> The guys at DX Engineering have a 1:1 balun for this
purpose, which they of
> course say works better They also say to use 300 ohm
line instead of the
> 450 line.
Steve,
The impedance at the end of an open wire line feeding a
mutiband antenna can be about anything. It can range from a
few ohms to several thousand ohms.
There are two basic types of baluns.
One style acts as a common-mode coke, and is mostly passive
for desired differential voltages and currents. It only
appears as a short length of transmission line. This
provides balance because the balun terminals appear as a
high common mode impedance (or "ground independent) current
source. The core of the balun is only subjected to the
minimum magnetizing forces required to keep the system
balanced. NONE of the differential or transmission mode
power influences the core of the balun.
This is called a current balun, and is commonly available
in 1:1 ratios, although it can be found in 4:1 and higher
ratios. It has the widest frequency range and highest power
handling for any given material cost. It achieves the
desired load current balance required for all transmission
lines (coax or balanced) regardless of the load being
balanced, unbalanced, or anything in between. Most antennas
are actually between the two ideal cases.
See http://www.w8ji.com/verticals_and_baluns.htm
The second type of balun is a voltage balun. This type of
balun forces the core to be magnetized by the voltage
appearing across the transmission line. Regardless of load
balance the core is always under maximum stress. Since the
output terminals appear as a grounded center-tapped winding,
if the antenna is not perfectly balanced this type of balun
will actually aggravate feedline radiation. It rarely
provides balanced operation even with a perfectly balanced
load, because the transformer action is nearly-perfect only
over a very narrow frequency range and over very restricted
impedance ranges. There are exceptions to this, but this
type of balun is normally available in the inexpensive 4:1
baluns common used in or sold as "tuner baluns" or ladder
line baluns.
The very **LAST** thing you ever want to do is feed an
unknown impedance with a 4:1 voltage balun.
All of the DX Engineering baluns are current baluns, even
the 4:1 and higher ratios. The 1:1 DXE balun is probably the
better choice if you don't know what the impedance presented
to the balun is on every operating frequency.
You can see a comparison of baluns at:
http://www.w8ji.com/Baluns/balun_test.htm
73 Tom
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|