bpnjensen wrote:
wrote:
dxAce wrote:
But, is it 'sucking it out' or merely propagating it somewhere else other than
that particular spot where your antenna is?

And that 'somewhere else' might not be very far away, but merely a few
wavelengths in distance.

dxAce
Michigan
USA
Before satellites carried most of the milcom they used "diversity
receivers".
Two, or more, receivers tuned to the same frequency but located some
distance apart.
The logic being that when the singal faded at one location, the other
didn't fade at the
same time. The more important a comm cicuit the more receivers spread
over a wider
area.

A friend and I played with our receivers feeding phone patches and
since we live 30
miles apart it was clear this approach was workable. With signals that
experienced
deep fades we were able to listen to nearly all of the time. Real
(commercial or
military) had AGC based voting systems to decided which signal to pass.
We ran
into issues of our audio phases shifting producing very odd sounding
"flanging"
effects.

I have often thought about trying this with receivers whose antennas
are only a few
hundred to thosand feet apart. I never have gotten around to it.


The military also used freqeuncy diversity, sending the same singal on
more then one
frequency. Kind of like listening to WWV on 5 10 and 15MHz at the same
time.

Terry

Fascinating. It sounds like a couple of antennae, maybe even on the
same property but spaced some modest distance apart, maybe a few
hundred feet, and phased into the same radio, might also be a solution
to the problem.

Anyone try this with a 50-acre lot and a phasing harness?

Bruce Jensen


Diversity reception has been a well established practice since the
early days. Hallicrafters produced a diversity receiver, which was
actually two receivers diplexed into a single audio stage, fed by
separate antennae.

May have been a bit of overkill. Separate antennae, if electically
isolated from one another, diplexed into a single input can produce
similar results: reducing selective fading before it reaches the receiver.

When I lived in Rockford, I rented a two bedroom home on a private
estate west of town. The rental property included several acres on a
hilltop, and access to the private lake on the estate.

Of course, I went antenna crazy. And using multiple antennae into the
BC-794, was able to mitigate a good deal of the selective fading
throughout most of the HF spectra.

Each antenna was connected to an RF preamp with a gain of 2-6db. The
outputs of the preamps were combined through resistive pads (for
isolation) into the RF input of the BC-794. The result was nothing
short of amazing, with fading distortions dramatically reduced, and
program listening, was quite pleasant. Even my wife was no longer
critical of SW listening.

Make no mistake, it wasn't FM quality. But it was fine wideband (when
conditions permitted) AM quality. And though there WAS some latent
fading remaining, it was, by far, less objectionable, and often barely
noticeable than a single antenna on the same receiver.

Diversity, at least in this case, is something to be implemented with
affirmative action.

dxAce wrote:
I don't think that would work properly. In practice, I think you need the 'voting
machine' that works on two receivers AGC to pick the best signal.

I do recall some folks trying to emulate this to a certain degree by having two
receivers, two antennas widely seperated (more than a wavelength), and feeding the
audio to headphones (one receiver in the right ear, one in the left).

dxAce
Michigan
USA

I fed the output of the phone patch intop one ear and my radis audio
into the other.
Gave me a splitting headache that turned into a miagrane. So I cheated
and
just fed the patch aduio into one speaker and used the local receiver's
speaker.

Like I mentioned it worked very well, but the aduio sounded very odd.

A "Rolling hollowness is the best discription I can come up with.

Seehttp://www.harmony-central.com/Effects/Articles/Flanging/

Terry

D Peter Maus wrote:
Diversity reception has been a well established practice since the
early days. Hallicrafters produced a diversity receiver, which was
actually two receivers diplexed into a single audio stage, fed by
separate antennae.

May have been a bit of overkill. Separate antennae, if electically
isolated from one another, diplexed into a single input can produce
similar results: reducing selective fading before it reaches the receiver.

When I lived in Rockford, I rented a two bedroom home on a private
estate west of town. The rental property included several acres on a
hilltop, and access to the private lake on the estate.

Of course, I went antenna crazy. And using multiple antennae into the
BC-794, was able to mitigate a good deal of the selective fading
throughout most of the HF spectra.

