Strong Signals Fading In And Out?

[intrepid97]

What would cause strong signals approx. 35-50 miles away go from really strong to really weak and back to strong?

This happens on some VHF frequencies in my area.

Mainly the St. Louis City Fire Dept. and some Police Agencies too.

TIA for any answers

 

[gmclam]

It depends on how you are receiving the signal in the first place. Can your antenna "see" the antenna that is broadcasting the signal? It typically means that either you or the broadcaster's antenna is at a good elevation.

When the antennas can see each other, reasons for fading can include weather conditions (heating and cooling in the atmosphere) or perhaps aircraft in line with your signal.

Sometimes you are picking up the signal from a reflection off an object, rather than directly from the source antenna.

More likely, especially in low band VHF, is that your antenna can't see the source antenna. Signals bounce (or skip) in that band and vary in quality and distance all the time.

In order for a signal to travel ~35 miles directly, one or both antennas (yours and the transmitting antenna) need to be at some elevation. That's why they put repeaters on mountain tops. So, for example, if the transmit antenna is at 1000' above sea level, and you are at sea level, you could have line-of-sight without the curvature of the Earth getting in the way.

Just because a signal "sounds strong" does not mean it is. We might be able to give a better answer with more detail of your situation (elevation, elevation of transmitting antenna, etc).

 

[intrepid97]

It depends on how you are receiving the signal in the first place. Can your antenna "see" the antenna that is broadcasting the signal? It typically means that either you or the broadcaster's antenna is at a good elevation.

When the antennas can see each other, reasons for fading can include weather conditions (heating and cooling in the atmosphere) or perhaps aircraft in line with your signal.

Sometimes you are picking up the signal from a reflection off an object, rather than directly from the source antenna.

More likely, especially in low band VHF, is that your antenna can't see the source antenna. Signals bounce (or skip) in that band and vary in quality and distance all the time.

In order for a signal to travel ~35 miles directly, one or both antennas (yours and the transmitting antenna) need to be at some elevation. That's why they put repeaters on mountain tops. So, for example, if the transmit antenna is at 1000' above sea level, and you are at sea level, you could have line-of-sight without the curvature of the Earth getting in the way.

Just because a signal "sounds strong" does not mean it is. We might be able to give a better answer with more detail of your situation (elevation, elevation of transmitting antenna, etc).

VHF 154-155 range.

I think you hit on something about the signals reflecting off of buildings etc.

My antenna is right at 30ft. in the air. The stations are mainly coming from the St. Louis City area.

I listen to quite a lot of traffic from that area and it seems the fire and only a couple of other stations do this.


I am running a RS Pro-197 and the signal meter will fluctuate.

The audio is always there and never degrades to where I can't hear it.

I am really thinking the signal reflection thing is what is going on here.

Thank you for responding to my question

 

[gewecke]

VHF 154-155 range.

I think you hit on something about the signals reflecting off of buildings etc.

My antenna is right at 30ft. in the air. The stations are mainly coming from the St. Louis City area.

I listen to quite a lot of traffic from that area and it seems the fire and only a couple of other stations do this.


I am running a RS Pro-197 and the signal meter will fluctuate.

The audio is always there and never degrades to where I can't hear it.

I am really thinking the signal reflection thing is what is going on here.

Thank you for responding to my question

Try a different antenna,designed for vhf high if you haven't already? Your telescopic pulled out to about 15" should work great at 154-155 mhz. and may even work better with the ATT turned on if you have any intermod at all.


73,
n9zas

 

[jackj]

It could be "ducting" where temperature inversions bend the signal either away from or towards your antenna. The ducted signals might also be either adding to or canceling out the signal you receive directly from the station. Ducting usually doesn't occur all day or even every day. It is caused by a layer of cold air on top of a layer of warm air and the sun will disperse the layers usually by mid morning.

It could also be multipath interference that can have some of the same effects as ducting.

 

[kb2vxa]

Sounds to me like airplane flutter. A reflection recombining with a direct wave (multipath reception) can cause a slow fade to a rapid flutter or a combination of both, once the aircraft passes out of the area between transmitter and receiver all is back to normal. St. Louis being a high traffic area it should be fairly common.

