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Author Topic: AM Broadcast/LW/MW frequency standards  (Read 2430 times)
N3DT
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« on: November 12, 2017, 10:08:14 AM »

I'm looking for an AM broadcast station in the NOVA area that I can use to compare with my GPSDO. I've been using Spectrum Lab to measure the difference between my Service Monitor locked to my GPSDO and HF stations. WWV (Colarado) appears to be about 50-100 mHz high at this time of day, CHU is better, especially 3.330, it's within 3 mHz average. But I notice that the AM Broadcast stations seem to be all over the map. I need one I know is good. Unless there's another close by MW or LW station that's a standard I don't know about. Might be something out of Annapolis or similar?
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K8AXW
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« Reply #1 on: November 12, 2017, 10:37:40 AM »

At one time the frequency of any AM broadcast station had to be +/- 10Hz.

Pick a station that you wish to use and call that station on the phone and speak to their engineer. Explain to him what you are doing and he will be happy to tell you exactly what frequency they are presently on.

(You might find that the smaller stations don't have an engineer.  The FCC has relaxed this requirement - Larger stations with more sophisticated/higher power transmitters probably still keep an engineer on site)
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K5TED
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« Reply #2 on: November 12, 2017, 11:33:44 AM »

You may or may not find this interesting as it pertains to your HF frequency measuring http://www.qsl.net/zl1bpu/IONO/doppler.htm

Frequency tolerance on commercial broadcast stations, per FCC rules, is +/-20 Hz for AM radio, +/- 2000 Hz for FM radio, and +/- 1000 Hz for the audio and video television carriers, and as a dimensionless tolerance of 0.0015 % for international shortwave broadcasters.

 

« Last Edit: November 12, 2017, 11:54:27 AM by K5TED » Logged
N3DT
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« Reply #3 on: November 12, 2017, 12:18:10 PM »

Thanks for that link TED. Yes, Iv'e been noticing for some time that trying to measure the freq of WWV is nearly impossible here within a couple Hz due to the ionospheric Doppler. That's why I was looking for a local station that may give me a better indication of what my GPSDO is doing. What I'm doing is generating a GPS locked signal 100 Hz lower than the measured freq and letting SpectrumLab do the calculations of the difference in freq, trying to eliminate any variations of the RX master osc (which is actually also locked to the same GPDSO), the BFO and any sound card drifting. I can actually reliably measure the drifting of the sound card, and the RX BFO within .005 mHz. I'm finding that 3.330 CHU gets me within .003 Hz average of 3,3000,000 over several hours, average. Now I still don't know if that's some Doppler shifting, or my GPSDO is off by that much. I think I will go looking for some LF marine transmissions and see what I come up with there. CHU is the best I've done so far. My motivation is to find out how close my GPSDO is to a real standard, if that's actually possible. Time will tell.

I know my BFO drifts ±10 mHz with the fan on and off and the Asus sound card drifts .030 mHz over an hour or so.

I've also got an old Rubidium standard I want to try. I used it years ago, I hope it still works.
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N3DT
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« Reply #4 on: November 12, 2017, 04:02:42 PM »

I tried 60KHz WWVB, but that seems to be too erratic at least with the SpecLab so I went back to CHU at 3.330. I'm still within .006 Hz with that, average. I may have to call a local AM broadcast station and see if they can give me any clue. There is something at 198KHz but I'm not sure what it is. I could probably live with only knowing freqs within ±.006 Hz, given enough time.
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K0OD
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« Reply #5 on: November 12, 2017, 05:17:23 PM »

Frequency tolerance on commercial broadcast stations, per FCC rules, is +/-20 Hz for AM radio...

Several years ago. when I was into Frequency Measuring Tests, I measured the accuracy our local St. Louis AM broadcasters and determined that all of them were impressively on the mark especially clear Channel 50,000 watter, KMOX. All were easily within the FCC standard. As I recall, the worst one was off by about 3 Hz.

My only frequency standard was WWV or perhaps CHU and I did pretty well in FMTs, within a few tenths of a Hz. Yes in FMTs Doppler becomes the main problem. I used nothing but a stock Flex-5000 and its built-in scope.

There's a Yahoo Frequency Measuring Group where you can get all your questions answered. 
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G3RZP
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« Reply #6 on: November 13, 2017, 06:04:32 AM »

198kHz is the frequency of the BBC Radio 4 broadcast transmitter at Droitwich, about 20 miles or so south west of Birmingham in the UK. I believe there is another BBC one on the same frequency with the same programme in the North of the UK. I believe they use rubidium standards.
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DL8OV
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« Reply #7 on: November 13, 2017, 08:58:37 AM »

The one 'up North' is the 60 KHz timekeeping signal that used to be transmitted from Rugby and they use atomic clocks. The site is at Anthorn in Cumbria Coordinates: 54.91°N 3.28°W.

I prefer DCF77 on 77.5 KHz as they are only 30 Km away from me.

Peter DL8OV
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N3DT
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« Reply #8 on: November 13, 2017, 09:06:57 AM »

Thanks RZP, what ever I'm receiving at 198KHz shows exactly 2.009 Hz below 198000 on my measurements. Pretty consistent. I don't hear it today. Oops it just came on just before 1700Z.