Each antenna was connected to an RF preamp with a gain of 2-6db. The
outputs of the preamps were combined through resistive pads (for
isolation) into the RF input of the BC-794. The result was nothing
short of amazing, with fading distortions dramatically reduced, and
program listening, was quite pleasant. Even my wife was no longer
critical of SW listening.

Make no mistake, it wasn't FM quality. But it was fine wideband (when
conditions permitted) AM quality. And though there WAS some latent
fading remaining, it was, by far, less objectionable, and often barely
noticeable than a single antenna on the same receiver.

Diversity, at least in this case, is something to be implemented with
affirmative action.

I will have to try this tonight!

I have 2 very nice active dipoles seperated by about 100'. Right now I
am using
a "phaser" to "rotate" the beam/pattern. It will be very easy to
coulple them via a
Mini Circuit power divider/combiner!

Thanks for the hint. I had always assumed you needed "fancy"
electronics to
do this.

Terry

Ok, I got it. I kinda figured the band was very narrow where you would
lose carrier. The fade is probably moving around the transmitted
spectrum like a game of musical chairs. I figured I was going to have
to play with AGC some so I'll probably throw an agc amp on the board as
well.

73
NEO


HFguy wrote:
N9NEO wrote:
Greetings,

I have just got a sony ICF sw7600GR and it is a very nice radio. The
sync detector seems to take care of a lot of the distortion, but the
audio continues fading in and out and is quite annoying. Could the
fading be mitigated to any extent by using another stage of agc? I am
going to be doing some experiments with the 455kc if out on my Red Sun
RP2100 whenever it gets here. Detectors, filters, SSB, etc... I thought
that along with other experiments I might want to try some outboard
agc.

regards,
NEO

The sync' is doing what it's designed for by reducing the distortion
caused by selective fading, but you need a longer time constant (release
time) for the AGC, to help smooth out the fading. You could lengthen the
time constant of the AGC circuit in the 7600GR but then it wouldn't work
well for other conditions where a shorter time constant is needed. This
is why a good table-top receiver has more than one AGC rate, which can
be selected by the user. The AGC in the Drake-R8 series uses a decay
rate of 300-ms for the fast setting and a much longer rate of about
2-seconds in the slow mode. The latter really aides in reducing the
effects of rapid fading.

wrote:
D Peter Maus wrote:
Diversity reception has been a well established practice since the
early days. Hallicrafters produced a diversity receiver, which was
actually two receivers diplexed into a single audio stage, fed by
separate antennae.

May have been a bit of overkill. Separate antennae, if electically
isolated from one another, diplexed into a single input can produce
similar results: reducing selective fading before it reaches the receiver.

When I lived in Rockford, I rented a two bedroom home on a private
estate west of town. The rental property included several acres on a
hilltop, and access to the private lake on the estate.

Of course, I went antenna crazy. And using multiple antennae into the
BC-794, was able to mitigate a good deal of the selective fading
throughout most of the HF spectra.

Each antenna was connected to an RF preamp with a gain of 2-6db. The
outputs of the preamps were combined through resistive pads (for
isolation) into the RF input of the BC-794. The result was nothing
short of amazing, with fading distortions dramatically reduced, and
program listening, was quite pleasant. Even my wife was no longer
critical of SW listening.

Make no mistake, it wasn't FM quality. But it was fine wideband (when
conditions permitted) AM quality. And though there WAS some latent
fading remaining, it was, by far, less objectionable, and often barely
noticeable than a single antenna on the same receiver.

Diversity, at least in this case, is something to be implemented with
affirmative action.

I will have to try this tonight!

I have 2 very nice active dipoles seperated by about 100'. Right now I
am using
a "phaser" to "rotate" the beam/pattern. It will be very easy to
coulple them via a
Mini Circuit power divider/combiner!

Thanks for the hint. I had always assumed you needed "fancy"
electronics to
do this.

Terry



You rarely need 'fancy' electronics to experiment. And as I recall
(I'll have to dig out my notes on this--I did a lot of experimenting,
there, due to the RF quiet...including using an AK Model 30 as a high
fidelity air monitor for airchecking...LOVE those infinite impedance
detectors) it really was this simple.

