Does anyone have any tips on increasing sensitivity for a medium wave
receiver trying to distinguish navtex 170Hz phase shifts on a low
power carrier at 518kHz?!

Ive been on and off trying to do this project for my boat for a year.
I say off and on, its become most frustrating! I can't use a loop
antenna because its just too big and too directional for use on a
boat. So I'm currently using a Nasa Marine active aerial (it is just
a small plate attached to a standard fet common source driver
circuit). I have that going down some coax through capacitive
impedance transformer and inductor creating some extra front end
selectivity, to the input of a 612 mixer, getting 6kHz IF (after being
mixed with a lo). I'm driving the 612 differentially, as its a slight
improvement over single ended. From here Ive tried using several op
amp filter circuits, and they all perform adequately.

Heres my problem, the receiver just picks up too much noise.
Everything interferes with it, even the scan on my oscilloscope. Even
without this, taking the aerial out of the room, the actual noise in
the system is too high to pick up anything but the strongest navtex
signals (which are decoded). Of course AM radio broadcasts are way
above the background noise, and navtex signals are much lower power.
And AM radio has the help of our ears to tune out the noise.

It works better when closer to the transmitter of course, but the
navtex spec says you should be able to pick up stations 400 miles
away, and i'm only just decoding the local one 100 miles away.

So my question is, does anyone know of any special techniques on the
RF side that can improve my noise handling/signal integrity? (my
expensive sony worldband receiver seems pretty good at it on SSB).
And....does anyone know how the upper market receivers get such a good
signal? My receiver just samples the input frequency (as does the
Nasa low end I think), so is very susceptible to any interference.

Grateful and very interested to hear views on this.

Many Thanks
Andy

bigorangebus wrote:
Does anyone have any tips on increasing sensitivity for a medium wave
receiver trying to distinguish navtex 170Hz phase shifts on a low
power carrier at 518kHz?!

Ive been on and off trying to do this project for my boat for a year.
I say off and on, its become most frustrating! I can't use a loop
antenna because its just too big and too directional for use on a
boat. So I'm currently using a Nasa Marine active aerial (it is just
a small plate attached to a standard fet common source driver
circuit). I have that going down some coax through capacitive
impedance transformer and inductor creating some extra front end
selectivity, to the input of a 612 mixer, getting 6kHz IF (after being
mixed with a lo). I'm driving the 612 differentially, as its a slight
improvement over single ended. From here Ive tried using several op
amp filter circuits, and they all perform adequately.

Heres my problem, the receiver just picks up too much noise.
Everything interferes with it, even the scan on my oscilloscope. Even
without this, taking the aerial out of the room, the actual noise in
the system is too high to pick up anything but the strongest navtex
signals (which are decoded). Of course AM radio broadcasts are way
above the background noise, and navtex signals are much lower power.
And AM radio has the help of our ears to tune out the noise.

It works better when closer to the transmitter of course, but the
navtex spec says you should be able to pick up stations 400 miles
away, and i'm only just decoding the local one 100 miles away.

So my question is, does anyone know of any special techniques on the
RF side that can improve my noise handling/signal integrity? (my
expensive sony worldband receiver seems pretty good at it on SSB).
And....does anyone know how the upper market receivers get such a good
signal? My receiver just samples the input frequency (as does the
Nasa low end I think), so is very susceptible to any interference.

Grateful and very interested to hear views on this.

I've already answered this on sci.electronics.design. Please get onto
Wikipedia and read their article on "cross posting". They'll tell you
about "multiple posting" in there, which is what you've done.

Cross posting is often appropriate, when you have an item of interest to
two or three groups.

I can't think of a case where multiple posting is appropriate, and it is
mildly but generally frowned upon, to the extent where normally
mild-mannered newsgroup denizens will suddenly appoint themselves as net
cops and write posts like this one.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

On 12 Feb, 20:43, Tim Wescott wrote:
bigorangebus wrote:
Does anyone have any tips on increasing sensitivity for a medium wave
receiver trying to distinguish navtex 170Hz phase shifts on a low
power carrier at 518kHz?!

