clvrmnky wrote:
On 10/03/2005 12:31 PM, clvrmnky wrote:

On 02/03/2005 8:57 PM, Roy Lewallen wrote:

Try this simple test.

Tune your receiver to a part of the dial where there's no station.
Disconnect the antenna from your receiver. If the noise level drops,
impedance matching won't help your signal/noise ratio, it'll just
make everything louder. You can accomplish the same thing by turning
up the volume control.

Pulling the wire just attenuates the noise. So, a more balanced match
will not help with selectivity or overworked front-ends?

If this is the case, and a balun is most useful for transmitters
trying to manage SWR, then I guess it doesn't matter.

... in which case my only real choice is to shorten the antenna, or
attenuate the signal coming to my front-end a little ...

Hmmm. More to think about.

You're on the right track, but more information might help.

Your antenna is picking up signal and noise. When you disconnect your
antenna, the antenna's signal and noise are removed, leaving only the
receiver's internal noise. The fact that the noise dropped means that
the noise being coming into the receiver from the antenna is larger than
the internal receiver noise. This is nearly always the case at HF and
nearly never the case at VHF and higher. As long as the noise from the
antenna dominates over the receiver noise, then improving the impedance
match, which is what the original question and answer were about, will
only make both the signal and noise (both coming from the antenna)
bigger. This is exactly what you get when you increase the volume, and
doesn't help the signal to noise ratio one bit. It therefore doesn't
help you hear signals.

There are other ways to help the signal to noise ratio, though. If a
signficant amount of the noise is originating locally, improving
feedline balance with a balun might help. Horizontal polarization will
often be less sensitive than vertical to moderately local noise sources,
because vertically polarized waves can propagate by surface waves while
horizontally polarized waves can't. Feedline balance also improves
rejection of vertically polarized waves when the antenna is horizontal.
This is because it prevents pickup from the feedline itself, which is
often partially, at least, vertical.

Increased antenna directivity will help if the signal and noise are
coming from different directions. But this requires a mechanically or
electrically rotatable antenna unless you're only interested in
listening in certain directions. Short wave listeners sometimes use a
small rotatable loop, which has sharp nulls which can be pointed toward
a single dominant noise source.

You mention an "overworked" front end. If your front end is getting
overloaded from strong signals, which usually manifests itself as
cross-modulation -- "ghost" signals created from existing signals at
different frequencies -- either a passive preselector or an attenuator
is needed. The former is better only if the loud signals are on
frequencies reasonably removed from the frequency where you're
listening. Adding attenuation is fine, until you add so much that the
receiver noise begins dominating. You can use the antenna-disconnection
test to check for that condition. Another way to attenuate the signal is
to *degrade* the impedance match. That's right, if overload is the
problem, you'll do better with a lousier match.

You also mention selectivity. A preselector can take care of attenuating
strong, out-of-band signals. But nothing you can practically do outside
the receiver will help you separate two signals which are close
together. That requires internal modification to the receiver.

Forget about SWR. It's not of any use in improving your ability to hear
signals at HF where the noise from the antenna dominates.

Roy Lewallen, W7EL