Generally speaking, with components of the same Q, an L network will be the
lowest loss solution to any matching problem. The network Q is set only by the
two impedances to be matched. (I did once contrived a situation where matching
the very low R, high C gate impedance of an FET could be done with lower loss
using a series pair of L networks, but this is unusual.)
In correspondence with Dean Straw 20+ years ago when we were prepping my article
on ladderline, I pointed out to him that the problem with T networks with three
variables was that there are an infinite number of solutions, with only one
giving the lowest loss and some giving huge losses, with the operator being
clueless. I believe the same would be true with the Match Box. (By what I'm
sure must be coincidence, it was shortly after that an ARRL favored author wrote
article on tuner losses.)
Any rejection due to an antenna matching system is just serendipity and
depending on it for that purpose is foolhardy, IMHO, of course.
To pick one nit with Scott, a pi-network can be high pass and a tee-network can
be low pass.
Wes N7WS
On 12/22/2018 7:21 PM, K9MA wrote:
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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