Following is a simple solution for a balanced tuner. While I use a pair of
ganged rotary inductors, tapped inductors could also be switched with a 2
pole multi-position switch (the inductance must be the same in each series
leg). The variable capacitor (in the shunt leg) is a 250pF variable unit.
The balun is made from large ferrite beads slipped over RG58 coax.
/---A----Inductor-------C-------
/ |
/ |
xmtr--balun Capacitor
\ |
\ |
\---B----Inductor-------D-------
While I prefer L networks, the T network can also be transformed into a
balanced tuner.
--------Cin-------------Cout---------
|
|
Inductor
|
|
--------Cin-------------Cout---------
In this case the 2 Cin (and the 2 Cout) capacitors are gang tuned.
73
Ed, W1AAZ
>"If you haven't already done so, you might download IBALUN.TXT from
>ftp.teleport.com/pub/vendors/w7el, which explains why a current balun
>placed on the input side of an unbalanced tuner has the same effect on
>balance as one placed on the output side."
>
> /---A---tuner---C-------
> xmtr--balun |
> \---B-----G-----D-------
>
>If the currents at C and D are not balanced, the system is not balanced.
>If the currents at C and D are balanced then, because of the phase shift
>through the tuner, the currents at A and B are unbalanced. So the balun
>sees the same amount of unbalance whether it is at the tuner input or
>output. Nothing's changed except the tuner chassis is RF hot. A >balanced
>tuner puts the same amount of phase shift in each side and therefore,
>does not have the above problem.
[
QRP-L Archive |
]
[
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000
]