The tolerable level of SWR depends on the feedline used. With ordinary
coax, I would try to avoid anything over 3:1. The problem caused by high
SWR is dielectric loss, plus the possibility of exceeding the rf voltage
reading at high voltage points, and overheating marginally sized cable at
high current points. With open wire line, a SWR of 10:1 or more is not
unusual, and if the wire size is large enough and the spacing between wires
is wide enough, there is no problem. The feedline loss of open wire tuned
feeders is very low even at high SWR.
By definition, a "tuned" feedline is a feedline with substantial SWR. A
"flat" feedline theoretically has a 1:1 SWR. It's a matter of properly
designing the antenna/ feedline system. Coax is too lossy to use as a tuned
feeder, but the loss in open wire is usually not a problem, even at high
SWR. But there is nothing magic about running a flat feedline. As long as
the current and voltage limits of the coax are not being exceeded, anything
below 2.5:1 or 3:1 should be ok. But at those extreme limits of SWR, the
coax exhibits some aspects of a tuned feeder system. The impedance the
transmitter sees into the feedline will vary according to the length of the
feeder. That explains why sometimes the performance of a coax-fed antenna
may vary depending on the length of the coax feedline.
A "transmatch" is used between the transmitter and coax feedline to
transform the impedance seen at the transmitter end of the feedline to
something within the range the transmitter is designed to work into. In
amateur work, modern solid state rigs typically are designed to work into
50-ohms, non-reactive, and tolerate very little variation from that figure
of load impedance. According to the manual, the Gates BC1-T and later ones
in the Gates series of 1 kw broadcast transmitters, are designed to work
into a 50 to 70-ohm load. With 2.5:1 SWR on the coax feedline running out
to the base of the tower, I could not get mine to load up to full power at
the extreme edges of 160, so I designed and built a simple variable
L-network to use as a transmatch between the transmitter and coax feedline.
Open wire tuned feeders are a different matter. An "antenna tuner" is
required. This matching network usually has to more radically transform
the impedances than would a simple transmatch. The best antenna tuner
circuit is the classic link-coupled balanced tuner with split stator tuning
capacitor. The tuned circuit may be wired as series resonant to feed a line
that presents a low impedance to the matching network, or wired in parallel
to present a high impedance. Intermediate impedances may be matched by
tapping the feeders down on the coil in the parallel tuned configuration,
although this can be an "iffy" proposition. Better to use the appropriate
lenghth of feeder to present a high voltage point (high impedance) or a
high current point (low impedance) to the matching network. The worst
possible circuit to use as an antenna tuner to feed open wire balanced line
is what most commercial tuners use: an unbalanced L- or T- network, working
into a bal-un on the feedline side of the tuner. Baluns are not designed to
work into highly reactive loads, and usually are designed to work into a
very limited impedance range. The highly reactive load, at widely varying
impedance, that appears at the transmitter end of a typical amateur
open-wire tuned feeder installation, is inevitably far beyond the range of
the balun at certain frequencies on certain amateur bands. This results in
excessive RF loss and possible overheating of the balun, especially if it
uses a ferrite core. I have heard of ferrite core balun coils literally
exploding like firecrackers at high power. In addition, the ferrite core in
a balun may be overdriven into saturation at the peaks of the RF sinewave,
distorting the waveform, and causing the radiation of RF harmonics, which
could result in interference to non-amateur services (including, but not
limited to TVI), and possibly an FCC citation.
If the open wire feeder is properly balanced, there should be no feedline
radiation. Unbalance in the current in the feeders indicates COMMON MODE
current in the feedline. Per the laws of physics, the rf feeder current
must be balanced, just as both wires of a two-conductor cable feeding a
resistor must carry the same current when DC is applied from a battery. But
in addition to the RF feeder current, the feeders may also carry "antenna
current" (common mode) which means that the entire feedline is acting as a
single conductor. Superimposed on the feeder current, this will appear as
an unbalance between the RF current on one feeder vs current on the other
one. This common mode, or antenna current on the feeder will radiate when
transmitting, or pick up signal when receiving. With coax, common mode
current will appear as a current on the outside of the coax braid, and will
radiate just as would a solid piece of wire carrying the same current. If
there is no common mode current, the balanced current will reside on the
centre conductor of the coax and the inside of the braid, and there will be
no radiation. With open wire balanced line, if there is no common mode
current, the currents in the feeders will exactly balance, and there will be
NEGLIGIBLE radiation. That means a tiny fraction of the energy will be
radiated because, with open wire line, both conductors cannot occupy
precisely the same space, and thus the open feeders act like a two-element
antenna, but with the elements so closely spaced that the radiation of one
virtually cancels out the radiation of the other.
As you go higher in frequency, there will be more and more radiation from
balanced open wire feeders. Try using 6" spaced 600-ohm open wire line,
very commonly used at 1.8 - 30 mHz, at 300 mHz, and you will have very
substantial radiation. That same feedline when used on 160m will have
essentially zero radiation if the feeders are closely balanced (carrying no
"antenna," or common mode current).
With coax, a choke balun near the feedpoint of a centre-fed dipole may
eliminate the common mode current. There will inevitably be at least a
small amount of common mode current when a coax line directly feeds a
balanced dipole, since at the feedpoint of the dipole, the RF voltage at
each leg cannot be zero (if it were zero that would mean zero energy was
being fed into the antenna), so therefore, the coax braid cannot be at zero
RF voltage where it feeds the antenna. This common mode rf voltage will
form standing waves along the coax braid, just as it would on a single wire
antenna. That is why the common mode current is sometimes called "antenna
current." It is current that makes the feedline act like an antenna ard
radiate.
With open wire feeders, it's a matter of physical balance, such as getting
the wire lengths exactly the same on both sides of the dipole, making sure
the two conductors making up the line are the same length, keeping both legs
of the dipole the same distance above the ground and the same distance away
from large metal objects and other feedlines and antennas.
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