Questions from a Liberal Arts Major

[Don Rasmussen]

>> I'm trying to figure out how to "hear" an S_Meter, but I suppose we  
>> all "listen" in different ways.

It's true Grant, the received signal has a rythm of amplitude that can be seen on the S-meter, but that effect is greatly reduced by the AGC if you are only listening for a change in the level of the AF. The effect is also reduced greatly by displaying 5 db chunk bars only as on/off/flicker values.

The fading on SSB and CW modes can tell you a lot about what the band is doing.

To me, this is an issue that has merit, as some others have suggested.

You will really enjoy your K3, and having been an Orion pioneer it will
be an interesting comparison.


[Elecraft] Questions from a Liberal Arts Major
Grant Youngman nq5t at tx.rr.com
Thu Nov 13 12:27:51 EST 2008

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> Five DB steps that can be on or off with a flicker fall way short  
> for me as well, a convincing analog meter is 50% of the listening  
> experience at this QTH.

I'm trying to figure out how to "hear" an S_Meter, but I suppose we  
all "listen" in different ways.

I'm really surprised, after happily monitoring this list in stealth  
mode for some time while waiting for my K3 to arrive, how much it has  
begun to sound from time to time like every other vendor/product list,  
all of which I finally retired from after tiring of these sorts of  
discussions.  Or perhaps I just haven't yet realized how many serious  
problems my new K3 has :)

(returning to stealth mode)

Grant/NQ5T

UR RST 5 8.39974 9



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[Ralph Parker]

>The fading on SSB and CW modes can tell you a lot about what the band is
doing.

Indeed, but I've always done this by ear, rather than by watching the S-meter.
If you can't hear the difference, perhaps the AGC is adjusted too 'tightly'.

Rapid CW QSB can easily be seen on an audio recording monitored in audio
editing software (at least it can on my previous radio).

VE7XF

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[Jim Brown-10]

>The fading on SSB and CW modes can tell you a lot about what the
>band is doing.

Yes, but not necessarily what you think. Most of the "in and out"
fading we hear on HF is the result of multipath. That is, the signal
travels over two paths that differ a bit in time, which puts them
out of phase. When they out of phase by some odd multiple of 180
degrees, they cancel, and by an even multiple of 180 degrees, they
add.

The time interval of the fading depends on the time difference of
the arrivals and the wavelength of the signal. We're used to hearing
this kind of thing on mobile signals, and also when an aircraft
reflects a VHF or UHF signal. Those VHF/UHF wavelengths are short,
so the fading has a fast characteristic (an interval often less than
a second). Down on 160M, it's common for the fading interval to be
tens of seconds or a minute.

The DEPTH of fading (that is, the weak part of the fade) depends on
perfect cancellation of the signal, which happens when the two
arrivals are almost precisely equal in strength.

73,

Jim Brown K9YC


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[Don Rasmussen]

>>>If you can't hear the difference, perhaps the AGC is adjusted too 'tightly'.

Yes, as Wayne will confirm, I keep a minimal AGC, the loosest that
anyone might imagine.

Assume Wayne checks into the SSB net and your ears say - very strong - so
you look at the S-meter. S7 peaks to S8.

Ten minutes later Eric checks in, you say, very loud too, and take a look at the S-meter. S9 to S9+5.

You now have a visual relative benchmark on Wayne and Eric and can fairly
easily follow the changes on each.

By ear, well, that would be impossible for me.

How do you make a note in the log on Wayne that would have any
meaning later with respect to Eric? Just one example, I am pretty sure
many others do it similarly.

Some may or may not care as long as they can hear them at all, that's the preference part.

[Elecraft] Re: Band condx Questions from a Liberal Arts Major
Ralph Parker ve7xf at dccnet.com
Thu Nov 13 13:49:08 EST 2008

>The fading on SSB and CW modes can tell you a lot about what the band is
doing.

Indeed, but I've always done this by ear, rather than by watching the S-meter.

If you can't hear the difference, perhaps the AGC is adjusted too 'tightly'.

Rapid CW QSB can easily be seen on an audio recording monitored in audio
editing software (at least it can on my previous radio).