Ive been on and off trying to do this project for my boat for a year.
I say off and on, its become most frustrating! I can't use a loop
antenna because its just too big and too directional for use on a
boat. So I'm currently using a Nasa Marine active aerial (it is just
a small plate attached to a standard fet common source driver
circuit). I have that going down some coax through capacitive
impedance transformer and inductor creating some extra front end
selectivity, to the input of a 612 mixer, getting 6kHz IF (after being
mixed with a lo). I'm driving the 612 differentially, as its a slight
improvement over single ended. From here Ive tried using several op
amp filter circuits, and they all perform adequately.

Heres my problem, the receiver just picks up too much noise.
Everything interferes with it, even the scan on my oscilloscope. Even
without this, taking the aerial out of the room, the actual noise in
the system is too high to pick up anything but the strongest navtex
signals (which are decoded). Of course AM radio broadcasts are way
above the background noise, and navtex signals are much lower power.
And AM radio has the help of our ears to tune out the noise.

It works better when closer to the transmitter of course, but the
navtex spec says you should be able to pick up stations 400 miles
away, and i'm only just decoding the local one 100 miles away.

So my question is, does anyone know of any special techniques on the
RF side that can improve my noise handling/signal integrity? (my
expensive sony worldband receiver seems pretty good at it on SSB).
And....does anyone know how the upper market receivers get such a good
signal? My receiver just samples the input frequency (as does the
Nasa low end I think), so is very susceptible to any interference.

Grateful and very interested to hear views on this.

I've already answered this on sci.electronics.design. Please get onto
Wikipedia and read their article on "cross posting". They'll tell you
about "multiple posting" in there, which is what you've done.

Cross posting is often appropriate, when you have an item of interest to
two or three groups.

I can't think of a case where multiple posting is appropriate, and it is
mildly but generally frowned upon, to the extent where normally
mild-mannered newsgroup denizens will suddenly appoint themselves as net
cops and write posts like this one.

--

Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com

Posting from Google? Seehttp://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details athttp://www.wescottdesign.com/actfes/actfes.html


I assumed that not all radio amateurs would read electronics, and vice
versa, sorry if this is seen as inapropriate to post in the two groups

bigorangebus wrote:

On 12 Feb, 20:43, Tim Wescott wrote:

bigorangebus wrote:

Does anyone have any tips on increasing sensitivity for a medium wave
receiver trying to distinguish navtex 170Hz phase shifts on a low
power carrier at 518kHz?!

Ive been on and off trying to do this project for my boat for a year.
I say off and on, its become most frustrating! I can't use a loop
antenna because its just too big and too directional for use on a
boat. So I'm currently using a Nasa Marine active aerial (it is just
a small plate attached to a standard fet common source driver
circuit). I have that going down some coax through capacitive
impedance transformer and inductor creating some extra front end
selectivity, to the input of a 612 mixer, getting 6kHz IF (after being
mixed with a lo). I'm driving the 612 differentially, as its a slight
improvement over single ended. From here Ive tried using several op
amp filter circuits, and they all perform adequately.

Heres my problem, the receiver just picks up too much noise.
Everything interferes with it, even the scan on my oscilloscope. Even
without this, taking the aerial out of the room, the actual noise in
the system is too high to pick up anything but the strongest navtex
signals (which are decoded). Of course AM radio broadcasts are way
above the background noise, and navtex signals are much lower power.
And AM radio has the help of our ears to tune out the noise.

It works better when closer to the transmitter of course, but the
navtex spec says you should be able to pick up stations 400 miles
away, and i'm only just decoding the local one 100 miles away.

So my question is, does anyone know of any special techniques on the
RF side that can improve my noise handling/signal integrity? (my
expensive sony worldband receiver seems pretty good at it on SSB).
And....does anyone know how the upper market receivers get such a good
signal? My receiver just samples the input frequency (as does the
Nasa low end I think), so is very susceptible to any interference.

Grateful and very interested to hear views on this.

I've already answered this on sci.electronics.design. Please get onto
Wikipedia and read their article on "cross posting". They'll tell you
about "multiple posting" in there, which is what you've done.

Cross posting is often appropriate, when you have an item of interest to
two or three groups.

