Hello Allan and the LF-group,
I have been reading all the mail about monitoring cw signals and optimum
frequency etc..
My receiver for Lf is a Kenwood R5000 with standard CW filter.
I also have a Siemens D2055 ( 80 Hz bandwidth) and a Wandel & Golterman
TFPM76 with approx. 100 Hz bandwith. These are carrier level meters, but
perform excellent for Rx on the Lf band.
I noticed that when I turned my head whilst listening to CW (with the
R5000) that the
signal almost disappeared and came back, this depending on the actual
position of my head.
My first impression was that there was something wrong with my ears. But
then I thougt that it had something to do with reflections and absorption on
different nearby objects, so
I thougt that it had to be possible to place some sort of diffusor on top of
the speaker and obtain a more equal soundpattern.
Then I did the following experiment: The loudspeaker of the R5000 is
located in the
top of the cabinet. I placed a cardboard ring (82 mm diameter and 50 mm
height) on top of the cabinet above the speaker. Then I took a (empty)
glass (jelly jarr - don`t know the
excact english word), held it above the cardboard ring and (while listening
to a cw signal)
lowered it in the cardboard ring. When I reached a certain position in the
ring I had the following result:
First of all the cw sounded twice as hard. Second , most of the noise was
strongly reduced and finally I could turn my head without hearing the signal
fade . I made some
measurements with a microphone connected to a oscilloscope to see the
variation of the amplitude and indeed , without the "acoustical filter" the
signal on the scope was about
5 mV I believe and with the "filter" about 10 to 15 mV. Of course is this
not a very scientific method but I am very pleased with the result. My
brother-in-law who was here for a weekend ( and doesn`t know anything about
radio or cw) was impressed that it sounded so loud !! I must say that I have
experimented with a lot
of different shapes and lenghts of glass and one model was outperforming
all.
Overall hight approx. 9 cm, largest diameter 7 cm , smallest diameter 6 cm ,
its conical and difficult to describe. This is al lot of text, and if
someone is interested I can take a picture of the setup and put it on the
reflectorside.At the moment the R5000 is out of order, there is something
wrong with the PLL system. Have to check it. I did not do any test on other
speakers but I am sure that it must work too.
73 Ruddy ON6UX
From: Alan Melia <[email protected]>
To: rsgb_lf_group <[email protected]>
Sent: Monday, May 22, 2000 12:05 AM
Subject: LF: Morse tones and spacial filters
Dave and Larry raise an interesting effect, that I noticed myself when
idly
monitoring the band on the speaker (move your head around to select the
signal you want to copy). The effect is caused, I think, by the selection
of
a position of the ear where multiple audio paths add in phase for the
required tone. The noise is not affected or may even have it high and low
frequencies attenuated by the size of nearby objects in relation to the
wavelength and absorption by furnishings (or tent walls on FD). I believe
the effect is an analogue of the technique used in digital filtering
called
Finite Impulse Reponse (FIR) filters, where delayed copies of a signal
are
added together can produce a very selective output.
I have noticed that whilst I often copy signals at tone frequencies up to
1800Hz, if I find the going difficult I can read the same signals better
at
around 600-800Hz. Strangely enough this is not where the ear is supposed
to
be the most sensitive (1000 Hz to 1400Hz I seem to remember) which may be
why the is a lot of reference to 1000Hz tones.
In the case of static-crashes I agree that getting as much as possible
over
between the crashes with repetitions is undoubtably the best way, but I
don't think you work really weak stations under those conditions. I have
listened to pairs of continental stations late in the evening who are
probably good strength with each other pushing along at a reasonable
speed.
They were down in the normal band noise (not crashes) with me making copy
very difficult as I seemed to miss the 'dots'. In these conditions I find
the slow deliberate sending easier to read, and I use a pencil. (BUT then
I
am not by any stretch of the imagination a morse operator..... but I do
try!) The problem I found with Jim was that having the AGC on to give an
S-meter reading, meant the crashes carved up the copy (with the AOR 7030
it
is not easy to flip the AGC on and off....ergonomics is not one of its
strong points)
All the comments have been most instructive, it keeps me trying, and maybe
my ability is improving slightly ( I hope). I look forward to learning
more
from the Thesis that Dick mentioned.
Cheers de Alan G3NYK
[email protected]
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