Questions about Linrad and polarity

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Earl Shaffer

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Jan 8, 2021, 11:51:33 AMJan 8
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Hi All.

MAP65 is quickly becoming obsolete so I am looking at alternatives. This brings me to Linrad so I have questions about Xpol and polarity handling in Linrad. Since WSJTx is being actively updated we can always expect it to have up to date features and modes and performance levels. The first question is about the wide band waterfall. For a system where both vertical and horizontal signals are presented to Linrad, how does the wide band waterfall display signals of both polarities? Is it some sort of digital mix from both channels? Will it be as sensitive as a polarity matched input to a single receiver? 
Maybe I missed where this is written about. A link would be welcome. There is little information on the internal workings of Map65. Does the wide band waterfall of Map65 simply copy Linrad or is it different?

The next question is about adaptive polarity. I understand adaptive polarity was designed for CW EME using an Xpol system.  What I have noticed while using an Xpol system with it and monitoring noise, it will seek to minimize one channel while maximizing the other channel. In my case it is noise. I can see that for the case of CW one might use a narrow bandwidth perhaps 100 hz and the CW signal may then show a significant signal to noise ratio and the adaptive polarity will correctly maximize one channel while minimizing the other.  I would be passing JT65B signals through it at a 600 to 1000 hz bandwidth. Given that these signals are often much weaker than CW signals the adaptive polarity may simply peak on noise instead of signal? Is this correct? What would be nice is if adaptive polarity could be used on Linrad to drive WSJTx giving a simple single frequency solution to automatic polarity control. I wonder how Map65 does this polarity control? Does it simply copy the adaptive polarity control of Linrad or perhaps it is done differently. Maybe it is done in digital and based on the sync signal. Is there a link detailing the workings of this?  Failing the adaptive polarity control working for this, I need to duplicate the W3SZ experiment where multiple programs are run for various polarities.  I hope I don't have to do that. I'm not sure I have enough room for the monitors to keep track of all of that. 

Leif Asbrink

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Jan 9, 2021, 12:11:55 AMJan 9
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Hi Jim,

Answers inline below.

> MAP65 is quickly becoming obsolete so I am looking at alternatives. This
> brings me to Linrad so I have questions about Xpol and polarity handling in
> Linrad. Since WSJTx is being actively updated we can always expect it to
> have up to date features and modes and performance levels. The first
> question is about the wide band waterfall. For a system where both vertical
> and horizontal signals are presented to Linrad, how does the wide band
> waterfall display signals of both polarities? Is it some sort of digital
> mix from both channels? Will it be as sensitive as a polarity matched input
> to a single receiver?
Linrad computes the power of the polarization that is present in each
frequency bin. This becomes powerful when there are several spectra
for each line in the waterfall. On the other hand narrower bin widths
improves S/N while making the waterfall slower. Optimum is to set
the bin width to match the line width of the signal of interest.

On 144 MHz, for CW, the bandwidth of the carrier is about 0.2 Hz due
to doppler spread but frequency stability (chirp) typically makes
the bandwidth larger. For the sync tone of jt65 the bandwidth is
5 Hz if I remember correctly. That means that each FFT spectrum spans
0.4 seconds when sine squared windows are used. Then spectra overlap
by 50 % so a new spectrum would arrive each 0.2 second. If averaging
is set to 25 a new line would appear every 5 th second and a complete
transmission would produce about 10 lines. Then sensitivity would be
extremely good. The 25 spectra would give two average power spectra
plus one correlation spectrum (real and imaginary part.) I wrote the
code long ago and I did not make it quite right - and when now looking
I did not find that particular code snippet, but the formula I
adopted was the sum of the horisontal power and the vertical power
plus a function of the correlation spectra that would give a signal
with 50% of the power in H and 50% of the power in V not the sum
of H^2 and V^ (50% of what a signal in either antenna would give)
but also the power of the correlation spectrum so that a signal
at 45 degree polarization would give the same power as a signal
in one or the other antenna.