But reading Steve's comment about switching and AGC in diversity
reception, I'm thinking there may have been an additional stage in my
installation. Though I don't specifically recall this. I do recall that
the AGC was swithed off, and the RF gain was manually set.

But, it was 30 years ago, after all. I'll have to dig out my notes.

Regardless, there is nothing to be lost in the attempt. And if you
find something that works, that streamines what I did, please share the
wealth.

wrote:
dxAce wrote:
But, is it 'sucking it out' or merely propagating it somewhere else other than
that particular spot where your antenna is?

And that 'somewhere else' might not be very far away, but merely a few
wavelengths in distance.

The selective fading results because of multiple paths from the
transmitter site. With multiple signals at slightly different
propagation paths, you can get a very sharp cancellation effect which
moves around as the delay paths change by 1/4 of a wave or so.

It's observable by anyone listening to a MW AM station just outside the
service area at night. That's where you'll hear nearly equal strength
from the skywave signal and the ground wave signal. They'll often
cancel each other out at the carrier. This results in a very distorted
sound from an envelope detector.

dxAce
Michigan
USA

Before satellites carried most of the milcom they used "diversity
receivers".
Two, or more, receivers tuned to the same frequency but located some
distance apart.
The logic being that when the singal faded at one location, the other
didn't fade at the
same time. The more important a comm cicuit the more receivers spread
over a wider
area.

A friend and I played with our receivers feeding phone patches and
since we live 30
miles apart it was clear this approach was workable. With signals that
experienced
deep fades we were able to listen to nearly all of the time. Real
(commercial or
military) had AGC based voting systems to decided which signal to pass.
We ran
into issues of our audio phases shifting producing very odd sounding
"flanging"
effects.

I have often thought about trying this with receivers whose antennas
are only a few
hundred to thosand feet apart. I never have gotten around to it.


The military also used freqeuncy diversity, sending the same singal on
more then one
frequency. Kind of like listening to WWV on 5 10 and 15MHz at the same
time.

Let me suggest using identical antennas spaced at some multiple of 1/2
wave apart, preferably starting at 3/4 of a wavelength or more away from
each other. In other words, 3/4 wavelengths apart is good, 5/4
wavelengths is good, etc... Use the same lengths of coaxial cable to
each antenna. Then you'll have a decently identical array, offset by
1/4 wave. You probably shouldn't have the antennas at 1/4 of a wave
apart because you might radiate enough LO from one receiver in to the
other to cause problems.

Your receivers ought to have a switch for turning off AGC; however, if
you don't have that, you could just turn up the RF gain so that the AGC
doesn't have any affect.

Ideally, both receivers should be tuned from the same VFO. However,
with envelope detection, this isn't really a big deal.

This method will work particularly well with Active antennas. Two
Active antennas spaced apart by 3/4 of a wave should yield good
diversity. It will also have a nearly identical omnidirectional
patterns, which should help when trying to cancel out multiple paths.
The key to understand here is that when one spot has a null on the
carrier, the other place will have constructive interference.


73,

DE AB3A

In article ,
dxAce wrote:

Carter-k8vt wrote:

wrote:

Why do you insist that the atmosphere treats the carrier differently
from the rest of the signal?

Because it does. See below...

Geez. You have a spectrum produced by
modulation. If the modulation is AM, then a carrier is present. Now you
are saying the atmosphere is sucking out the narrow band carrier and
leaving the wideband spectrum untouched. Fiction at best.

Bzzzzt. Wrong!

Yes, the "atmosphere" [ionosphere] DOES "suck out a narrow band" or even
a single frequency. Ask any amateur radio operator that has used RTTY
(radio teletype).

The RTTY "modulation mode" used is FSK or frequency shift keying. At any
given instant, the transmitter is sending either a "mark" or "space",
essentially two carriers if you will, 170 Hertz apart that represent the
5-level Baudot code as used in ham RTTY.

As an aid to tuning, an oscilloscope is used as a tuning indicator; the
mark signal from your RTTY decoder is connected to the horizontal plates
of the 'scope, the space signal to the vertical plates. On the screen of
the CRT (due to the persistence of the CRT phosphors and your eyes),
this shows what appears to be a "+" sign, also known as the classic
"cross display". When you see the cross on your screen, you know you are
tuned in properly.

