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High Pressure

O

ojmurphy

Guest
I have a question about the effects high pressure has on radio signals.

During a high pressure, high humidity day recently, 2 FM stations from the same building went off the air at the same time. Initially broadcasting nothing, the signals returned gradually - as if a CD was skipping (digitally cutting in and out). This cutting in and out continued for some 45 minutes. The explaination given by these stations was that high pressure was affecting their services.

How is this possible? I'm not technically minded, so I'd love to know if the problems I described could really have been caused by high pressure, or if I was being spun a story!
 
Owen Murphy said:
I have a question about the effects high pressure has on radio signals.

During a high pressure, high humidity day recently, 2 FM stations from the same building went off the air at the same time. Initially broadcasting nothing, the signals returned gradually - as if a CD was skipping (digitally cutting in and out). This cutting in and out continued for some 45 minutes. The explaination given by these stations was that high pressure was affecting their services.

How is this possible? I'm not technically minded, so I'd love to know if the problems I described could really have been caused by high pressure, or if I was being spun a story!
Click to expand...

marginal digital STL, high pressure could possible cause fresnel problems, bending the path...especially with a fairly long path...
 
It could have been a temperature inversion. But it's the wrong season for it - here around Buffalo, at least. A temperature inversion happens when the air is cool, still, and humid, usually just after dawn in the spring and fall. If the air near the ground stays cooler as the sun warms the upper atmosphere, and the warming does not disturb the air, the boundary between the two air masses works like a mirage on a hot summer day. For instance, when you look a long way down a hot road, you can see the sky reflecting back at you. The same thing happens in the sky - except the STL signal gets reflected away from its proper path. And if the STL is digital, and frames of data are being lost, then yes, it will sound just like a skipping CD. If the STL is analog, the signal will get hissy as the signal drops. If it drops far enough, the STL receiver may squelch out and drop the signal. The phenomenon rarely lasts for more than 15-20 minutes, but since it is totally reliant on natural atmospheric forces, it could conceivably last longer.
 
I think we may be dealing with this phenomenon ourselves today. STL forward power is showing 100%, and so is the xmtr -- we just keep having these quick dropouts that last for only a split second. Pilot light goes out and everything.
 
whit979 said:
I think we may be dealing with this phenomenon ourselves today. STL forward power is showing 100%, and so is the xmtr -- we just keep having these quick dropouts that last for only a split second. Pilot light goes out and everything.
Click to expand...
You can mitigate that problem by loosening the squelch or 'mute' setting in the STL receiver. Being on the air with a trace of noise is far superior to being off the air. When you say that the stereo light extinguishes, I assume that you are using a composite STL. If that's the case, the above suggestion will help. Once that setting is changed, if this problem shows up for an extended period, turning off the stereo pilot at the studio will dramatically reduce the noise listeners hear during a fade. None of these are a substitute for more height at one or both ends of the STL path, but they are proven tricks you can use to stay on the air until a permanent fix is employed.
 
You might also check for a new "obstruction" in the STL path. I had a station several years ago that was a turnkey install during the winter months and then during the summer the oak tree nearby got full of leaves and grew a little bit - enough to knock out the STL link.

Also, not knowing your setup - is it possible that the STL recievers are using a preamp? The peramp might be starting to drift or go by the wasteside.
 
Owen,

Are you in a major city and are the stations' stransmitter sites up pretty high? I know that in LA they can develop problems with RF STL links up to Mt. Wilson, etc. because the SMOG will create a bit of a barrier between the studio and transmitter. Temperature inversions and the like could have a similar effect I suppose. I know of several stations that have had problems withe the inversion problem actually SOLVE the problem by putting another dish up another 100 feet or so on the same tower for diversity on the receive side. Believe it or not one will be in when the other is out of useable signal... A relay system to switch over makes it all work.
 
I used to know of an FM in Houston that would switch to a different main FM transmit antenna when inversion would become severe.

I think it used different wave spacing and was located far lower on their tower.
 
OKCRadioGuy said:
Owen,

Are you in a major city and are the stations' stransmitter sites up pretty high? I know that in LA they can develop problems with RF STL links up to Mt. Wilson, etc. because the SMOG will create a bit of a barrier between the studio and transmitter.
Click to expand...

This reminds me of a TV station that had a problem with a microwave link - there was a power plant nearby the line of the link and sometimes the steam from the plant would hang low enough with little drift and cause problems with the link.
 
sparks794 said:
OKCRadioGuy said:
Owen,

Are you in a major city and are the stations' stransmitter sites up pretty high? I know that in LA they can develop problems with RF STL links up to Mt. Wilson, etc. because the SMOG will create a bit of a barrier between the studio and transmitter.
Click to expand...
This reminds me of a TV station that had a problem with a microwave link - there was a power plant nearby the line of the link and sometimes the steam from the plant would hang low enough with little drift and cause problems with the link.
Click to expand...
Then there was the case of a 950mhz FM STL that worked great until it rained on a hot day & steam rose off of the roads. Raising one end by 100' cured that one here in relatively flat Central indiana.
 
sparks794 said:
You might also check for a new "obstruction" in the STL path. I had a station several years ago that was a turnkey install during the winter months and then during the summer the oak tree nearby got full of leaves and grew a little bit - enough to knock out the STL link.

Also, not knowing your setup - is it possible that the STL recievers are using a preamp? The peramp might be starting to drift or go by the wasteside.
Click to expand...

The problem stopped after just one day. Must have been weather after all. Yes, we have a number of things that are potential obstructions to our STL path. Receiver antennas are still pointing at a building where the station USED to be, STL is having to shoot over a hill full of trees, and we can't make either tower higher because it's in the landing path of an airport. *sigh*
 
I'd consider at least getting things lined up right. If a person get's ducting or some other abnormal interference, the sheer fact your dishes aren't lined up right COULD cause a problem where it normally wouldn't be.
 
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