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Old 13th Sep 2018, 12:25 pm   #1
stevehertz
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Default FM signal strength change over a short period?

This morning I noticed that the FM signal strength of a station increased noticeably over a one hour period, between 10 and 11am. The high quality tuner in question has a digital readout of signal strength and it went from 47 to 55dB. Perhaps the moisture in the air 'burning off' as the sun strength increased was the cause? The signal comes to me over/through the forest area of Cannock Chase (high humidity?).

Can FM signal strength vary over relatively short periods of time?

In general, what are the best/worst conditions for FM transmissions and does the time of day - as it does with AM - affect them?
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Old 13th Sep 2018, 1:10 pm   #2
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Default Re: FM signal strength change over a short period?

Looks like you've encountered 'Ducting' ! - It affects VHF and UHF, more noticeably at the higher frequencies, during settled weather.
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Old 13th Sep 2018, 1:22 pm   #3
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Default Re: FM signal strength change over a short period?

The fastest variation is probably aircraft flutter where a multipath situation changes rapidly.

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Old 13th Sep 2018, 1:27 pm   #4
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Default Re: FM signal strength change over a short period?

David, no, nothing to do with aeroplanes in this instance I'm sure. We're talking over an hour period, signal strength gradually increased. Even the slowest aircraft would be over in much less time than that.
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Old 13th Sep 2018, 1:35 pm   #5
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Default Re: FM signal strength change over a short period?

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Originally Posted by SteveCG View Post
Looks like you've encountered 'Ducting' ! - It affects VHF and UHF, more noticeably at the higher frequencies, during settled weather.
Hmm. Just googled it and it does seem as though it could well be the 'culprit'. See tropospheric ducting: https://en.wikipedia.org/wiki/Tropospheric_propagation

Mind you, the station in question is being transmitted just a couple of miles from my location, but I'm in a valley, behind a large hill, 'out of sight' of the transmitter, so the effects described may well be applicable to what I'm experiencing.
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Old 13th Sep 2018, 1:53 pm   #6
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Default Re: FM signal strength change over a short period?

There has just been a solar storm as well, so another possibility is this dissipating.

When a solar storm is active in our atmosphere, radio signals can be attenuated and conditions can change rapidly.
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Old 13th Sep 2018, 2:09 pm   #7
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Default Re: FM signal strength change over a short period?

You have probably explained it to yourself.
Behind the hill there is no dominant direct path but probably several indirect ones.
It needs just two to be equal in level to produce either 6dB enhancement or complete cancellation depending on exact path length/phase.

As the world warmed up I think the phase slowly shifted.
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Old 13th Sep 2018, 2:38 pm   #8
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Default Re: FM signal strength change over a short period?

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Originally Posted by stevehertz View Post
David, no, nothing to do with aeroplanes in this instance I'm sure. We're talking over an hour period, signal strength gradually increased. Even the slowest aircraft would be over in much less time than that.
My fault, I wasn't clear. I think you've come across tropo ducting. which fits the sort of thing you've come across.

Aircraft flutter is just the fastest mechanism and is a variety of multipath. You get cyclic variations of several per second down to several seconds. The effect doesn't linger for more than a minute at most.

Some amateurs are communicating on 1296MHz using reflections from regular scheduled flights.

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Old 13th Sep 2018, 4:14 pm   #9
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Default Re: FM signal strength change over a short period?

"Tidal fading" when the path is over a sea estuary etc is also a possibility, but I concede you can't get more inland than Rugeley, Staffordshire.

Another possibility is a moveable object, say the jib of a crane, between you and the transmitter. Likely to be relatively close to one or the other, Fresnel zones and all that.

Don't discard the possibility that a fault, or engineering work may have temporarily reduced the ERP.
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Old 13th Sep 2018, 9:23 pm   #10
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Default Re: FM signal strength change over a short period?

Temperature inversion causing diffraction of the signal perhaps? Maybe the interface between the cold and warm air in the valleys affecting the signal path, which would gradually change as the height of the interface varied due to warming of the air? We have a non-line-of-site path profile on Freeview and we can get all sorts of unexpected signal dropouts, depending on the weather and ground conductivity. Just good old radio theory playing it's part I suppose.
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