While I seriously doubt any rejection of nearby frequencies by a tuner
is likely to have any effect on the K3 receiver,
I'd point out that any tuner configuration other than an L network can
provide a match over a wide range of Q. One generally tries to tune them
for minimum Q to minimize losses. If tuned to a high Q, however, both
the T and pi networks generally will provide some rejection of adjacent
frequencies. Just how much is impossible to predict, unless you know
just how the antenna impedance varies with frequency. At far removed
frequencies, of course, a T does act like a high pass, and a pi like a
low pass, but in neither case do they match the antenna to 50 Ohms,
unless it happens to actually be 50 Ohms at some frequency.
That said, is suppose it IS possible that a tuner/antenna combination
just happened to have a deep null right on the frequency of a nearby
broadcast station which was causing intermodulation. If that were the
case, a more reliable solution would be a trap or stub.
73,
Scott K9MA
On 12/22/2018 08:52, Bob McGraw K4TAX wrote:
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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