Wind and Reception

[KSPILOT536]

We had some 40-50 mph winds here last night and heard very little police activity,does wind have anything to do with signal reception in the digital world?

 

[popnokick]

Only if an antenna was knocked down.

 

[Boatanchor]

Multipath fading caused by wind

Strong wind can cause havoc on UHF signals.

I've seen 450Mhz signals quickly go from full scale to nothing from the multipath fading.
It is more noticeable on analogue channels, but it still occurs on digital channels too, you just don't hear the fade, the audio just cuts out.

And, it's not caused by your antenna moving either. I have a theory that it is either air currents, or possibly foliage moving around.

Here is an example of multipath fading on a P25 control channel on a windy day:

https://www.youtube.com/watch?v=eSywTifsZQE

Both RX and TX antennas are stationary.

 

[popnokick]

I stand corrected... a Google search shows several scholarly papers regarding the effect of wind-blown foliage on VHF and UHF propagation. Haven't had time to read the details.... but I'm guessing that they don't draw the conclusion that VHF / UHF public safety and other comm users should abandon the VHF/UHF radios when the wind is blowing ;-) Is the effect more pronounced with digital systems? Someone can propose a study and likely get a grant for it.
[edit] Had time to skim one of the papers. While it appears there is an effect, it would be small enough to have the most impact on signals that already were weak or have a high degree of path loss due to foliage. And now I'm guessing the OP is going to write, "... but I live in the desert and there isn't significant foliage, so how could the wind be a factor?" And anticipating another question: Yes, heavy rain/snow can increase path loss.

 

[Boatanchor]

Smoke and convention currents above wildfires also causes similar, if not worse problems.

Interestingly, in my experience the effect of Rayleigh / Multipath fading is much more pronounced on frequencies above 300Mhz. VHF (160Mhz and lower) signals received from the same sites as the UHF signals will be relatively stable, while 400 & 800 Mhz signals will swing wildly.

Another reason why, in rural settings, VHF is clearly a much better choice (providing you can live with poorer building penetration).

Of course, 'they' don't mention these taboo issues when selling expensive UHF trunking systems to wildfire fire and land management agencies.

 

[DaveNF2G]

Multiplexed microwave paths can also be disturbed by extremely high winds. If the dishes are blown out of alignment, then transmissions can be heard on the wrong channels (usually just for short bursts) or not at all.

 

[newsphotog]

Yeah, the 1s and 0s get blown around quite a bit. I have heard instances of the 1s and 0s being found as far as 325 miles away from their transmitter site.

Dits and dahs, on the other hand, weigh much more than 1s and 0s, so they are not affected by gusty winds as much.

 

[zz0468]

Multiplexed microwave paths can also be disturbed by extremely high winds. If the dishes are blown out of alignment, then transmissions can be heard on the wrong channels (usually just for short bursts) or not at all.

After several decades in the microwave business, I never experienced that. Dispersive fading, where there is phase distortion of a multiplexed analog signal can cause crosstalk, but it will usually manifest as noise, not intelligible audio. On a digital multiplexed microwave radio, the dispersive fade margin might be reduced, but so long as the received signal remains above that margin, the bit error rate is unaffected.

High winds and dispersive conditions are generally mutually incompatible. A dispersive fade is caused by multipath, usually within multiple stable atmospheric layers. High winds churn the atmosphere, so those stable layers can't usually exist. High winds can cause scintillation of a signal, but the effect is usually minor, and the microwave radio adaptive equalizers will cancel it out so it's not service impacting. Unless you dig deep into the radio, you won't even know that's happening.

Dishes getting blown off path equates to a flat fade, unless co-channel interference is increased with the off-heading antenna. In a flat fade, so long as the received signal stays above the flat fade margin, the bit error rate is unaffected. So, an antenna can be quite a few degrees off azimuth before it becomes service impacting, depending on the path geometry. With everything tightened down the way it's supposed to be, I've seen 100+ mph winds with zero service impact on microwave paths.

What I've seen with high winds is loose connections getting noisy, improperly tightened antennas getting blown off path, and sometimes static charges building on antennas that noise up VHF and UHF receivers.

To the OP, the most likely cause of low police radio activity during high winds is that the wind is keeping the bad guys from coming out from under their rock.

 

[DaveNF2G]

I used to dispatch through a system that was positionally multiplexed. During extreme high wind events (which were rare), we would occasionally get audio from one channel coming out over the wrong console or channel selector. If many channels were busy at the time, then the result could be very confusing as several dispatchers were suddenly (though briefly and intermittently) hearing a channel other than the one to which their console was assigned.

 

[jfhtm350]

Over the years I have noticed the wind have an effect on scanners and on cellphones. Does it have to do with the wind blowing the antenna around, the wind blowing the signal around or something else? I dont know the reason. I just know it happens.

 

[mancow]

My money is on human behavior. When the weather changes, the winds pick up, it starts to get cold, people hunker down and move less.

Also, if you get a chance to stare up at a tower on a really windy day it can be interesting. I've seen multibay antennas wobbling like crazy. That might have a bit to do with signal propagation.

 

[DickH]

Yeah, the 1s and 0s get blown around quite a bit. I have heard instances of the 1s and 0s being found as far as 325 miles away from their transmitter site.

Dits and dahs, on the other hand, weigh much more than 1s and 0s, so they are not affected by gusty winds as much.

That's why Morse Code is much more reliable than digital.

 

[RonnieUSA]

I know my antenna wobbles in the winds, but it doesn't affect my receive, digital or analog. I like when it's foggy or raining, for best receive. :lol:

 

[zz0468]

I used to dispatch through a system that was positionally multiplexed.

What, exactly, do you mean by "positionally multiplexed"? It's not a term I've encountered in the context of any practical transmission systems, either in school, or several decades in the telecommunications business. A Google search on "positional multiplex" brings many references in terms of DNA and cell research, and lock tumblers.

A Google search on the quoted term "positionally multiplexed" brings one result - this thread.

The two common multiplexing methods in use for the past 60 years or so are frequency division multiplex, and time division multiplex.

I suspect that you've mislabeled something else. Spatial multiplexing, perhaps? You've left me scratching my head...

 

[mfn002]

I've noticed the same thing around here with one of the analog 800 systems we have. When the wind picks up, I start to notice a "whooshing" sound that increases in volume as the wind speed increases. It can get bad enough that I loose the signal altogether.


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