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Author Topic: 40 Meter Signal Duration  (Read 523 times)

K6AER

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40 Meter Signal Duration
« on: November 20, 2022, 06:43:15 PM »

I have been a ham for over 63 years and with the exception of High School and College, for the normal reasons, I have been active on HF for the whole time. I thought I have seen it all.

Last night on Forty meters at 6 PM mountain time I decided to get on 40 meters and make some contacts There was a contest going on and finding a frequency required patience.

I was running QRO and my noise floor out here in the high desert is S-Zero. My antenna is a center feed Zep at 50 feet. It favors East and West.

I am able to hear what most cannot.  During the various contacts a strange phenonium was tsking place. It didn’t matter wither I was talking to the East coast or the West coast.  Typical signal levels with stations were S9 when the were running 100 watts with my signal report at S9+15 to 20. On each contact, after about 5 minutes, the path would go to no signal heard at all to the noise floor at my QTH (127dBm). In one case the operator called me on my cell phone ask what happened? Other signal on the water fall display would stay the same.

I understand normal HF propagation but this was very unusual. Any thoughts what would cause such an unusual phenonium?

(I know I am near Roswell, NM but I don’t think that is the problem.)
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KD6VXI

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Re: 40 Meter Signal Duration
« Reply #1 on: November 21, 2022, 07:45:52 AM »

It was only the person on the frequency you where speaking with that went up and down?  You stated the other signals on the waterfall stayed the same....

That, to me, would point to something in the radio.  I'd say faulty AGC, but if the noise floor stayed the same then AGC is kinda out in left field.

I've seen that on greyline back when I was a CBer as a kid and stayed up all night (couldn't talk during the day, TVI and telephone interference saw to that) but it would be ALL signals in the band.  Never a spotlight like a couple khz.

Interesting.

--Shane
WP2ASS / ex KD6VXI
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W1VT

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Re: 40 Meter Signal Duration
« Reply #2 on: November 21, 2022, 07:59:38 AM »

The ionosphere interacts with signals to generate RHCP and LHCP signals.  It sounds like there were cancelling each other out on your linearly polarized center fed Zepp.  I've considered putting up a CP array but this optimally requires  switching between transmit and receive polarizations.

https://digital.wpi.edu/downloads/g445cg54z?locale=en
Improving SNR in a HF Link with Circularly Polarized Antennas

DEC 2010 - QST (PG. 33)
Gimme an X, Gimme an O, What's that Spell? -- Radio (feature)  Author: Nichols, Eric, KL7AJ
http://www.arrl.org/arrl-periodicals-archive-search  An ARRL membership benefit is the availability of old articles for download.
« Last Edit: November 21, 2022, 08:12:20 AM by W1VT »
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AC2EU

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Re: 40 Meter Signal Duration
« Reply #3 on: November 21, 2022, 08:12:11 AM »

Quote
Other signal on the water fall display would stay the same.

Did you tune over to the other signal to verify that you could receive it?

WA3SKN

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Re: 40 Meter Signal Duration
« Reply #4 on: November 21, 2022, 08:39:59 AM »

Since I don't see anything in that time frame on spaceweather.com, you might start monitoring your transmit output for a while.  Maybe get some benchmarks on a dummy load.

-Mike.
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KA2DDX

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Re: 40 Meter Signal Duration
« Reply #5 on: November 21, 2022, 08:46:18 AM »

I agree with wa3skn....... But I would not rule out a flare even if not reported by spaceweather.com.
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KE6SLS

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Re: 40 Meter Signal Duration
« Reply #6 on: November 21, 2022, 08:53:12 AM »

Hi Mike,

I see condx like this frequently on 75m.  Signals are 59 then, over perhaps sixty seconds, the signal drops below my noise floor -- 100% zero copy.  I have confirmed we both loose the path.  Power levels do not change help--whether qro/qrp/barefoot.

This fascinates me to no end!  Hamming for 30 years and I still can't put my finger on WHY condx can change with such rapidity.

I've tested with public online SDR's to find out "where did MY signal go" and most often, the path becomes 2x or 3x longer.  Stations closer than say 800 miles are no longer heard.  For me on the extreme west coast, I found my signals clearly heard in Alabama! 

I work 40 far less frequently than I do 75m in evening time.  Most evenings/nights are on 75m in my shack.

I've also seen that band simply drop.  Like it turned into 10m but at night.  Not a signal to hear or work.  I've used WWV to test during these condx--even their monster signals disappear. 

Wintertime condx . . .

73
Jaye
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KA2DDX

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Re: 40 Meter Signal Duration
« Reply #7 on: November 21, 2022, 02:44:48 PM »

This may partially explain....

https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts

But I think it raises more questions.
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WB0DZX

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Re: 40 Meter Signal Duration
« Reply #8 on: November 21, 2022, 10:31:11 PM »

1. When other signals near in frequency coincide with a faded signal, the faded path has suffered from what is called a "frequency selective fade." With the irregular nature of the ionosphere, this can be rapid and lasting, especially when a band is about to drop out entirely, as many have experienced. It is usually an entire band of frequencies that fades when the MUF changes.
   
