lost in/concerned over this minutia), it explains how, as bad as
rubber ducks are, how the do seem to 'work' in some fashion after all. A
3:1 SWR or more is quite typical for a rubber duck. Yet, they work, (even
though I say they are -10 (1/10) to -20 dB ((=1/10^[20/10]) = 1/100 = 1%!)
All this being said, coax and connectors never get "better" with age.
Coax cables and connectors, (and baluns, line isolators, wires,
connections. etc.) are always "best when new." So, record the numbers
(measured coax loss before you put it "up" out of reach, SWR, etc.) when
put up new, and date it! Then, if/when the SWR gets better at some time
in the future on its own, you can bet your bottom dollar the loss in the
coax has increased, certainly not decreased on its own. (Water in the
coax, something broke, etc.) We come back to more coax loss reads out as a
better SWR when it really shouldn't. 10 dB of coax loss would be only 1%
Reflected Power (up and back) measured in the shack = 1.2:1 SWR from a
completely open-ended or shorted-at-end coax with no antenna attached!
In fact, this is one way to check new coax cable, open-ended or
shorted, if low loss, and "all" the test power into the coax gets reflected
from the open or shorted end (and it all should be), then a (very) high SWR
reading means the Reflected power is NOT getting lost in the coax. The
higher the SWR for open (or short) ended coax cables, the better (lower
loss) the coax is! A 2.0:1 SWR into 100 feet of RG-8X tested (-4.5
dB/100', up and back = -9 dB) at 146 MHz is bad news for VHF.
http://www.universal-radio.com/catalog/cable/coaxperf.html And that is
just the loss-to-reflected-power that has NOTHING to do with the antenna
radiating efficiency (or not!). Presuming the antenna IS radiating some
power, the SWR readout will be even lower. A true 5.5:1 SWR for the
antenna proper would make this example's readout in the Shack about a 1.6:1
SWR. Everything is fine, right??????