AI4QJ wrote:

I think you were right, it is a front end overload issue. But I think adding
an attenuator that simply reduces all input signal strength in a linear
manner does not act the same as de-tuning. Comments from others on their
experiences would be interesting. It is easy enough to try out out; just
find a strong 75m station and using a tuner, tune out the noise and see if
you can still receive (and more pleasureably). Is this the same as
attenuating all input signals 20dB? I lack a technical explanation and we
haven't agreed that the effect is real so I will drop out of this discussion
unless someone else wants to pick it up.

Let's take a look at what mismatching does.

At the frequency you're listening to (and for practical purposes some
bandwidth on either side), mismatching does the same as an attenuator --
it simply reduces the strength of the incoming signals and noise by the
same factor. (It has to reduce both by the same factor because neither
the antenna nor the tuner has any way of distinguishing which is which.)

At other frequencies the story is a bit different. Assuming you don't
want to listen to other frequencies at the same time, everything
off-frequency can be categorized as noise. Adjusting your tuner will
cause the match to improve at some frequencies, increasing the noise at
those frequencies which reaches your receiver. It will also degrade the
match at other frequencies, decreasing the noise from those frequencies
which reaches your receiver. A good quality receiver will reject the
noise at all frequencies except the bandwidth you're intentionally
listening to, so increasing or decreasing the noise at those frequencies
shouldn't make any difference, and again the result won't be any
different from an attenuator. Only if off-frequency noise is strong
enough to cause your receiver to distort will the mismatching have any
effect that might be different from an attenuator. If that's the case,
different mis-tunings will have different effects depending the
frequencies of the overloading signals and which frequencies are favored
or not favored by the mis-tuning -- mis-tuning could make matters better
or worse.

There's an added wild card here if the receiver is being overloaded and
creating intermod. Two off-frequency signals ("signals" in the broad
sense, here, which includes both noise and intentionally transmitted
signals) can combine to produce an in-band intermod product which you'll
hear and regard as noise (this also in the broad sense, meaning anything
you don't want to hear). This intermod product isn't on frequency until
after the receiver front end, because that's where it's created. Messing
with the tuning could possibly attenuate one or both the offending
signals more than an attenuator would, so you'd see more of an
improvement in S/N ratio than with an attenuator. I think this would
only be an occasional lucky happenstance, though, and not something
you'd consistently see. But a plain attenuator can provide quite a
dramatic improvement when intermod is present. Adding 10 dB of
attenuation ahead of the receiver will reduce your desired signals and
in-band noise by 10 dB. But it'll reduce second order intermod products
by 20 dB, third-order products by 30 dB, etc. So an attenuator (or
mis-tuning or a smaller antenna) can provide a real S/N ratio
improvement when some of the noise is intermod being created by your
receiver. A preselector can also be helpful in this situation. Again,
though, you won't see this with a quality receiver unless you have some
remarkably huge off-frequency signals. I wouldn't be too surprised to
see it on 40 meters in Europe, for example.

Roy Lewallen, W7EL