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
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Joined Sep 15, 2006 Messages 6,320 Location Fair Oaks, CA
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).
neohrožený97
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Připojeno 23. června 2006 Zprávy 178
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
Completely Banned for the Greater Good
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Připojen 29. ledna 2006 Zprávy 7,452 XNUMX Místo Illinois
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.
jackj
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Joined Jul 19, 2007 Messages 1,548 Location NW Ohio
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
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Joined Mar 22, 2005 Messages 6,100 Location Point Pleasant Beach, N.J.
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
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Připojeno 18. července 2009 Zprávy 1,763
Priority on?
kruser
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Joined Nov 25, 2007 Messages 4,967 Location Central St Louis Cnty, MO
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
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Připojeno 6. února 2009 Messages 40
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.
neohrožený97
Člen
Připojeno 23. června 2006 Zprávy 178
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.
neohrožený97
Člen
Připojeno 23. června 2006 Zprávy 178
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
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Joined Nov 25, 2007 Messages 4,967 Location Central St Louis Cnty, MO
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.
neohrožený97
Člen
Připojeno 23. června 2006 Zprávy 178
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
neohrožený97
Člen
Připojeno 23. června 2006 Zprávy 178
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
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Joined Nov 25, 2007 Messages 4,967 Location Central St Louis Cnty, MO
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.
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