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FM reception

B

billy2047

Hi all, can road overpasses "amplify" weak FM signal? I've noticed sometimes a very weak distant FM signal can suddenly become very strong for a split second (or even less) going through an overpass. It's very similar to picket fencing but in very short duration, usually strong enough to understand a syllable or two before it fading back into white noise. How can an ordinary overpass "amplify" VHF 88-108MHz signal? Don't overpasses or bridges tend to reduce signal because multipath and or metal absorption etc? Thanks!
 
I've noticed that too. I think it is because the overpass is higher up than your car the signal is stronger up in the air. The superstructure of the bridge then reflects the stronger signal back down to your car.

Also if there is a co-channel station that whose signal is starting to interfere with the station you're listening to, the overpass will briefly shield your car from that signal. Hence the fading station is briefly on its own.
 
It would help if you were more descriptive.

Is the overpass an older all-metal structure, or a concrete overpass?

Are you driving over it, or underneath it? You only said "through," which isn't clear.

If underneath, can you see to the horizon in all directions while directly under it, or does the road you're on dip beneath it?
 
While on the road in southwest Utah, I was in contact with my neighbor in Los Angeles on forty meters.
Signals were weak until I drove under an overpass - his signal spiked way up for a second or two.
I would have liked to park under the overpass, but couldn't do it safely.
 
The New Chico said:
CentralFL is onto something here. Whatever the material, it will block a lot of co- or adjacent-channel noise coming form random directions. Thus "focusing" signal as it tend to eliminate all of the extranneaous stuff out there.
Click to expand...
If you mean intermodulation generated within the receiver splattering onto the desired frequency, that would certainly explain things, although automotive radios would be the variety I'd least expect this to happen with, given their well-known design standards for dealing with challenging environments.
 
CentralFL said:
I've noticed that too. I think it is because the overpass is higher up than your car the signal is stronger up in the air. The superstructure of the bridge then reflects the stronger signal back down to your car.

Also if there is a co-channel station that whose signal is starting to interfere with the station you're listening to, the overpass will briefly shield your car from that signal. Hence the fading station is briefly on its own.
Click to expand...
Interesting.. so is this similar to how sometimes high rise buildings can reflect a weak distant FM signal causing a momentarily spike in signal strength?
 
FB8614 said:
Signals were weak until I drove under an overpass - his signal spiked way up for a second or two..
Click to expand...
That's exactly what I've been experiencing too listening to distant staticky FM station while driving. Overpasses or bridges must be really good reflectors because it won't do it if i'm driving directly on that overpass
 
Hopefully relevant here ....
One foggy 2 AM night driving home from some south shore place on Long Island there was trope all over the underdash Panasonic FM-cassette I'd recently bought. Stopping for coffee, I spotted an empty LIRR station (Lynbrook) parking lot where overhead wires spanned one part of the lot. Nearby LILCO power sub-station of some sort.
With me spending over an hour maneuvering ..... the car two feet this way, maybe four feet perpendicular to that, backing up three feet at another angle, finding other 'hot spots' to align with whatever they had to offer : I was able to pick up every Philadelphia station except WIFI 92 (92.5) and the two co-channel curiosities with NYC (101.1 and 100.3). Long Island's WBLI on 106.1 was nowhere to be heard anyplace; just Philly's WWSH on the same frequency.
The pretty downtown village of Lynbrook (named so by its original settlers from Brook-lyn) is well within line of sight of the Empire State Building tip where the FM stations were at the time.
A cop cruiser showed up, idly proofed me and asked, politely, what I was doing there. I did my best to explain DXing and the Totals list and tropo. He seemed to be tolerating things when his vehicle unit started squawking. I started to launch into the parallels between his receiver and mine when he said 'That's for me', went over to his 2-way job, came back and told me he was going for coffee, would be back in ten minutes, and that he didn't expect to see me there when he returned.
I haven't been in Lynbook at all very much, and never again at 2 AM. So I don't know if the fog or the trampoline of overhead wires was the bigger factor for such directional- and separation-clear audio conditions. I suggest both were, and daresay in equal measures.
 


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