So, "What does this have to do with the discussion above?" you ask.

Remember, you are looking at essentially two "carriers", 170 Hz apart,
one on the horizontal axis and one displayed on the vertical axis.
During disturbed ionospheric conditions, many times you will see one
signal or the other disappear; i.e., the cross turns into a single line,
either a "-" or a "|", depending if the mark or space faded--and yes,
sometimes both fade, but it is more common to see one or the other
disappear.

This phenomenon is known to hams as "selective fading", is quite common
and is interesting to observe.

So, yes, the ionosphere CAN suck out one signal separated from another
by as little as 170 Hz.

But, is it 'sucking it out' or merely propagating it somewhere else other than
that particular spot where your antenna is?

And that 'somewhere else' might not be very far away, but merely a few
wavelengths in distance.

The answer is multi-path signals sum or cancel based on phase as I wrote
he

"Phase cancelation is a frequency dependent phenomenon and the
modulation scheme or part of it does not matter.

When phase cancelation is narrow and right at the carrier frequency
then the carrier level goes down where the rest of the side band signal
is still good."

This is called selective fading.

This is not an absorption or refraction problem.

--
Telamon
Ventura, California

Carter-k8vt wrote:
wrote:

Why do you insist that the atmosphere treats the carrier differently
from the rest of the signal?

Because it does. See below...

Geez. You have a spectrum produced by
modulation. If the modulation is AM, then a carrier is present. Now you
are saying the atmosphere is sucking out the narrow band carrier and
leaving the wideband spectrum untouched. Fiction at best.

Bzzzzt. Wrong!

Yes, the "atmosphere" [ionosphere] DOES "suck out a narrow band" or even
a single frequency. Ask any amateur radio operator that has used RTTY
(radio teletype).

If the carrier is effected, the odds are signals nearby (i.e. the
audio) will be effected. I still stand by the statement that the only
advantage to sync demod is that you have cut the bandwidth in half. I
never bought the "fresh carrier" story.



The RTTY "modulation mode" used is FSK or frequency shift keying. At any
given instant, the transmitter is sending either a "mark" or "space",
essentially two carriers if you will, 170 Hertz apart that represent the
5-level Baudot code as used in ham RTTY.

As an aid to tuning, an oscilloscope is used as a tuning indicator; the
mark signal from your RTTY decoder is connected to the horizontal plates
of the 'scope, the space signal to the vertical plates. On the screen of
the CRT (due to the persistence of the CRT phosphors and your eyes),
this shows what appears to be a "+" sign, also known as the classic
"cross display". When you see the cross on your screen, you know you are
tuned in properly.

So, "What does this have to do with the discussion above?" you ask.

Remember, you are looking at essentially two "carriers", 170 Hz apart,
one on the horizontal axis and one displayed on the vertical axis.
During disturbed ionospheric conditions, many times you will see one
signal or the other disappear; i.e., the cross turns into a single line,
either a "-" or a "|", depending if the mark or space faded--and yes,
sometimes both fade, but it is more common to see one or the other
disappear.

This phenomenon is known to hams as "selective fading", is quite common
and is interesting to observe.

So, yes, the ionosphere CAN suck out one signal separated from another
by as little as 170 Hz.

Carter
K8VT

If the selective fading is as tight as you indicate, then there would
be "holes" in the audio spectrum of the recovered AM, much like a comb
filter. Sync demod won't fix that problem.

There are too many people that think sync demod will cure everything.
It's just not true. Now if you have a nearby signal bleeding into the
desired signal, then pick the sideband the furthest away from the
interfering signal. Here, sync works great. If you have fading, you can
narrow band the signal by using one sideband. It helps a bit, but the
signal will still fade.

All this assumes your sync is decent, and not a growler. Otherwise, all
bets are off.

