[Elecraft] Baluns and 450 ohm line

[Ron D'Eau Claire]

Scanning this thread, I think it would be useful to point out something
about the use of balanced feedlines to reduce losses, especially with the
Elecraft KAT2 and KAT1.

There are two ways to reduce feedline losses. One is to keep the SWR at
something reasonable. On the lower frequencies, even coaxial line will have
reasonably low losses if the SWR is kept to, say 5:1 or less, if it is good
quality coax.. Now, you still need an ATU to keep the K2 or K1 transmitter
happy. The output filters in the K1 or K2 want to see an SWR of 2:1or less
to work properly, but once the line is matched to the transmitter using an
ATU, the feedine will work reasonably well even with a moderate SWR. How
high? Antenna books will show you charts. The losses vary widely according
to the type of coax, the length of coax and the frequency used.

The second approach to minimizing feeder losses is to use an inherently low
loss feedline such as true open wire feedline. True open wire feedline has
almost all "air" insulation. "Air" is the lowest loss dielectric/insulator
available. The minimal insulation that does exist to hold the wires the
proper distance apart is of very high quality in good line. With this sort
of feedline you do not need to care about the SWR. The highest SWR an
antenna can produce in a line hundreds of feet long on, say, 10 meters, will
not have signfiicant losses. This makes the antenna "design" very easy. You
only need a radiator long enough to be efficient (at least 1/4 wavelength at
the lowest frequency), high enough to avoid ground losses if it is
horizontal,  and a good wide-range ATU to handle the matching of the feeders
to the transmitter.

So-called "window line" and even "twinlead" are compromise "open wire"
lines. They have a lot of insulation, so their losses are much higher than
true open wire line. Still, in most high-swr applications they will have far
lower losses than coax.

One other thing is important if you want open wire feedline to act as a
feedline instead of a radiator. The currents in the feedline must be
balanced. That is, the currents in each wire must be equal and flow in
opposite directions. The "balancing" has nothing to do with reducing losses
in the line. Its function is to keep the feedline from radiating or picking
up energy (such as noise). When the antenna is designed to provide a
balanced load and the feedline is fed from a balanced ATU, the r-f field
created by one wire is exactly cancelled by the other wire. This prevents
radiation and, by the same process, prevents pickup. Any r-f picked up will
appear in phase on both wires - what engineers call "common mode" currents.
These "common mode" currents get cancelled out at the balanced circuit at
the transmitter end of the feeder. That leaves only the "out of phase"
currents - the signals coming down the feeders from the antenna - to be
passed on to the receiver input.

Balance is not always critical or even desirable! Some popular antennas such
as an "off center fed" doublet and "Carolina Windoms" that use open wire
line assume that the "feedline" is part of the radiating antenna. Lack of
feedline balance does not affect their efficiency as long as they are
installed to avoid losses from objects near the feedlines.  Some of these
designs use a balun at the end of the open wire feeder section. The purpose
of the balun is to avoid currents flowing on the outside of a length of coax
connecting it with the rig inside the shack. The unbalanced load on the
feeders at the antenna end will ensure that the feeders radiate (and pick up
r-f) in spite of the balun.

When two coaxial lines are used in parallel as a sort of "shielded" balanced
feeline, the fact that the line may be balanced does nothing to reduce the
losses. The paralleling of two coaxial lines in a balanced system is done to
raise the feeder impedance which can reduce the SWR in a certain antenna
designs, as Stuart pointed out. Managing the SWR in such a system is still
critical if you want low losses. The r-f field around each wire is still
trapped in the dielectric between the center conductor and the shield in
each coaxal line. That's where the losses occur, but it also makes the
balance less critical since there won't be any radiation or pickup from the
line because of the shielding.

With true open wire line, these concerns about keeping a moderate SWR  to
avoid excessive losses do not exist. The KAT1 or KAT2 ATU's in the Elecraft
rigs have an excellent matching range, and are capable of handling *almost*
any impedance a true open-wire feedline is likely to present. But there is
one concern for operators using the KAT1 or KAT2 ATUs to be concerned with.
That is providing balanced feed if they do not want the feed line to
radiate. The common way to do this using a "single ended" ATU like the
K-rigs have is to put a balun at the output. Baluns get unpredictable with
very high SWR's. More often than not they seem to work fine, but there is no
guarantee that common torodial baluns will work if your feedine impedance at
the transmitter is extremely low or high.

When using open wire line in this manner, you don't care what the impedance
of the line is, nor is there an obvious link between feedline impedance and
the impedance presented to the rig in practical designs. Back in the 1930's
when open wire lines were the most common sort of feeders, few Hams had any
way of measuring SWR nor did they care. They discovered that an efficient
antenna system sometimes presented an impedance to their ATU's that made
adjustment very touchy. Sometimes they could not affect a match. The more
efficient the antenna, the worst this situation was at certain frequencies.
(Lossy antennas seldom have large changes in SWR. A good dummy load presents
a 1:1 SWR across the spectrum, for example). If you peruse the manuals for
center fed doublet antennas using good open wire feedlines, you will find
tables showing combinations of antenna plus feedline lengths to avoid.
That's why. Those are the lengths likely to produce such extreme impedances
at the ATU that it may be difficult to find a match.

The same is true today.  The impedance presented to the ATU will be a
function of the frequency, feedline length and antenna system and will vary
over a very wide range - perhaps from a few ohms to several thousand ohms.
You may find that it is difficult to find a match on some bands even using
the KAT1 or KAT2. In that case, it may be necessary to change the length of
the feedline (or antenna) a little. It's a bit more difficult, nowadays,
because of all the new bands we have, but a compromise that allows one to
operate on all bands can be found... usually <G>.

Ron AC7AC
K2 # 1289