I can't think of a case where multiple posting is appropriate, and it is
mildly but generally frowned upon, to the extent where normally
mild-mannered newsgroup denizens will suddenly appoint themselves as net
cops and write posts like this one.

--

Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com

Posting from Google? Seehttp://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details athttp://www.wescottdesign.com/actfes/actfes.html



I assumed that not all radio amateurs would read electronics, and vice
versa, sorry if this is seen as inapropriate to post in the two groups

Posting the same message on more than one group is perfectly fine. It's
the way you do it. What's inappropriate is to post the same message to
more than one group _separately_. Even with Google Groups you can
cross-post, which puts the same message on multiple groups, but with the
message tied together so that all groups get all the replies. This
gains you considerable synergy, and keeps folks who are on both groups
from answering the same things twice.

You should be able to do a web search to figure out how to cross-post
from Google -- check the link in my sig line, it may even be in that FAQ.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Feb 12, 10:19�am, "bigorangebus" wrote:
Does anyone have any tips on increasing sensitivity for a medium wave
receiver trying to distinguish navtex 170Hz phase shifts on a low
power carrier at 518kHz?!

Ive been on and off trying to do this project for my boat for a year.
I say off and on, its become most frustrating! *I can't use a loop
antenna because its just too big and too directional for use on a
boat. *So I'm currently using a Nasa Marine active aerial (it is just
a small plate attached to a standard fet common source driver
circuit). *I have that going down some coax through capacitive
impedance transformer and inductor creating some extra front end
selectivity, to the input of a 612 mixer, getting 6kHz IF (after being
mixed with a lo). *I'm driving the 612 differentially, as its a slight
improvement over single ended. *From here Ive tried using several op
amp filter circuits, and they all perform adequately.

Heres my problem, the receiver just picks up too much noise.
Everything interferes with it, even the scan on my oscilloscope. *Even
without this, taking the aerial out of the room, the actual noise in
the system is too high to pick up anything but the strongest navtex
signals (which are decoded). *Of course AM radio broadcasts are way
above the background noise, and navtex signals are much lower power.
And AM radio has the help of our ears to tune out the noise.

It works better when closer to the transmitter of course, but the
navtex spec says you should be able to pick up stations 400 miles
away, and i'm only just decoding the local one 100 miles away.

So my question is, does anyone know of any special techniques on the
RF side that can improve my noise handling/signal integrity? (my
expensive sony worldband receiver seems pretty good at it on SSB).
And....does anyone know how the upper market receivers get such a good
signal? *My receiver just samples the input frequency (as does the
Nasa low end I think), so is very susceptible to any interference.

Grateful and very interested to hear views on this.

Andy, you may hate yourself after reading this, but there IS
a solution to improving signal-to-noise ratio with a loop:

Mount it HORIZONTALLY. :-)

A loop can be an advantage when it has an electrostatic
shield included in it. The pattern will be roughly omni-
directional. With horizontal mounting it won't be high to
hit either sail booms or be taken away by wind as easily
as a vertically-mounted loop.

In 1967 I proved that to myself with a 60 KHz loop intended
for the new WWVB service from (then) NBS (now NIST).
13 x 13 feet in the attic space above my house' center
room. Had lots of noisy appliances in the house then,
plus harmonics from old tube-type TVs' horizontal sweep
(two). Good signal despite all that, just didn't have the
good digital hardware to decode it. BTW, a roll of
Reynolds' Wrap aluminum foil was sacrificed to make a
nice electrostatic shield over #12 AWG electric power
wire used for the loop itself.

What I use now is a vertically-mounted loop of #14 wire
2 1/2' in diameter, tuned to 60 KHz. The resonating
capacitor for tuning is 10 feet below the loop, using some
of 75 Ohm coax capacitance as part of that tuning.
Balanced tuning with coax shields together as the center
common, aluminum foil as an electrostatic shield
connected only to the common. Works well to Fort
Collins about a thousand statute miles away.

An "AM BC Band" tuning is easily possible although the
number of turns will be reduced (and thus cutting down
on developed RF voltage output), but lots of those have
been made by others over the years. Mounting the
loop horizontally might cause a slight detuning but
nothing severe and be roughly omnidirectional so the
boat could be in any heading without changing signal
strength.

Good luck on that,