On 5/4/10 13:28 , Brian Gregory [UK] wrote:
"D. Peter wrote in message
...
On 5/4/10 05:03 , Brian Gregory [UK] wrote:
wrote in message
m...

How laughable.

Who buys into the whole "active antenna" thing? Some of my best SWLing
has
been with a 5 or 6 foot hunk of wire laying on the floor.

Put your effort into a getting a good alignment. That will do you far
better than these red herring gadgets that are just like a shiny lure to
a
sucker.

When you amplify, you amplify EVERYTHING...noise, distortion and often
these gadgets inject products into the mixer that screw up your IF
tracking, reducing the inherent performance of your receiver.

Quit looking for a "magic pill" and get back to basics: 1. Good
alignment.
2. Wire antenna in unobstructed free space.

That's all you need.




You're basically right that the TinyTenna can't achieve miracles.

But what do you mean (or think you mean) by
a) Alignment.
b) IF Tracking.


A series of tuned circuits, such as those found in a superheterodyne
receiver are said to be in alignment when all paths are tuned to the
correct frequencies. Correct alignment raises output of desired signals,
while reducing noise floor. A receiver in proper adjustment-- proper
alignment -- will perform often remarkably better on the same antenna, in
the same location as the same model receiver out of alignment. Often by
only a very small amount.

IF tracking is the property of two variable tuned circuits to produce a
single output frequency across their entire range.

Do you mean the ability of the RF tuned and local oscillator tuned circuits
to stay separated by exactly the IF frequency?
That's just part of alignment.


Yes, on both counts. I did say it was an oversimplification for
those without the technical understanding.



Forgive me if this is something you already know. A gross
oversimplification, here, if you don't, will explain.

IF, or intermediary frequency, is a property of a superheterodyne
receiver that improves selectivity, improves rejection of undesired
signals, and produces a single, easily manageable signal for detection. IF
is produced by mixing the incoming desired signal with a locally generated
signal of a frequency above the desired signal that is a fixed difference
from the desired incoming signal.

By mixing these two frequencies, the intermediary frequency is generated
the frequency of which is the exact difference between the incoming and
the locally generated frequency. There is also a frequency generated that
is the sum of the two mixed frequencies, but that is rejected by the tuned
intermediary frequency circuits which are tuned to a single fixed
frequency.

In order for the intermediary frequency to pass through the tuned IF
circuits, the intermediary frequency needs to be the same as one tunes
incoming signals across the dial. That means that the mix frequency needs
to vary along with the incoming frequency to remain precisely higher than
the incoming frequecy by the amount of the IF. This is referred to as
tracking. One signal tracks the other to retain the proper relationship as
the tuning dial is moved.

Implements, like the Tiny Tenna, in some inexpensive receivers, can, if
overloaded, produce their own mix frequencies that, in turn mix with
incoming and locally generated frequencies to yield un expected mix
products that produce variations in the intermediary frequency. In some
receivers, a very strong signal alone can cause the IF to shift.

Maybe in a really ancient receiver. In all my receivers the local
oscillators are either crystal controlled or locked by PLL to crystal
controlled references.


Many of mine, as well, yes. But there are those, even today which
are not so.




So, what the poster was referring to is proper adjustment of tuned
circuits within the receiver, and the injection of instability by
frequencies created by cheap active antennae disturbing the frequencies
that must pass through those tuned circuits.

Again, apologies, if your technical understanding exceeds these
oversimplified descriptions.

A good modern receiver will stay adequately in alignment unless it's been
fiddled with, all the important selectivity will be in crystal or ceramic
filters anyway.

Alignment and IF tracking seem like really weird things to quote as
important nowadays.

They might help improve sensitivity, and reduce intermodulation problems but
other things like adding a preselector will produce more exciting results.


The original poster's point is that with a properly aligned
receiver, one begins with an optimized platform that will respond to
less complex configurations with more satisfactory results. Better
performance, lower cost, on the same radio.

My personal choices include preselectors, and a variety of
antennae, active and passive.