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Author Topic: What RTTY Station Is That on 4905 KHz?  (Read 8055 times)

Offline ultravista

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Re: What RTTY Station Is That on 4905 KHz?
« Reply #15 on: April 29, 2018, 1502 UTC »
Token - found the signal @ 4905 khz again this morning.

With my Yeasu FT-950, tuning while in CW mode seems best. The signal, as you described above, remains constant when switching from USB-CW to LSB-CW. It stays strong and 'centered' in HDSR.

With that said, is it logical to use CW mode for this?

Offline Token

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Re: What RTTY Station Is That on 4905 KHz?
« Reply #16 on: April 29, 2018, 1604 UTC »
Token - found the signal @ 4905 khz again this morning.

With my Yeasu FT-950, tuning while in CW mode seems best. The signal, as you described above, remains constant when switching from USB-CW to LSB-CW. It stays strong and 'centered' in HDSR.

With that said, is it logical to use CW mode for this?

How you tune such signals is totally up to you.  Particularly encrypted signals like this, since there is no correct or incorrect way based on how decoding must be done.

However, I would not use CW mode at all for these, I would use USB for this specific signal.

Why?  Because the defined center frequency for this kind of signal (STANAG 4481 FSK) is 2000 Hz.  That means there is a correct tuned frequency for the signal.  If the center of the data is on 4905 kHz then tuning to 4903 kHz in USB mode will put the center of the data at 2000 Hz, or the space at 1575 Hz and the mark at 2425 Hz.

And yes, most military and utility modes are tuned in USB.

So for this signal it would go in my log book as 4903 kHz, USB mode, c/f (center frequency) 4905 kHz.  But I would report it to people in chat as FSK on 4905 kHz c/f.  Not confusing at all, is it?  Also, tuning this way (4903 kHz USB mode) will allow you, without adjusting software from defaults, to use software to strip the KG-84 headers and confirm it is indeed STANAG 4481 FSK and encrypted.  If, for some reason, the transmission was not encrypted it would also allow you to receive the text.

It is kind of like the question of WEFAX, do you report center freq?  Or do you report tuned freq?

T!
T!
Mojave Desert, California USA

Offline ultravista

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Re: What RTTY Station Is That on 4905 KHz?
« Reply #17 on: April 29, 2018, 1611 UTC »
Thank you!

I am surprised that the little PA0RDT active mini whip can pickup so much here in Nevada. In the AM, I can tune at least 50+ APAC stations. 

One day I'll put up another antenna to see what else I can get.

I am on a postage stamp sized lot in a tightly packed neighborhood. Do you have any 'wonder' antennas for HF RX - something that may outperform the mini whip?

Offline Token

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Re: What RTTY Station Is That on 4905 KHz?
« Reply #18 on: April 29, 2018, 1706 UTC »
I am on a postage stamp sized lot in a tightly packed neighborhood. Do you have any 'wonder' antennas for HF RX - something that may outperform the mini whip?

Unfortunately, small space antennas are always a compromise.

I do,  however, like loops when talking about antennas for small space, urban or higher noise, environments.  While it may not outperform the mini-whip in all cases, a loop, something like the Pixel Pro or the Wellbrook 1530+, when mounted on a rotor will allow you to null offending local noise sources.  Where the mini-whip is omni, the loops have a null in their pattern.  If you tune to a signal and there is locally (in the house or neighborhood) generated noise on the same freq you can turn the loop to point the null at the noise source, often greatly improving reception of the desired signal.

Such a loop is more expensive and more complex (you need a rotor as well as the antenna) to set up, but in my opinion often very much worth it.

T!
T!
Mojave Desert, California USA

Offline MDK2

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Re: What RTTY Station Is That on 4905 KHz?
« Reply #19 on: April 29, 2018, 1730 UTC »
Don't forget the W6LVP loop. I'm not sure exactly how well it compares to the others, but it works well for me in my urban environment, and Larry Plummer (W6LVP) is terrific about service and answering any question you have. Just have to toss that out there.
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W6LVP active loop, 20m dipole, homebrewed mag loops.
eQSLs appreciated wickerjennie at gmail

Offline Token

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Re: What RTTY Station Is That on 4905 KHz?
« Reply #20 on: April 29, 2018, 1741 UTC »

However, I would not use CW mode at all for these, I would use USB for this specific signal.


So why did I say this?  In the past some radios did have CW/RTTY specific modes, often using the same switch position on the radio.

CW mode typically selects a narrower filter, CW being a narrow mode of operation.  Such a narrow filter might very well clip out portions of the digital / image / data mode you are trying to receive.  This is less an issue today with DSP radios and adjustable filters, but when radios used hardware filters (or modern radios with hardware filters) this was a potential problem.

Another issue is that CW mode introduces a frequency bias.

