The Johnson Matchbox configuration is indeed a banpass filter meaning it
attenuates both above and below the frequency to which it is tuned. I
use mine at Field Day to provide attenuation to stations operating both
above and below the band being used. The amount of attenuation does
vary as it is not symmetrical in nature.
Probably you were using a less than optimum balun which had little
common mode rejection or poor balance. The best way to check the two
configurations is to measure the current in each leg of the balanced
feed line. Many baluns do not do a good job or making a "balanced"
feed. The work of DJ0IP {see his website} has a lot of information
from real field measurements on baluns, good ones and bad ones.
73
Bob, K4TAX
On 12/21/2018 11:03 PM, Victor Rosenthal 4X6GP wrote:
I just replaced my single-ended T-network tuner plus balun with a
massive old Johnson Matchbox. It is very selective, unlike the T
network. It is as if there is a sharp bandpass filter between the
antenna and the rig.
This may be totally imaginary, and there's no easy way to A/B test it,
but it seems as though the K3 sounds "cleaner" in some sense.
Could it be that since the mixer sees a much narrower spectrum, there
are fewer spurious responses?
Do those of you who use bandpass filters for SO2R or multi-transmitter
contesting notice such an effect?
It also seems that the better balance (my antenna system is a dipole
fed with balanced line) has reduced RF in the shack and possibly local
noise pickup.
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