2. Non-faded signals on or near the faded frequency, being from diverse locations, have taken other paths via the ionosphere. They are subject to other simultaneously existing ionospheric conditions affecting those paths.
   
3. Frequency selective fading can be enhanced by simultaneous other forms of fading that may not be noticed prior to the dropout. The following link, although discussing forms of fading relating to equatorial paths, discusses some of those:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2001RS002495
   
4. Frequency selective fading of critical paths can only be counteracted by a frequency diversion system, simultaneous use of multiple frequencies. Non-critical paths suffering from prolonged fading must continue communication on a predetermined alternate frequency.
   
5. Changing polarization or using polarization diversity would not have much effect on frequency selective fading. It would have an effect on certain other types of fading and, of course, polarization loss in the absence of profound frequency selective fading.
   
6. Having a waterfall or spectrum display to view after signal loss discounts a strong continuous carrier overloading the receiver. Such a signal causes loss of the received signal, but the noise floor usually disappears, too.
   
7. Any solar or geomagnetic disturbance would affect a range of frequencies  instead of just one.
   
8. However, rapid, profound, and lengthy signal loss during a QSO in or near Roswell, NM raises the intriguing possibility of other causes!
   
Mike WBØDZX
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KH6AQ

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Re: 40 Meter Signal Duration
« Reply #9 on: November 22, 2022, 07:12:26 PM »

"Typical signal levels with stations were S9 when the were running 100 watts with my signal report at S9+15 to 20. On each contact, after about 5 minutes, the path would go to no signal heard at all to the noise floor at my QTH (127dBm). In one case the operator called me on my cell phone ask what happenened. Other signal(s) on the water fall display would stay the same."

The last sentence makes me suspect the transceiver frequency suddenly shifted.
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WB0DZX

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Re: 40 Meter Signal Duration
« Reply #10 on: November 22, 2022, 07:38:24 PM »

In my previous post, I failed to catch an auto-corrected word in point 4. To prevent any confusion, change the term "frequency diversion system" to "frequency diversity system."
   
Mike WBØDZX
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WB6BYU

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Re: 40 Meter Signal Duration
« Reply #11 on: December 07, 2022, 01:44:48 PM »

Some years ago I had a long QSO between Oregon and Hawaii,
probably on 20m.  Fading was quite noticeable at about a 5 minute
rate, and I could bring the other station back by switching between
a long wire antenna and a beam.

Clearly the cause was polarization rotation along the path that
changed with time, so sometimes the received signal was
cross-polarized to our antennas.

There wasn't necessarily any correlation with the timing of when
our signals peaked at either end vs. the antenna I selected:  our
signals didn't peak at the same time, but the sequence seemed
to repeat as conditions changed over time.

So it is possible that the station would come back in a few minutes.
Other stations on the waterfall wouldn't necessarily fade synchronously,
but might still show a similar variation over time.


But it also may have been a brief peak in conditions prior to the
skip zone extending past the other station.  That might require
that your contacts were worked at roughly increasing distances.

W5HRO

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Re: 40 Meter Signal Duration
« Reply #12 on: December 28, 2022, 06:30:58 AM »

I have been a ham for over 63 years and with the exception of High School and College, for the normal reasons, I have been active on HF for the whole time. I thought I have seen it all.

Last night on Forty meters at 6 PM mountain time I decided to get on 40 meters and make some contacts There was a contest going on and finding a frequency required patience.

I was running QRO and my noise floor out here in the high desert is S-Zero. My antenna is a center feed Zep at 50 feet. It favors East and West.

I am able to hear what most cannot.  During the various contacts a strange phenonium was tsking place. It didn’t matter wither I was talking to the East coast or the West coast.  Typical signal levels with stations were S9 when the were running 100 watts with my signal report at S9+15 to 20. On each contact, after about 5 minutes, the path would go to no signal heard at all to the noise floor at my QTH (127dBm). In one case the operator called me on my cell phone ask what happened? Other signal on the water fall display would stay the same.

I understand normal HF propagation but this was very unusual. Any thoughts what would cause such an unusual phenonium?

(I know I am near Roswell, NM but I don’t think that is the problem.)

The band has been going very long this year and it usually happens right after dark. It's worse on my end because I'm running a full height (66.5' tall) 80-meter vertical tuned for 40 so I've noticed the same thing only worse. Last night even right before dark Australia was booming in along with parts of the UK. I live in northern California now. Last winter I could work my buddy in Oklahoma right after dark for an hour or two before it would happen, but this year it’s happening right away. You wont hardly here anyone here in the U.S. when it happens, only stuff overseas. The band is open, but if the other side or the world is still dark when it happens you might not hear very much because people are still in bed then. But again, the band has just been going really long this winter.
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