Today I think a better choice would have been to make a linear
combination of the signals to get new polarizations that would
have zero correlation. Then display the strongest one only.
A frequency bin with only noise in it would have almost zero
correlation if many anough spectra are averaged for each waterfall
line so then only the strongest of H or V would be displayed.
With noise only they would be equal so noise only would be
reduced by 3 dB. Likewise, by use of the correlation spectra one
could do a transformation into two uncorrelated signals and then
skip the weakest.

> Maybe I missed where this is written about. A link would be welcome. There
> is little information on the internal workings of Map65. Does the wide band
> waterfall of Map65 simply copy Linrad or is it different?
I am pretty sure it is different.

> The next question is about adaptive polarity. I understand adaptive
> polarity was designed for CW EME using an Xpol system. What I have noticed
> while using an Xpol system with it and monitoring noise, it will seek to
> minimize one channel while maximizing the other channel. In my case it is
> noise.
Yes. .

> I can see that for the case of CW one might use a narrow bandwidth
> perhaps 100 hz and the CW signal may then show a significant signal to
> noise ratio and the adaptive polarity will correctly maximize one channel
> while minimizing the other.
I made the algorithm for use with the other enhancements of Linrad.
For CW the bandwidth would be in the range 15 to 25 Hz. Seldomly above
18 Hz. The AFC that looks both backwards and forwards in time would
guarantee that the signal is centered in the passband even when it
is well below what can be copied even with the coherent detect.
(Equivalent to synchronous detection of AM.)

> I would be passing JT65B signals through it at
> a 600 to 1000 hz bandwidth.
That would be useless. The JT65B bandwidth is something like 400 Hz.
If you set 800 Hz you loose 3 dB. Linrad would compute power spectra
and channel correlations averaged over the time constant you have chosen.
Maybe 10 seconds. That means you would average 10000 times so the
S/N improvement would be sqrt(10000) or 20 dB. Assuming an S/N of
6 dB is needed for a reasonably accurate polarization one should
need -14 dB in 800 Hz or -17 dB in the JT65 scale at 2.4 kHz.
That is a pretty strong signal...

BUT - the main problem is that this works ONLY in case there is no
polarized interference - so forget it.

> Given that these signals are often much weaker
> than CW signals the adaptive polarity may simply peak on noise instead of
> signal? Is this correct?
YES!

> What would be nice is if adaptive polarity could
> be used on Linrad to drive WSJTx giving a simple single frequency solution
> to automatic polarity control. I wonder how Map65 does this polarity
> control? Does it simply copy the adaptive polarity control of Linrad or
> perhaps it is done differently.
It is different. Linrad has an equal sensitivity for linear and circular
polarization. For CW there is no loss associated with that. MAP65 has
a 3 dB loss for circular.

> Maybe it is done in digital and based on
> the sync signal. Is there a link detailing the workings of this? Failing
> the adaptive polarity control working for this, I need to duplicate the
> W3SZ experiment where multiple programs are run for various polarities. I
Yes. This is the only solution unless the adaptive polarization algorithm
has a decoder for each mode of interest. It is highly efficient however
and only four instances of WSJTx and one instance of Linrad would need
to be visible on your screens. The maximum polarization error would be
22.5 degrees - but you would have two largely independent channels to
detect a weak signal in one or the other so the loss would be less than
the the 0.7 dB associated with a 22.5 degree polarization mis-match.

You could also tell linrad to supply three instances of WSJTx with
polarizations H, +60 and +120 degrees. The maximum error would be 30
degrees wioth an associated loss of 1.3 dB. Somewhat less, because there would
be two channels with the same loss and tha chance to fail in both for
a marginal signal is less (probably far less) than 100%

> hope I don't have to do that. I'm not sure I have enough room for the
> monitors to keep track of all of that.
On some microwave bands people use circular....