VE7XF





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[Jack Smith-6]

Ionospheric reflection seems to be generally N-ray fading, not two-ray
fading. Two-ray fading gives you a Rician distribution when looking at
the probability density function, and N-ray fading is Rayleigh
statistics with a Rayleigh PDF.

I've looked at this  recently but not nearly in the detail I would like
to, but what I have seen is closer to Rayleigh than Rician fading. The
standard references, including both of Kenneth Davies' books on
ionospheric propagation say Rayleigh fading as well.

Rician fading is more common in microwave where a direct and reflected
path will exist due to abnormal atmospheric conditions or mid-path  
reflections. (Paths are normally designed not to have a reflected path,
but sometimes they occur.)

I've put some data up at
http://www.cliftonlaboratories.com/signal_statistics.htm but as I say,
it needs quite a bit more work and analysis to verify Rayleigh versus
Rician fading. Over  the years, I've collected a lot of signal data at
VHF and UHF, going back to when we drove a chart recorded from the
speedometer cable and there's no doubt whatsoever that in the urban
environment, fading follows Rayleigh statistics once you are more than a
kilometer or so from the transmitter.

Jack K8ZOA


Jim Brown wrote:

>> The fading on SSB and CW modes can tell you a lot about what the
>> band is doing.
>>    
>
> Yes, but not necessarily what you think. Most of the "in and out"
> fading we hear on HF is the result of multipath. That is, the signal
> travels over two paths that differ a bit in time, which puts them
> out of phase. When they out of phase by some odd multiple of 180
> degrees, they cancel, and by an even multiple of 180 degrees, they
> add.
>
> The time interval of the fading depends on the time difference of
> the arrivals and the wavelength of the signal. We're used to hearing
> this kind of thing on mobile signals, and also when an aircraft
> reflects a VHF or UHF signal. Those VHF/UHF wavelengths are short,
> so the fading has a fast characteristic (an interval often less than
> a second). Down on 160M, it's common for the fading interval to be
> tens of seconds or a minute.
>
> The DEPTH of fading (that is, the weak part of the fade) depends on
> perfect cancellation of the signal, which happens when the two
> arrivals are almost precisely equal in strength.
>
> 73,
>
> Jim Brown K9YC
>
>
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[Jim Brown-10]

On Thu, 13 Nov 2008 14:50:34 -0500, Jack Smith wrote:

>not two-ray
>fading. Two-ray fading gives you a Rician distribution when looking at
>the probability density function, and N-ray fading

While there may be many (N) arrivals, near-perfect cancellation occurs
when two arrivals dominate, are equal, and are either some odd multiple of
180 degrees out of phase or in phase and out of polarity.

73,

Jim Brown K9YC



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[John Magliacane]

--- On Thu, 11/13/08, Jim Brown <[hidden email]> wrote:

> Yes, but not necessarily what you think. Most of the "in and out"
> fading we hear on HF is the result of multipath. That is, the signal
> travels over two paths that differ a bit in time, which puts them
> out of phase. When they out of phase by some odd multiple
> of 180 degrees, they cancel, and by an even multiple of 180
> degrees, they add.

Absolutely correct!

Being a on-air frequency measurement "nut" (originally started with my K2),
I recently started looking into the issue of fading, and the effects it has
on making accurate frequency measurements.

Here's a short video clip illustrating WWV's carrier phase plotted against a local
frequency standard as a Lissajous pattern as received over a 1622 mile path.

Everything you described can be seen through careful observation of this clip:

http://www.youtube.com/watch?v=nf9KWfS9QBs
 
> The DEPTH of fading (that is, the weak part of the fade) depends
> on perfect cancellation of the signal, which happens when the
> two arrivals are almost precisely equal in strength.

Sometimes the second path "wins out" after the fade, and the resultant signal
emerges 180 degrees out of phase with what it was before the fade.

Aside from multipath, much of the slow and shallow fading seen on low HF frequencies
during the day can be the result of variations in D-layer absorption.


73, de John, KD2BD

--
Visit John on the Web at:

        http://kd2bd.ham.org/
.


     
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