Posted by
George, W5YR on
Aug 06, 2004; 5:19pm
URL: http://elecraft.85.s1.nabble.com/Performance-of-baluns-tp455789p455793.html
The current balun can be readily demonstrated to provide equal and opposite
currents to highly unbalanced loads. A simple test is to connect a balun to
a load consisting of a 50 ohm resistor connected to a common or ground place
and similarly a 100 ohm resistor. Measurement of the r-f voltage across the
resistors will show that equal currents are being forced through each as the
voltage across the 100 ohm resistor will be twice that of the 50 ohm
resistor.
Roy Lewallen W7EL and Walt Maxwell W2DU have both discussed this test and
Roy described a test fixture for demonstration. I have built the fixture and
carried out the test with a simple W2DU bead balun and found almost perfect
balance with equal currents being delivered to unequal load resistances.
Simple 1:1 current or choke baluns such as the bead baluns are relatively
insensitive to load impedance. I use them to transition from ladderline
feeding multiband balanced antenna loads to coax to enter the shack to the
tuners. The load seen by the baluns varies widely with band as would be
expected.
The Elecraft 4:1 balun is unique in its design and is able to provide an
approximate 4:1 transformation over a wide range of frequencies and load
impedances. Thus there is little need to restrict its connection to the
input of a tuning unit.
Teacher wasn't wrong, Geoffrey - it has just been a long time!
73, George W5YR (58 years a ham the end of this month)
Fairview, TX
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http://www.w5yr.com----- Original Message -----
From: "Geoffrey Mackenzie-Kennedy" <
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To: <
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Cc: "Elecraft Discussion List" <
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Sent: Friday, August 06, 2004 10:47 AM
Subject: [Elecraft] Performance of baluns.
Hello all,
Have noticed over the recent years that current baluns are used in places
where the load/source Z is not purely resistive (OK will allow a little
reactance) , example - an incoming transmission line that looks like R +j
(quite a lot) goes to a balun then the to an ATU then to the TX. I was
brought up to understand that in such a case, outgoing transmitter power
would NOT result in equal and 180 degrees out of phase currents on the
transmision line wires. i.e. the antenna would see an "unbalanced source" I
was always told to put the balun between the Tx and the ATU, and to make
sure that the ATU presented the balun with a R +/- j0 load , or simply put
was "tuned up". (Loved those swinging links).
I appreciate that there are a lot of "but ifs", but was Teacher right in the
fundamentals? If so, it might go some way to explain why some antennas that
are looking for a balanced feed are misbehaved.
73
Geoff GM4ESD
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