It's too bad WWVB is so weak here, the low power part of the signal does not even register which makes it unusable with SpectrumLab. Too much erratica. Yeah, I've thought about a Flex or similar DSP RX, but haven't taken the plunge. I was thinking about the Anon, but it looks to be a real project to get going for the computer semi-illiterate.

As far as AM broadcast, I find them pretty much all over the place, 2 Hz is not acceptable to me when I can measure at least to .01 Hz with consistency on a few signals.

So far my best  source is CHU on 3330 K. On that one over 2 hours, I get an average of 3,329,999.992 with a +.275 and -.164 and a standard dev of .0224. I'll have to try it longer.

I hear slow CW on 60KHz but nothing on 77.5. Thanks Peter.
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K0OD
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« Reply #9 on: November 13, 2017, 10:30:26 AM »

WWVB exclusively transmits digital nowadays but it can sound a bit like CW if very weak. I believe it ID'd on CW and perhaps even voice many years ago.

You won't hear the BBC station on 198 during daytime even on the east coast. Whether 60 KHz or 198 you'd still have Doppler to contend with from far away stations.   

AM broadcast stations using the HD format are supposed to be quite accurate. Phone or email the head engineer at a local HD broadcaster and ask him about their accuracy. He may well be a ham.
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N3DT
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« Reply #10 on: November 13, 2017, 01:00:25 PM »

According to the NIST site, WWVB is now a legacy AM/PM signal, so it's still heard as an AM modulated CW signal, but the low part of the AM is 17dB below the peak, but it's still a modulated AM carrier with PM on it. https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwvb I thought I read somewhere that because of the low frequency, it can be used for freq reference with much better accuracy than the WWV signals. But yes, because of the modulation, I get a very erratic average with the WWVB signal.

This is what their publication says:
"If  you  use  WWVB  for  frequency  measurements  or  calibrations, there  is  no  need  to 
estimate  or  compensate  for  the  path  delay.  For  frequency, the  important  issue  is 
path stability, or  the  changes  in  the  path  delay  that  occur  over  time.  Part  of  the  signal  that leaves  the  WWVB  transmitter  travels  along  the  ground  (groundwave)  and  another  part  is  reflected  from  the  ionosphere  (skywave).   Groundwave  reception  provides  better results  than  skywave  reception.  The  reason  is  simple—the  groundwave  signal  follows  a direct route to your receiver, and therefore the path length doesn’t change very much."

Anyhow, It doesn't seem to work very good for me, but I may try making a tuned ferrite antenna for it to see if it improves.

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K0OD
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« Reply #11 on: November 13, 2017, 02:39:31 PM »

I'm no expert on FMTs although I played in a few of them. I don't recall anyone mentioning calibrating a ham radio on WWVB. Of course few radios tune down to 60 kHz. Older Flex receivers like my 5000 are worthless below the AM BCB due to intermod. I use a Palomar converter to receive longwave. (BTW WWVB  is almost always loud at my place in Missouri)

In FMTs I always calibrated to the closest WWV to the test frequency. Or some hams recommend using the highest WWV you can hear which might be 20 MHz. Most often I've used the 5 MHz transmitter which is usually loud and stable here.

Not sure about WWVB's mode, but it doesn't transmit anything in Morse code or voice even on the hour. 

I doubt you can hear WWVB on groundwave very far. Here's an eHam discussion on a similar  subject:
https://www.eham.net/ehamforum/smf/index.php?topic=34199.0
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K0OD
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« Reply #12 on: November 13, 2017, 03:22:17 PM »

Everything you always wanted to know about the complex and ever-changing WWVB including...

"Since WWVB's low frequency signal tends to propagate better along the ground, the signal path from transmitter to the receiver is shorter and less turbulent than WWV's shortwave signal, which is strongest when it bounces between the ionosphere and the ground. This results in the WWVB signal having greater accuracy than the WWV signal as received at the same site."

https://en.wikipedia.org/wiki/WWVB

Does that mean anything for huge majority of us outside the groundwave range of WWVB?
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N3DT
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« Reply #13 on: November 14, 2017, 05:49:26 AM »

According to the NIST map, WWVB groundwave covers the entire US just fine. I've got one of those WWVB 'Atomic' clocks downstairs where I can't get cell coverage and it works fine. I can hear it on my TS2000 with the 80M antenna, but it's not strong.
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K0OD
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« Reply #14 on: November 14, 2017, 08:04:39 AM »

WWVB runs 70 KW on 60 KHz. Why can't I hear 50 KW stations in the nearby AM BCB 100 or so miles away during daytime? Is it something about signals that low. I'm puzzled. Might be worth a telephone call to Colorado.

There are other strong signals at the bottom of longwave, those megawatt military data transmitters around 25 kHz. I don't know a thing about their accuracy, but they're more powerful than WWVB and they use gigantic antennas.  One is in Maine.

A good longwave source on the US east coast might be the French AM broadcaster on 162 KHz. Last year it shut down regular programming on longwwave, but it still transmits data which is required as a time/frequency standard in France. In wintertime I can hear that station at my place at night. Not certain about nowadays, but it used to run one megawatt.   

BTW, I don't see Colorado hams, who are certainly in groundwave range, dominating FMTs.
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