R: Fw: Re: [TowerTalk] DB's, ad nauseam

[Maurizio Panicara]

Preface, if ionosphere wouldn't be a linear media we should be in grat
trouble with the zilions of "gost" signals generated by reciprocal mixing of
all radio waves there.
An apparently unlinearity is instead produced when a signal, present and
existing but masked because insufficient to generate any detectable S/N,
goes beyond a stated level and starts to become instrumentally or "brain"
detectable.
Again (why to talk about "DB's ad nausem" if not....) either an instrumental
device like a receiver detector or an induividual ability to understand
information in the noise, get an advantage for incremental S/N changes when
S/N is at the lowest detectable limit.
In other words, 3 dB S/N compared with 0 dB S/N is a different story than 67
dB S/N compared with 70 dB S/N.
Althought  both S/N increments are produced doubling the power, the first
effectively starts to transform a signal in a partly detectable one while
the latter enhances something that's already so perfect to be in practice a
useless (and not detectable) improvement.

73,
Mauri I4JMY



----- Original Message -----
From: Tom Rauch <w8ji@contesting.com>
To: <towertalk@contesting.com>; <wa4dou@juno.com>
Sent: Monday, July 24, 2000 5:04 PM
Subject: Re: Fw: Re: [TowerTalk] DB's, ad nauseam

> While focusing antennas on a target area is always desirable, the
> change in signal level is absolutely linear with the gain change in
> the direction and angle of the target.
>
>
> There are two exceptions:
>
>
> 1.) The signal has multipath.
>
>
> In this case there can be destructive interference between two
> paths. The main effect of multipath is fading increases, along with
> distortion of the signal's modulation. You can hear this using SSB
> on amateur bands when people change antennas and their audio
> seems to change, or on AM as the signal has "selective fading"
> and the carrier or sidebands are nulled randomly making the audio
> distorted with fades. This is a fading problem, not a level problem.
>
>
>
> 2.) The transmitting system is so powerful it actually changes the
> ionosphere. That requires many millions of watts ERP.
>
>
>
> <italic><color><param>0000,0000,0000</param>Last month, in
>
> > QST, Tom Schiller(hope i spelt that correctly) wrote an article which
>
> > recognizes this phenomonon.
>
>
> </italic></color>Because something appears in print, that does not
guarantee it
> correct or factual. That's especially true if the goal is selling
> something instead of a technical article.
>
>
> <italic><color><param>0000,0000,0000</param> I have personally experienced
it. Basically
>
> > its this: An antenna with well known and predictable characteristics,
>
> > compared to another lesser antenna(also with known and predictable
>
> > characteristics), producing results via ionspheric propagation
>
> > disproportionate to and far in excess of any known  differences between
>
> > the two. Do you have your own stories to tell about this
> phenomonon Tom?
>
>
> </italic></color>I don't doubt people "think" they have seen that effect,
given the
> fading on the bands...poor calibration of S meters....a tendency to
> exaggerate changes...etc.
>
>
> For example, I did a series of A-B tests on 75 meters changing
> from a G5RV to a dipole. In virtually every case, the receiving
> station reported the dipole was better. The amazing thing was I
> never changed antennas! The test was really to see how much
> difference I could get with only an antenna "name" change.
>
>
> Sorry, but this stuff about non-linear amplitude response in the
> ionosphere is pure unsliced bolognia.
>
>
> <nofill>
> 73, Tom W8JI
> w8ji@contesting.com
>
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