 

[sfd119]

Priority on?

 

[kruser]

Where you located about? I'm not near 35-50 miles out but I'll give a listen to the city fire freqs and see if I see any fluctuations. I'm only out in Chesterfield so I should get a fairly decent signal.
What other VHF freqs or agencies are you seeing this on? Can you hear the city fire simulcast on 460.175? That one is weak here but my antennas are not really designed for UHF. I've not monitored the city vhf fire freqs for years but I'll give them a listen.

One thing does come to mind, how are your coax connections? Any chance they are going bad from swaying in the wind? This will happen over time if the coax is not secured to the mast very well.
Are the outdoor ends sealed with a good coax sealer? Not electrical tape I hope as that does not work for long. Moisture in your coax should not really cause a fluctuating signal though but will cause a overall lower signal at all times. I doubt the problem is moisture but just wanted to throw that in as many people do not think about sealing the outdoor connections.

The reflection theory is also highly likely if it is mainly on the signals out of St. Louis city like you seem to indicate.

 

[phatboy48]

How rapid is the fade? Does it happen quickly (several times per second) or slowly over a period of hours?
Quick fade that varies in speed is airplane flutter caused by the signals bouncing from overhead aircraft (the principle that makes RADAR work). Slow fade over hours with peaks at morning and evening and usually poor signal level during midday is called tropo enhancement and is caused by signals being ducted or reflected by air masses of differing temperatures, which normally occurs around sunrise and sunset mainly during summer months.

 

[intrepid97]

I actually have an antenna designed for VHF with the ground plane kit.

I can monitor stations upwards of 80-100 miles on a daily basis without these weird flucuations.

This is only happening with some of the St. Louis City Fire and a couple of other stations.

Example, I can hear 155.370 Missouri Pt. to Pt. with no issues. St. Louis Co. State, St. Charles Co. Weldon Springs etc. with no issues.

I understand ducting and how it works and there are days I can hear 155.475 ISP dist. 22. 10, 14 etc.

Also get ducting on 155.340 etc.

SFD119. No priority on, I understand what that would do

Kruser, coax is in great shape, no swaying. Everything is nice and secure.

Jackj, yes, it could be multipath?

Anyway I just thought it was interesting how it mainly affects the city Fire

Thanks again guys for your answers.

 

[intrepid97]

How rapid is the fade? Does it happen quickly (several times per second) or slowly over a period of hours?
Quick fade that varies in speed is airplane flutter caused by the signals bouncing from overhead aircraft (the principle that makes RADAR work). Slow fade over hours with peaks at morning and evening and usually poor signal level during midday is called tropo enhancement and is caused by signals being ducted or reflected by air masses of differing temperatures, which normally occurs around sunrise and sunset mainly during summer months.

Phatboy, no it's not slow. It is most likely reflection

I know what you mean about the aircraft and how they can make signals all screwy lol

 

[kruser]

For the City Fire, does the fading happen on every transmission from what you can tell? Or does it go for several minutes with normal strong levels?

I've only heard a very short transmission since plugging them in and no fading observed.

I'll keep watching as time permits.

 

[intrepid97]

For the City Fire, does the fading happen on every transmission from what you can tell? Or does it go for several minutes with normal strong levels?

I've only heard a very short transmission since plugging them in and no fading observed.

I'll keep watching as time permits.

Kruser it's more often than not.

Example. I can hear North Central loud and clear on 154.325. They have always been a booming station here in my area for as long as I can remember.

Then some of the inner city stuff flucuates. I am convinced it is a reflection issue.

When I use to install wireless mesh networks, we would use buildings to bounce the signals

 

[intrepid97]

Let me put it this way. It is almost like a mobile unit when it is moving.

You get that flutter, but can hear the transmission just fine.

It's not like a mobile unit barely breaking the squelch

 

[kruser]

That is odd here, I put in the North County Fire and I get a lower signal level from it than I do from the city dispatch channel.
I'm much closer to the north county towers and the FCC data shows both systems are about the same regarding tower height and allowed power.
I also have way less obstructions to the north county towers than the cities tower locations.