If a CW signal is on the frequency of 7500.000 kHz and you tune to it in USB or LSB, on 7500.000 kHz, then you will hear nothing (almost nothing), until you tune off frequency slightly.  This is normal because in these modes the CW signal would be at "zero beat" when tuned to 7500.000 kHz, and so you hear no audio tones.  To hear the tones you tune below the frequency in USB mode and tune above the frequency in LSB.  The audio tones heard will be equal to the tuning offset.

So CW mode on a receiver intentionally introduces a frequency offset, often around 700 Hz.  With such an offset when a signal is on 7500.000 kHz and you tune to 7500.000 kHz in receiver CW mode you hear the dots and dashes as 700 Hz tones (many newer receivers have adjustable CW offsets, so you can set this tone frequency to whatever you are comfortable with).

The problem is that using CW mode on the receiver this offset also is applied to any data signal you have.  So now the audio will be the signal offset plus the CW offset.

Assuming your CW offset is 700 Hz if you tuned to the RTTY on 4905 kHz using CW-U and tuned to 4905.000 kHz, and you widened up the filter wide enough to get both Mark and Space the tones will be at 1125 Hz (700 Hz CW offset + 425 Hz from center) and 275 Hz (700 Hz CW offset - 425 Hz from center).  Using CW-L mode will result in the same audio tones, but invert the relationship of the Space and Mark.

But STANAG-4481 FSK has a spec of space and mark at 1575 and 2425 Hz, so CW-U or CW-L clearly results in different values.

Better to just use USB (or LSB, as required) with such signals, for pretty much anything except Morse.

T!
T!
Mojave Desert, California USA

Offline Josh

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Re: What RTTY Station Is That on 4905 KHz?
« Reply #21 on: April 29, 2018, 1745 UTC »
Here's some fsk logs from some time back;
04.005.00  850Hz 50Bd  FSK   0412  11.12.16 CV-768/Rockwell 700B

05.340.00  850Hz 50Bd  FSK   009Z  12.05.16 

06.487.00  850Hz 75Bd  FSK   0349Z 11.12 16 TST-4043/CV-768/Rockwell 700B/STANAG 4481

08.416.60  200Hz 100Bd SITOR 1358Z 11.14.13

08.471.60  200Hz 45Bd  FSK   0300Z 04.05.14 WLO

08.471.60  200Hz 100Bd SITOR 0300Z 04.05.14 WLO

09.473.00  200Hz 100Bd SITOR 2048Z 11.13.16

10.153.00  850Hz 75Bd  FSK   2226Z 11.15.16 CV-768/Rockwell 700B

11.685.50  850Hz 75Bd  FSK   1837Z 11.17.13


12.012.50  850Hz 75Bd  FSK   1834Z 11.17.13

12.117.50  850Hz 75Bd  FSK   1834Z 11.17.13

12.603.40  200Hz 100Bd SITOR 2230  06.11.14 GREEK

13.869.50  850Hz 75Bd  FSK   1739Z 11.17.13

14.928.50  850Hz 50Bd  FSK   2107Z 11.11.16

15.598.50  850Hz 75Bd  FSK   1736Z 11.17.13


16.122.50  850Hz 75Bd  FSK   1733Z 11.17.13

16.267.00  850Hz 75Bd  FSK   1733Z 11.17.13

16.806.60  200Hz 100Bd SITOR 1144Z 11.14.13




And my latest USN fsk sched;

USN HF FSK Network

NPN Barrigada Guam       
8654 14887 15601 16015

Croughton UK             
7477 9074 10286 11464

AFA/NSS Davidsonville Md
4005 4985 6487 6497 9830 10712 12015 13590 15959

NKW Diego Garcia BIOT   
4550 8093 8547 8550 22471 22910

NPG Dixon Ca             
4905 7597 8538 8694 9085 10320 10430 13870 16268

NAU Isabela PR           
8676 7455 7711 10153 10895 11688 12120 16123

NPM Lualualei Ha         
8676 9112 9960 14434 18558

NDT Totsuka Japan       
5304 6358 10830 14375
 
NSY Sigonella Italy     
5414 7420 9904 10974 18040 18411

Some channels have night/day ops qrg, example; Isabella 16123 day 10153 night.
« Last Edit: April 29, 2018, 1748 UTC by Josh »
Conveniently located near Vincennes Indiana.

Offline ultravista

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Re: What RTTY Station Is That on 4905 KHz?
« Reply #22 on: April 30, 2018, 0304 UTC »
Token - thank you again.

Josh - how are you able to determine the location?

Offline Josh

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Re: What RTTY Station Is That on 4905 KHz?
« Reply #23 on: April 30, 2018, 1652 UTC »
Josh - how are you able to determine the location?

Some of the headings came from ITU and hobbyist df fixes, some from osint, some came from man on the ground sleuthing. Kinda like when I lived in West Omaha, anything that came out of the Elkhorn Communications Detachment was more than obvious.

http://www.offutt.af.mil/News/Features/Display/Article/312829/rural-communications-site-has-global-mission/
https://www.facebook.com/Det2Elkhorn/
Conveniently located near Vincennes Indiana.