[Elecraft] Tuners and spurious responses

[Don Wilhelm]

Vic,

I have long been an advocate of the parallel tuned circuit matching 
network, and an oldie but goodie of that is the Johnson Matchbox.  It is 
a bandswitching version of the old plug-in coil tuners.  The matching 
range is not as great as that with the plug-in coils with movable taps 
on the coil, but it is quite useful even with the more limited matching 
range.

Yes, it provides a bandpass response which keeps out 'crud' from bands 
other than the one to which it is tuned.  The high pass filter of the 
"T" network tuner or the low pass filter of a "PI" network cannot 
provide that same filtering.

Our local club has used fixed bandpass filters for the last 3 years with 
a great improvement in station to station interference.  The first year, 
the 40 meter bandpass filter did not work as planned, and my Johnson 
Matchbox was used in its place and did a fantastic job.

I admit, I am enslaved by the convenience of the Elecraft "L" network 
ATUs, and I  have not evaluated the comparison of receive noise with a 
bandpass type ATU or filter.  I am fortunate to have a relatively low 
noise location for my station.

I cannot comment on the "RF-in-the-shack" aspects, but the balanced 
Johnson Matchbox likely presents a better balanced load to your antenna 
with parallel transmission line than any unbalanced tuner with a balun.

73,
Don W3FPR

On 12/22/2018 12:03 AM, 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.
>