Anyway, I've been monitoring the city dispatch on 154.130 with an Icom R9000 and I get a solid S9 signal level with no fluctuation at all.
So I'd say the reflection theory is high.
I do experience bad signal loss on the cities P25 system in the summer months. It is due to the foilage (leaves) on the trees during the summer months. During the winter months the signal levels here for the P25 system fluctuate like your VHF does but it is caused by the large tees that I must aim the beam through to even get a decent signal. When it gets windy, they sway in the wind and the signal levels come and go in rhythm with the wind! At one point, I used the wall of an apartment building to capture the reflected signal as it worked way better than a direct aim. That was with a smaller beam though. The 12 element I use now works well with true aim except during heavy rain or heavy fog. Then I lose all signal from the P25 system only.

I suppose being that you are still capturing the audio and it is not annoying, that you are good to go. Can you ever hear any of the mobiles from city fire? You never did say about where you are located but if you are 35-50 miles out, I would not think you hear the mobiles at all.

I'd also never paid any attention to the signal levels on 460.175 which is a simulcast of 154.130. Tonight, I found the signal levels here are really low even when I switched to another antenna that is UHF only. I'm thinking I read that 460.175 is used by one or two houses that have reception problems and the 460.175 helps that but it is a low power transmitter.

Short of getting your antenna higher or maybe using a VHF beam, I'm thinking you will just need to live with this one.

 

[intrepid97]

Kruser it's not an issue for me, just thought it was weird lol (original topic)

I can hear mobiles up to 20 miles give or take.

Kinda weird you are getting a low signal on North Central? They boom in here like they are next door to me.

I think the reflection thing is what i'm experiencing

 

[kruser]

I figured it was more of a curiosity thing from your comments about still getting clean audio even during the low level periods.

The only thing I can figure for north county fire here is that I'm south of two of the towers which I think one is their main tower. These two towers are located at a fairly high location here in St. Louis county but they seem to be located on the southern edge of their dispatch area. I suspect they are using directional antennas aimed towards the northern part of their dispatch area and that is why the signal levels are low here. I see they also show a tower much further north but I do not know which tower the signals are originating from. It is interesting that you receive them so well yet they are low signal here.

At my location, I can receive both Clinton and Williamson counties in Illinois. They both use 154.875 for Sheriff dispatch and that is the same freq that my area uses so I must use PL equipped radios if I wish to cut out the Illinois chatter. Most times I enjoy the Illinois chatter though so I listen with the old Icom. It does not have PL or DPL capability. I actually get a very good signal out of Clinton County, IL. Williamson is not that bad either but not near as good as Clinton.
I used to be able to monitor the Starcom site out of Belleville but they took that site down and no others can be heard here. I can faintly hear the control channel out of St. Clair county but not near good enough to decode anything. That blows as I used to love to monitor Illinois State mainly. Of course I did not hear much state traffic on the old Belleville site that I can recall. I miss the low band radio from Illinois and Missouri's will also be gone before long.

Which direction are you from St. Louis? You do any low band tuning?

 

[intrepid97]

I am in southeast Macoupin co.

Too bad you cannot pick up Starcom. Maybe an 800 Yagi will do ya some good

 

[kruser]

Too bad you cannot pick up Starcom. Maybe an 800 Yagi will do ya some good

That is exactly what I have! 12 element at that. I tried a 6 element at first but I found that I need 12 just to pull in the St. Louis City P25 system here during the summer months. I put the 6 element on another mast aiming west of my location as I like to monitor out in St. Charles county also. I also switch in a Stridsberg 800 MHz pre-amp for the STL city system. It is not needed in the winter but without it in the summer, I only get about 10% good decode rate with just the 12 element Yagi alone.

For the reflections, I've always wondered in a case like yours if the refelections are caused by buildings near the transmitter site or something nearer to your end or both.

Maybe someone that knows more about reflections will chime in and answer that one.

Being in Macoupin County, do you get decent signals from Clinton County on 154.875? Can you hear the St. Louis County Police on 154.875? I could almost hit that tower with a rock it is so close!

I can copy most of the Macoupin County repeater channels here pretty well.

 

[intrepid97]

Yeah I can hear St. Louis Co. and Clinton county quite well