[Antennas] Zo/Zl

2002年1月27日 11:26:36 -0600

It is hard to point to a reference, Steve, because an impedance mismatch at
the load with a balanced feedline DOES NOT cause feedline radiation. Any
reference that tells you that it does is incorrect. Such a statement is
tantamount to saying that any line with a SWR other than 1:1 will radiate,
and that is one of the more pervasive and totally incorrect urban myths
found commonly among CB ranks.
Such radiation occurs when the currents in the feedline are not equal and
opposite, that is when the line is unbalanced. It may physically be
"balanced" but electrically if the currents are not equal and opposite,
then the line is unbalanced and it will radiate. The extent of the
radiation is related to the extent of the unbalance in the line currents.
The portion of the current that is equal and opposite is called the
differential mode current of the line, and radiation is not caused by that
current. The remaining portion of the current is called the common-mode
current since it flows in both wires of the feedline and in the same
direction. This is the current component that produces radiation. Note that
the relationship between line Zo and antenna driving point impedance
affects the differential mode but has nothing to do with the common mode.
There are many causes for an unbalanced feedline and antenna system, such
as physical location, surrounding objects, etc., but the most common by far
is bringing the feedline away from the antenna at some angle other than 90
degrees.
If the feedline is brought away from the antenna at 90 degrees, and the
antenna is symmetrical (same length on both legs) and has no metallic
objects nearby to upset the balance, then most antenna systems will be
close enough to being "balanced" that little or no radiation comes from the
feedline. 
Using a current balun to drive the line at the transmitter end is one
approach to "forcing" balance since a current balun operates to produce
equal (and opposite) currents in the feedline wires. A voltage balun
produces equal and opposite voltages on the two wires which seldom does
much to assist in improving an imbalance situation.
Any antenna system fed with a feedline whose Zo differs from the
driving-point impedance of the antenna will be improperly terminated and
thus the line will have standing waves on it. However, this situation DOES
NOT result in radiation from the line. An open-wire line can operate with
an SWR of 20:1 with remarkably little loss and no radiation at all,
provided the entire antenna system is reasonably well balanced.
If you are reading anything that tells you that SWR on the line results in
line radiation, you need to put it aside. The ARRL Antenna Book is very
good on this topic as is the book "Reflections II" by Walt Maxwell, W2DU.
To summarize: line radiation is caused by common-mode current on the line
which in turn is the result of either a physical/electrical imbalance of
the antenna and feedline system, or in the case of an unbalanced line
(coax) feeding a balanced load (a dipole antenna, for example) by
common-mode current flowing along the outer braid of the coax. In that
situation a 1:1 current or choke balun at the antenna feedpoint is required
to minimize or eliminate the common-mode current and hence avoid line
radiation. 
And it is always important, regardless of type of feedline, to bring the
line away from the antenna as nearly at right angles as possible to avoid
having the line or the coax outer braid picking up r-f due to the spatial
imbalance of the line/antenna system.
72/73/oo, George W5YR - the Yellow Rose of Texas 
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe 
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771
Icom IC-756PRO #02121 Kachina #91900556 IC-765 #02437
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"Steve L." wrote:
>> Hello,
>> Can someone please point me to a printed or internet
> reference on balanced feedline radiation due to
> impedance mismatch at the load (antenna)? I'm reading
> this and don't understand it.