On 17 Jan 2007 20:54:27 -0800, wrote:


Telamon wrote:
In article . com,
wrote:

Michael Black wrote:
"N9NEO" ) writes:
Greetings,

I have just got a sony ICF sw7600GR and it is a very nice radio.
The sync detector seems to take care of a lot of the distortion,
but the audio continues fading in and out and is quite annoying.
Could the fading be mitigated to any extent by using another
stage of agc? I am going to be doing some experiments with the
455kc if out on my Red Sun RP2100 whenever it gets here.
Detectors, filters, SSB, etc... I thought that along with other
experiments I might want to try some outboard agc.

Synchronous detectors have never been about dealing with fading.
They are about ensuring there is enough "carrier" to beat the
sidebands down to audio.

Narrow band signal have less fading, thus sync demod will have less
fading. However, the result isn't all that significant since all you
have done is cut the bandwidth in half.

Narrow band signals do not have less fading.

So there's fading on the incoming signal. That means the amplitude
of the sidebands is varying with that fading. A locally generated
"carrier" at the receiver ensures that there is something to beat
those sidebands down to audio, even if the transmitter's carrier
has faded too much to do the proper job. But a constant level
"carrier" at the receiver beats the sideband down to audio intact,
ie an ideal mixer would not add anything to the signal. So if the
sideband is fading, of course the audio output of the receiver will
vary with that fading.

With an envelope detector, the carrier isn't beating down the
sideband. If you just look at the math of AM modulation, you would
see that the carrier is just there for the ride.

Selective fading occurs when conditions cause a very narrow band of
frequencies to be received at very low amplitudes where most of the
side band information is present at levels that your receiver can
ordinarily demodulate properly.

When part of the side band is being notched out it does not sound all
that bad but when the carrier gets weakened then the AM demodulator
can't process the side band information properly and there is
horrendous distortion. The carrier which is at the right frequency and
phase relative to the side band information keeps the detector in the
linear region so distortion is minimized.

A sync detector uses a local oscillator in a similar to the way SSB is
detected with the difference that it is phase locked to the signal
carrier and mixed with it so when the carrier fades out this near
perfect copy of the carrier allows the demodulator to continue to
detect the side band or bands without distortion during a carrier
fading condition. Here this necessary frequency and phase information
carried by the "carrier" is retained by the sync circuitry.

What the sync detector brings you is the ability to decode that
signal even if the carrier goes missing, because of selective
fading.


Snip

Michael has it right.

--
Telamon
Ventura, California

Why do you insist that the atmosphere treats the carrier differently
from the rest of the signal? Geez. You have a spectrum produced by
modulation. If the modulation is AM, then a carrier is present. Now you
are saying the atmosphere is sucking out the narrow band carrier and
leaving the wideband spectrum untouched. Fiction at best.
No, it is called selective fading, and it is a real phenomenon.
You can think of the two sidebands for AM as creating constructive and
destructive interference with the carrier. Any time the sideband
energy exceeds the carrier energy you get the equivalent of over
modulation. DSB and an envelope detector isn't a good combination.

As others have pointed out, what Synch detection does is to insure
that the sideband energy can never exceed the carrier energy. While it
is preferable to have it in phase, which a synch detector does, in
human speech, phase carries no information. As a result SSB can be
used to communicate. A product detector (which is used for synch
detection) simply produces phase distortion if the local carrier isn't
in phase with the original carrier. The phase distortion is a fact of
life in SSB-SC communication. If you Independent side band, and
generate the two independent side bands at quardrature, it is possible
to determine, and lock the local carrier in phase with the original
carrier.

wrote:

If the selective fading is as tight as you indicate, then there would
be "holes" in the audio spectrum of the recovered AM, much like a comb
filter. Sync demod won't fix that problem.

There are too many people that think sync demod will cure everything.
It's just not true. Now if you have a nearby signal bleeding into the
desired signal, then pick the sideband the furthest away from the
interfering signal. Here, sync works great. If you have fading, you can
narrow band the signal by using one sideband. It helps a bit, but the
signal will still fade.

All this assumes your sync is decent, and not a growler. Otherwise, all
bets are off.

If sync' detectors are only good for selecting the best sideband, how do
you explain the fact that the audio distortion caused by selective
fading can almost completely be eliminated with a good sync' detector?