73

Leif

Earl Shaffer

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Jan 9, 2021, 11:04:12 AMJan 9
to Linrad
Hi Leif.

Thanks for the nice explanation. It looks like the Linrad waterfall should be very effective for spotting EME signals of any polarity as it is.
It's too bad the adaptive polarity is not suitable for digital signals. That makes me wonder about Map65. People often compare WSJT to Map65 and reach different conclusions. Some of those results may be due to different sensitivities caused by gain or lack of gain in a setup. I think I will be able to tell for myself soon with a setup that should favor Map65. I  wonder how much frequency drift plays into those comparisons. I also wonder if Map65 has AFC and if so, how much and does it fail at some lower signal level? Perhaps Linrad ahead of WSJT should be providing AFC. Very often I see stations with 25 hz drift from the beginning to the end of a transmission. It seems that this could cause averaging to fail.  Maybe some programs do better than others in that respect. The failure of adaptive polarity for digital signals in Linrad makes me wonder about Map65. There may be some lower limit that it fails to resolve the polarity correctly. I have seen times where it suddenly flips its TX polarity suggestion to the opposite polarity and then back again for the next transmission. It seems like I do better when I ignore that brief change of mind. It would be nice to know for sure what the lower limit is for resolving polarity and how that might be influenced by noise. Noise and polarity control CAN be a reason for different results for different people when comparing the two. I will probably keep Map65 in my system as long as it supports the dominant mode for 2 meter EME. I may eventually only use the message window to know where the stronger stations are in frequency.  I may add in Linrad and other polarity options gradually. One interesting possibility might be to add an instance of Linrad that uses full adaptive polarity control at 5 Khz bandwidth to feed an instance of WSJT. Linrad would  feed the weaker output which would be the exact polarity to minimize noise. This should give a usable signal more than 50% of the time. I often find that I only decode EME signals when the EME signal  matches the polarity  with the lowest noise level.

I have been looking at old Linrad postings trying to learn what I could about Linrad and Map65 development.  I wonder if W3SZ ever figured out why Afedri seemed to do better for him than WSE? I think Map65 development moved off of the Linrad reflector almost a decade ago now and over to the WSJT development reflector but I could find almost nothing about Map65 there. For a few years there was some really good development of  Map65 happening on the Linrad reflector.  The problem I currently see with the strong signal  Noise blanker malfunction was noted in very early versions of Map65 and remains. I think W5UN was the last one to mention it. I asked a friend of mine about modifying Map65 but he says he knows nothing about QT. There are a large number of different programming languages used in Map65 and I guess this may be common practice for software. I still find it interesting to catch new stations on the air trying out their terrestrial stations on EME.
Noise is still my biggest challenge and will probably remain so. I don't know if I will try microwaves. A dish antenna is about as exciting as a dipole to me.

73, WB9UWA

--
There is an excellent Linrad User Guide by Gaetan, ON4KHG, at:
http://w3sz.com/Linrad%20Installation%20&%20Configuration%20User%20Guide%20-%20V1-0.pdf
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Joe Taylor

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Jan 9, 2021, 11:42:08 AMJan 9
to lin...@googlegroups.com, Earl Shaffer
Hi Earl,

> It's too bad the adaptive polarity is not suitable for digital signals.

I have no idea why you, as a user of MAP65, would make such a statement.

MAP65 is explicitly designed to implement reception of JT65 signals with
adaptive linear polarization. It matched the received polarization
angle of each and every decodable signal.

> That makes me wonder about Map65. People often compare WSJT to Map65 and
> reach different conclusions. Some of those results may be due to
> different sensitivities caused by gain or lack of gain in a setup. I
> think I will be able to tell for myself soon with a setup that should
> favor Map65. I  wonder how much frequency drift plays into those
> comparisons. I also wonder if Map65 has AFC

Yes

> and if so, how much and
> does it fail at some lower signal level?

Of course it must fail at some lower signal level.

> Perhaps Linrad ahead of WSJT should be providing AFC.

