On 13 dic, 22:01, Sébastien MEDARD wrote:
On Sun, 12 Dec 2010 12:25:59 -0800, Richard Clark wrote:
On 12 Dec 2010 13:59:06 GMT, Sébastien MEDARD wrote:

What you are talking about is common to DXing. *Get yourself a globe of
the world. *Light it up with an artificial "sun." *The edge of the
dark/light should be half way between you and the transmitter you want
to hear. *So, during the late day, your best signals are probably from
Eastern Europe, Turkey, and Asia. *During the early-to-late night, your
best signals are probably from the Americas. *During the late afternoon
to early evening you best signals are from the south or the north.

I understand better what is the purpose of this kind of map (on SDR-
Radio.com software for example). Where you can see an OM, you are pretty
sure to see this kind of map.

I need to know a little bit more about digital transmission modes... To
be able to recognize digital patterns (on the waterfall display) to be
able apply the good settings for decoding... Easy for CW... Easy for
RTTY used by HAM... But what is the pattern for Meteo Faxes for
example...?

When I used to teach RTTY there were only two tones.

The reason why they call you OM ? )

By the way... I searched for good free software to decode meteo faxes....
But it seems not easy to find something free and easy to use... Do you
know shere I could find something interesting?

Nope.

OK. I will try that later... For now I get some problems to use Linrad
with the SDR-IQ... Painful to boot on windows every time I want to listen
to shortwaves... And using a virtual system is not very efficient on my
old computer...

My next tries will be on a multiple turns loops. With or without
matching loop inside. I will try with simple wire but connection ribbon
cables could be a good alternative... (like old hard drive ribbon cable)

This will reveal the "Law of Diminishing Returns" (the more you add, the
less you get back).

Interesting. What would be the best ratio?

I was wondering... It seems loop antennas are a never ending story.

So, there you have a century of technical perspective on the topic.

)

Perhaps the "Joe Carr's Loop Antenna Handbook" may be interesting, but
it is quite expensive and an Amazon reviewer says it is not so
interesting... What about the "Joe Carr's Receiving Antenna Handbook" ?

He is a respected author. *I always look at the bad Amazon reviews to
see if something is terribly wrong with the product or terribly wrong
with the customer. *Quite often the customer is disappointed because
that customer is stupid. *If I encounter a smart customer that is
disappointed, I can well imagine all the good comments are not very
reliable.

I will try to find one when they are cheaper.

The thing about all these -um- books is that many of them serve up old
wives' tales about low noise and other magical characteristics. Remember
the lessons of a century worth of experience with loop antennas within
an inch of the guts of any radio. *That worked fine for a billion radios
and radios were not bought if they were "noisy."

OK. But today we probably ask more to radios than yesterday, no?

The real science of low noise is found in the distance to the source.

Lapalisse would have said exactly the same thing. He would have said that
the real science of good reception is found in the distance to the source
too )

Is the loop quieter because of its
magic design - or simply because it is 10 times further away (20dB) from
the noise?

OK for that.

We get writers here who pound the table because their magic loop in the
shack is noisy and their long wire 10M away in the back yard is quiet.
They have to be told to turn off the flourescent light in the shack if
they want their magic loop to work.
* *(Their magic loop low score on Amazon) = (stupid.)

Lesson one = learned.

Thanks.

Sebastien.

Hello Sébastien,

Regarding tuning, loops and (long) wires.

You probably locate your antenna far away from noise sources. Mostly
this is not where your receiver is.

In case of tuned loops, you have to go outside for each significant
frequency change. This is one of the reasons that I want my
preselection / tuning adjacent to the receiver.

Loops aren't magic things. A wire from your balcony to a tree or other
tie point may give better S/N ratio then the best (expensive) loop on
your balcony.

Best matching isn't required for reception at HF, just sufficient
signal. In case of a PI tuning / preselector device, you need to
adjust two capacitors in case of major frequency change. In my
opinion a single capacitor device is more pleasant to use.

(deep) Nulls in the radiation pattern at low frequency (AM BC and
below) are the main benefit of the loop if you are willing to change
the orientation frequently when tuning across the bands. At increasing
frequency, the nulls are less pronounced because of propagation
issues. Off course a small horizontally oriented dipole has also
figure-of-eight radiation pattern, but polarization isn't good for AM
BC and lower.

For experimentation / comparison, I used a 1m diameter non-tuned loop
in the attic with preselection / tuning adjacent to the receiver. In
most cases a 5 m outdoor wire with same preselector and receiver gives
similar or better S/N ratio. This may be because of the attic has
mains wiring and I didn't want to go to the attic for reorientation
of the loop.

If you want to experiment with some long wire (unbalanced) antennas,
make sure to have a ground (counterpoise) provision outside; otherwise
your coaxial cable will be part of the antenna. This may result in
more interference from indoor sources.

Whether or not you have sufficient signal from your antenna is easy to
check. Listen to a known station. Tune your preselector to maximum
signal. Now tune to a free frequency close to your station's frequency
so that you hear noise. Remove the antenna, the audio noise should be
less now.

If you use a tapped coil preselector (like the one in my link), and
have sufficient noise from the antenna, you may even change the taps.
The preselector will give more insertion loss, but it will be more
selective also. This can be of use when you have strong signals in
your neighborhood. Because of portability, I frequently use an AOR
AR8200. Without preselection, such a wide band receiver is useless
with an external antenna.

Best regards,


Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me very likely