Interesting Phenomenon Noticed During Intense Tropo (and Es!)

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John Faulkner, Skegness
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Interesting Phenomenon Noticed During Intense Tropo (and Es!)

Post by John Faulkner, Skegness » 30 Jun 2018 19:12

Just having a chat with Oliver Blackburn on Facebook, we were discussing the weird phenomenon we sometimes get during intense tropo conditions. Oliver has noticed it. I have noticed it. Other FM DXers have commented on it:

It's where semi-local transmitters somehow manage to fade down into the noise and disappear. They wouldn't do this under normal circumstances and they are so strong that tropo (or Es) are never strong enough to topple them. Oliver may write with his observations later.

In my own recent experience, during an intense Es opening to the Balkans, unshiftable Lincs FM 102.2 actually disappeared! There was nothing on channel to replace it. It just vanished, leaving a noisy, empty frequency. At the time.

I had a theory which I would have liked to put forward to explain what is happening, but Oliver had a different and, I think, a much better theory.

So what is happening?

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o_blackburn
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Re: Interesting Phenomenon Noticed During Intense Tropo (and Es!)

Post by o_blackburn » 01 Jul 2018 00:37

I noticed this a couple of nights ago during an intense tropo to Germany. Whilst tuning up the band, High Hunsley 95.9/96.9, and Trax FM Doncaster 107.1 (normally full signal on the XDR) had dropped into the noise! I actually thought the TX had gone off at Doncaster as DLF was only 1 bar at the time. Trax gradually faded back in to normal strength. When Viking had disappeared into the mush, I could hear an echo of 2 hearts (one being the old Chiltern) and German talking. 95.9 was only Humberside in the noise until it came back properly.

My theory is that these semi local signals could have been enhanced by conditions, leading to a phase cancellation, rather then something else overloading the receiver. I'm happy to be corrected if I'm wrong though!
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Oliver
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Tim Bucknall
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Re: Interesting Phenomenon Noticed During Intense Tropo (and Es!)

Post by Tim Bucknall » 01 Jul 2018 09:39

i notice this a lot

your theory is as good as any other i've seen, phase cancellation is a leading candidate for sure
i should not have been able to hear such a weak RCF on 97.1

for this to happen during E skip would require a tropo like enhancement during e openings which i and others have noted
possibly these 2 things are related

years ago there used to be a Nottingham Pirate called heatwave on 87.9 and if it faded up i knew e skip was coming
now there was no way this was short e or backscatter
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daveB
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Re: Interesting Phenomenon Noticed During Intense Tropo (and Es!)

Post by daveB » 01 Jul 2018 14:48

Or then again...
There is a phenomena called the 'capture effect' in FM. If two stations occupy the same or nearly the same frequency one of them only needs to be a couple of dB or so stronger to suppress the other.

So what happens when the signals are very close in strength - do we end up with hearing nothing? I can think of many instances when tuning the SDRs that there is a fairly strong signal showing on the spectrum, but there is poor/weak audio. BRFM on 95.6 is a local station (12 km / 25 watts) that has disappeared, and at other times been replaced by DX.

https://en.wikipedia.org/wiki/Capture_effect
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John Faulkner, Skegness
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Re: Interesting Phenomenon Noticed During Intense Tropo (and Es!)

Post by John Faulkner, Skegness » 01 Jul 2018 15:30

I went down the Capture Effect path with this. But it doesn't explain why a semi-local station can dip down into the noise with no signal reading.

Possibly a bit of both going on here.

We still aren't sure about this odd apparent tropo effect when we receive Es. It's the semi-local transmitters which become enhanced. The range could be anything from 100 km to 300 km and more. The stability of these signals, mixed with ground-wave signals could provide the phase cancellation, but I know how difficult it is to achieve this kind of total cancellation through traditional phase cancellation units. I have the HS Publications FM phaser and it just wasn't possible to fully achieve this. You'd get a null at best, not a total cancellation.

Although phase cancellation seems more likely, just think about it: The same signal from two different types of propagation are hardly likely to cause a cancellation of this level of perfection.

I don't think we have the explanation yet.

(Edited for reasons of coherence!)

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Re: Interesting Phenomenon Noticed During Intense Tropo (and Es!)

Post by daveB » 01 Jul 2018 21:11

In the case of tropo reception. To state the obvious. We should be familiar with the conditions that enhance tropospheric ranges by bending signals heading to space back towards the earth - so in my case the coastal station from Europe come in strongly - but not much beyond.. Ducting - where very distant stations come in, without there necessarily being any closer DX. My best example back in the 1970's when E36 from East Germany was received and not another German, or Dutch station was received. But you can have a form of tropo where the signal is bent back to earth more than usual - which I presume can mean that stations normally received fade out.

The whole phenomena needs more investigation and documentation.
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Re: Interesting Phenomenon Noticed During Intense Tropo (and Es!)

Post by Andrew Webster,Wigan » 01 Jul 2018 21:39

I get this a lot here first off I use Sony tuners they handle the high signal strengths received here I do suffer overload if I beam East due to Holme Moss causing intermod on various frequencies if I am beaming SE and if Sutton Coldfield signal is up to 80 db I end up with BBC R 2 on 87.7.

When Es are in strong Italy and Spain are the worst culprits I notice the locals/semi locals start to loose their RDS and then
fade into the noise examples are Heart 88.0 Wrexham Rhos 54 km usually solid and Rock FM 97.4/Heart 105.4 Winter hill 17 km sometimes they are gone and nothing there but twittering noises and sometimes there is DX on the frequencies.
The BBC powerhouses do suffer Winter hill and odd times Llangollen and more odd Sutton Coldfield Holme Moss never goes anywhere stands it's ground well.
I put this down to the tuners knocking the gain back due to high signal levels the overload problems occur on the horizonal FM 5 the vertical FM 5 doesn't suffer overload I also have a horizontal FM 3 mounted 2 M lower than the FM 5 and I switch to this when there's soup on the horizontal FM 5.

Now the capture effect another example is when I receive Newstalk 107.4 from Ireland on vertical I have Tower FM from Bolton 26 km and usually solid station Tower slowly fades down and replaced by Newstalk I see this effect more on tropo

A few weeks ago I had soup with Italy coming in and going up through the band I though wouldn't it be nice if my band stayed like this not a lot on it but coming back to reality this band is full to the rafters with stations the only listening place is 87.5 -8 when there's nothing on there RSLs are a pain.

Andy
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satnipper
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Re: Interesting Phenomenon Noticed During Intense Tropo (and Es!)

Post by satnipper » 01 Jul 2018 22:39

"I put this down to the tuners knocking the gain back due to high signal levels"

That's exactly what I was going to suggest - it's the tuner's AGC action.

Stuart

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