No. How could Linrad possibly do AFC corrections for each one of a
whole sub-band full of decodable signals?? Linrad's AFC focuses on an
individual signal.

> Very often I see stations with 25 hz drift from
> the beginning to the end of a transmission. It seems that this could
> cause averaging to fail.

Of course such drift could cause successful averaging to fail.

These days, there is little excuse for transmitting a signal with such
drift on the 144 MHz band.

> Maybe some programs do better than others in
> that respect. The failure of adaptive polarity for digital signals in
> Linrad makes me wonder about Map65. There may be some lower limit that
> it fails to resolve the polarity correctly. I have seen times where it
> suddenly flips its TX polarity suggestion to the opposite polarity and
> then back again for the next transmission.

Why should this surprise you ? Surely you must understand that
confidence in predicting the best Tx polarization angle depends on
measured parameters, the accuracy of which depends on SNR.

It seems like I do better
> when I ignore that brief change of mind. It would be nice to know for
> sure what the lower limit is for resolving polarity and how that might
> be influenced by noise. Noise and polarity control CAN be a reason for
> different results for different people when comparing the two. I will
> probably keep Map65 in my system as long as it supports the dominant
> mode for 2 meter EME. I may eventually only use the message window to
> know where the stronger stations are in frequency.  I may add in Linrad
> and other polarity options gradually. One interesting possibility might
> be to add an instance of Linrad that uses full adaptive polarity control
> at 5 Khz bandwidth to feed an instance of WSJT.

That would be a good solution for optimizing performance on one
particular signal at one particular time.

MAP65's strengths are very different. It gives you access to all
potentially decodable JT65 signals in the received subband.

-- Joe, K1JT

Earl Shaffer

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Jan 9, 2021, 1:59:28 PMJan 9
to Linrad
Hi Joe.

On Sat, Jan 9, 2021 at 10:42 AM Joe Taylor <j...@princeton.edu> wrote:
Hi Earl,

> It's too bad the adaptive polarity is not suitable for digital signals.

I have no idea why you, as a user of MAP65, would make such a statement.

MAP65 is explicitly designed to implement reception of JT65 signals with
adaptive linear polarization.  It matched the received polarization
angle of each and every decodable signal.

First I want to say a big thankyou for all you've done for EME. If not for digital, I would not be able to
do EME due to noise. I might only be able to work a few large CW stations and just how many times can a person do that?  I don't want to
seem ungrateful or minimize your large contribution to the state of the art.  I'm planning an upgrade path for myself, so I'm trying to look into my crystal ball just a bit.  My comment about adaptive polarity is for Linrad. I realize Map65 does it well. I'm just not sure how long Map65 will stay relevant. Aside from adding QRA64 to Map65 it has seen little or no development for nearly a decade now. As you have pointed out, there are only a few dozen people using Map65 and thousands using WSJTx. It seems the smart thing to do is to start using WSJTx as best possible and look for Map65 to become irrelevant at some point.  When that will happen. I don't know. Q65 may or may not be added to Map65. If it IS added then Map65 will likely stay relevant longer, especially if controls and features similar to WSJTx  that aid decoding sensitivity are added.  If QRA64 is any indication, I will need a faster computer. It really bogs down my little single core XP computer.
I am glad to see the sync tone is present on Q65 but I wonder about seeing shorthand tones on the waterfall.
I am in the habit of decoding those visually and it makes for much faster contesting (and QSO's) . I believe that EMEers could make valuable contributions to new modes and software in the planning stages. Some of these contributions (or lack of)  may impact the acceptance of new modes.
 I'm glad to see that Map65 has AFC. These sorts of things are hard to figure out even by looking over a decade of Linrad messages. 
My comment about AFC in Linrad would be for single frequency use with WSJTx. I presume that WSJTx already has AFC, so additional AFC in Linrad would simply be redundant.

73, Earl Shaffer, WB9UWA.

 
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