View Full Version : MFJ 911H balun - need schematic
ON3ZTT
02-07-2008, 08:16 PM
Can anyone who might own this switchable 4:1 / 1:1 balun/unun send me a schematic by opening it?
I would like to know how it is built so I can rebuild a better/more heavy duty version using large toroids and teflon covered wire.
I guess it is built around a Guanella current type 4:1 balun.
I've googled it but I did not get hold of a schematic.
Thanks in advance :)
Jan
ON3ZTT
W7ETR
10-29-2010, 04:35 PM
check this out
http://home.earthlink.net/~christrask/Trask4to1Balun.pdf
G3TXQ
10-29-2010, 06:23 PM
check this out
http://home.earthlink.net/~christrask/Trask4to1Balun.pdf
And then read http://vk1od.net/balun/gsc/
to see how Trask got it wrong!
Steve G3TXQ
G3TXQ
10-29-2010, 08:24 PM
And then read http://vk1od.net/balun/gsc/
to see how Trask got it wrong!
Steve G3TXQ
Perhaps I should amplify that comment.
Trask makes the point that a conventional 4:1 Guanella Balun can be constructed with the two 1:1 chokes wound on a single core providing that the load is fully floating. That is true, but as VK1OD points out "real life" antennas are not like that. And if the load were fully floating you would not need a balun to drive equal load currents - they would have to be equal anyway!
Trask's "improved" 4:1 Balun does indeed allow the two 1:1 chokes to be wound on a single magnetic circuit and still drive equal currents into an unbalanced load. However it introduces a new problem that we also see on the Ruthroff 4:1 Voltage Balun: the two voltages being added to create the load voltage are delayed with respect to one another by the length of the choke transmission line. That severely limits the high frequency performance in a way which isn't true of the 4:1 Guanella Balun wound on separate cores.
Steve G3TXQ
G3TXQ
10-29-2010, 08:38 PM
Jan,
Like you, I've been unable to find a schematic, but the description does refer to cores (plural) so I imagine it is a "conventional" Guanella 4:1 wound as two 1:1 chokes on separate cores.
I don't know how they switch the balun to 1:1, but I dislike the common practice of switching the two 1:1 chokes in parallel - all that does is halve the choke common-mode impedance !!
73,
Steve G3TXQ
