SWR Myths ..... The Facts And Fiction


"I don't know where all the feedline myths started, but I suspect it had
something to do with the do-it-yourself swr bridges which first became
popular back in the early 1950s. Until then, most amateurs didn't even know
about standing waves and, if they did, they didn't seem to care. However,
swr bridges soon caught on, and it wasn't too long before getting caught
with your swr up was synonymous with getting caught with your pands down!
Some hams dug into the books, but when they discovered that swr is caused by
a mismatched antenna, it only served to reinforce the myth. If a mismatched
antenna causes power to be reflected back down the line, they reasoned, this
power obviously wasn't radiated by the antenna.  Some even suggested that
the refelcted power got back into the transmitter tank and was dissipated
in heat.  Others apparently thought that reflected power was lost forever
tosome great swr heaven in the sky.  A few well informed amateurs tried to
nip these absurdities in the bud, but it was hopeless.....the disease spread
faster than the cure.  Transmission lines are too complex to be covered here,
but let's bury the myths.   First, reflected power is not lost nor does it
heat up the tank circuit of your transmitter.  Second, if the feedline has
low loss, as is in the case on the HF bands, increased loss due to swr is
so small that you can forget about it.  Since a 10:1 swr on 100 feet of RG8U
at 4 mHz increases loss by less than 1 db, don't worry about the fact that
the swr rises about 2:1 at the band edges....the station at the other end
won't be able to tell the difference. If your transmitter doesn't like to
load into a mismatch greater than 2:1, buy or build yourself an antenna
tuner and save yourself a lot of grief by forgetting the swr on the line
to the antenna if it's within reasonable limits, say 10:1.

"In the current vernacular, one could say that we have a severe swr hangup!
In many instances, from the viewpoint of good engineering, this hangup is
inducing us to concentrate our impedance matching efforts at the wrong end
of the transmission line. It is ironic that we should be in this situation,
because the amateur is generally quite practical when it coms to following
theoretical considerations.  In this case, though, we have been following
the perfect-match theory down the narrow path because many articles have
misled us to believe that all reflected power is lost, with never an inkling
that, properly controlled, reflections can be turned to our advantage in
obtaining increased handwidth, which we are presently throwing away.

Another accepted Ham antenna expert, Walter H. Anderson, VE3AAZ, writing
in HAM RADIO MAGAZINE, says, in part: "A UA9 I worked recently said he was
using a Zepp antenna.  It occured to me that at least one generation has
passed since the Zepp was, by far, the most popular antenna.  We didn't
realize it then, but the Zepp's standing wave ratio probably ran as high
as a 30:1.  However, history shows that the Zepp put out a good signal.
Thus, it would seem that the Zepp didn't really have the side-effects we hear
attributed to high swr nowadays....high plate dissipation, radiation loss
and all the rest. I don't suggest we dismantle our beams and go back to
Zepps.  Rather, I propose to show that transmission line theory, properly
understood, is free of the contradictions that arise when discussing swr,
reflected power, line losses and other phenomena associated with antennas
and feed systems."

Further along, Walter mentioned the transmatch: "It's easy to dispense
advice on obtaining low swr, but it's much more difficult to specify cures
for same. If you must live with kinky antenna impedances, then you might
consider using an antenna tuner. If air-dielectric capacitors and
silver-plated inductors are used, power loss from the tuner will be
negligible.  An antenna tuner will lower the impedance presented to the
transmitter to 50 ohms, and the transmitter will be be satisfied.  Such a
tuner also pays dividends on receiving."