[Elecraft] Antenna Feedline Question

[Ron D'Eau Claire]

Tom WB2QDG wrote:
I am running a barefoot K2 with various options, including the auto-tuner 
and am getting ready to put up a new antenna.  I've decided on a 40 meter 
dipole that I will feed with 450 ohm 'window' line.  This will be my only 
antenna, at least for a while.  I operate mostly on 40 meters (CW) but also 
a bit on 20 (psk31) and 15/10 (also CW) when they are open and hope to tune 
the 40 meter dipole to those bands with the auto-tuner.

 I am planning to run the 'window' line all the way into the shack to the K2

(which is about 5 or ten feet from the outside wall).  My question is should

I also use the BL1 at the K2 and will I have any trouble tuning to the other

bands.  I guess I don't understand what 4:1 means, in this case as I'll be 
using 450 ohm feedline to the K2.

----------------------------

The balun does two things: it provides an impedance transformation and it
helps reduce "common mode" currents on the feedlines. You may find one or
both of these functions useful, or neither of them may be needed. That is,
you may not need a balun at all. 

First, the 4:1 is the impedance transformation ratio into a resistive load.
For example if you had a folded dipole it'd show about 300 ohms at the
center. A 4:1 balun there would provide a 75 ohm termination (300/4) for the
rig. That is it would provide you with an SWR of about 1.5:1 at resonance. 

That holds true in general for such baluns, but not always, when you connect
them to a non-resonant antenna system. The 4:1 ratio can be useful if your
tuner can't find a match on some bands since the balun will change the
impedance it "sees". 

Your dipole would be 66 feet long if cut for 40 meters. The window line is a
decent feedline when operating the feeder at a high SWR. On any frequency
between 7 and 30 MHz, the SWR shouldn't exceed about 10:1 (if you tried
using 50 ohm coax it'd probably exceed 50:1 on some bands). It's that lower
SWR that makes the higher-impedance lines more efficient than coax and other
low-impedance lines. 

Some folks will say that the antenna will be badly unbalanced if a balun is
not used. That's the other function of the balun. It provides "push-pull"
currents to the feedline and it prevents non "push-pull" currents from
flowing through the balun. The non "push-pull" currents are called "common
mode" currents - they are identical or common to both wires. It works like
this. If, at some point along the feedline, there is a current of 1 amp of
RF current flowing up one side of the feeder, with an ideal system there
will be exactly 1 ampere flowing down the adjacent wire at that instant. Or
if there is exactly +50 Volts RF to ground on one feeder, there'll be
exactly -50 Volts RF on the opposite feeder at that instant. That's the
ideal way for a balanced feedline to operate since those opposite and equal
currents (or voltages) produce equal and opposite electric or magnetic
fields that cancel each other out. At a short distance from the feedline the
RF field produced by those opposing currents or voltages is zero. So no RF
is radiated (or picked up) by the feedline  - it all flows to the antenna. 

In the real world that is seldom, if ever, the case. First, the load at the
far end of the feed line must also be balanced. You get pretty close feeding
your dipole in the center, but proximity effects of the two ends as they
pass objects unbalances the system, so even with a balun there will be some
unbalance. Most dedicated balun users would be astonished to see just how
little the balun actually affects the balance of currents in the feedline,
especially at the antenna end. The length of the feeder itself acts as a
"balun" to ensure balanced currents at the feed point. 

Another thing your balun can do for you is to help isolate the antenna from
rig for common mode currents. Your feedline is in a high RF field, since it
runs right up to your antenna. Unless it comes away from a perfectly
balanced dipole at exactly right angles, RF currents will be induced in the
feed line by the strong field around the antenna. These currents flow along
both wires simultaneously in addition to the RF currents flowing from the
rig to the antenna. To these induced currents, your two feeders just look
like one "fat" wire, so the same voltage and current will be present at all
points along both wires; they are "common mode". The balun only allows
balanced or "push pull" currents to flow through it. So it will isolate your
rig from these common mode currents. Sometimes that's important. For
example, your feed line may be just the right length so the common mode
currents produce a high RF voltage loop right at the rig. In that case your
rig may tend to be "hot" with RF, producing bites when you touch it and
perhaps erratic behavior caused by RF voltages getting into the circuits.
The most immediate clue that you have a voltage loop at the rig is if your
tuning changes when you touch things. If that happens you can try changing
the length of the feedline, but that may not be convenient and it may
produce a voltage loop at the rig on a different band. So most operators go
for the "one size fits all" solution and put a balun at the rig end to help
isolate it from the common mode currents. And the balun can help keep the
currents balanced at the rig end, reducing pickup and radiation near the rig
end of the feed line.

So, if you experience a rig that seems "hot" with RF on some bands, try a
balun at the rig end of the feed line. Generally that seems to be a serious
problem only among those who must use a compromise installation with some
coax running from the end of the open wire feeders to the rig. That length
of coax often passes through the house where it needs to provide shielding
to prevent pickup or radiation of RF that may interfere with household
electronics. The coax shields the transmit RF currents, but he problem is to
keep common currents on the outside of the shield from causing trouble. The
balun will help a lot with that. Such installations work because they keep
that length of coax very short, but they still suffer significant losses in
most cases on the bands where there is a high SWR on the coax link.

The bottom line is that you may not need a balun at all. That's because you
probably do not need any impedance transformation and you may not need the
isolating effects of a balun, so you eliminate one device in the antenna
system. Everything in the system has some loss. A balun does not radiate, so
its losses are not contributing to anything. 

That said, typically a good balun like the BL1 will have low losses in most
cases, so it won't hurt.  

Ron AC7AC