AM Reception Sweet Spot

[FreddyE1977]

I have noticed an AM reception sweet spot in my daily commute and am trying to understand the
science of just how this may be working.

In my daily commute I travel down a 2-lane road which leads to a concrete tunnel.
The tunnel is probably 35-40 yards in length. On the other side you emerge into perhaps
25 feet of open sky before passing under a steel bridge that carries rail traffic. Then you
immediately go over a bridge which deposits you at the intersection with a main highway
on the other side.

I have noticed sitting in traffic inside that space between the concrete tunnel and the bridge
that my ability to receive AM stations on my car radio goes off the chart. All of the Class D
stations in town that broadcast less than 100 w. after dark and can barely be heard anywhere
else will suddenly start booming through the speakers as if the stick was next-door. The same
is true with the weakest or most distant station during daylight hours.

I am guessing the phenomenon is similar to those ferrite loop antenna enhancers that
have been sold for many years, but it's just a guess on my part. All and all it is interesting.

 

[kenglish]

I suspect that the railroad tracks are acting like a huge long-wire antenna, funneling all those signals toward your radio. The metal bridges may be helping a bit, too.
Do you hear much from distant stations? Distants may be overloaded by the strength of the locals, saturating your radio, though.

 

[FreddyE1977]

Not distant as in out of market DX. I should try that.
Mainly small AM's on the fringe of the metro and those running 5-75 watts after sundown.

 

[boombox4]

Railroad tracks can act as a beverage antenna-on-ground. I used to place an AM radio next to some tracks in my area of town and I could hear Alaska. Pull the radio away? No Alaska. And Alaska is difficult to hear in the PNW.

 

[FreddyE1977]

There is an urban legend around here about one of the local college carrier circuit AM's
hooking their antenna into the local streetcar system and being heard thousands of miles away.
Never found anyone to independently confirm that.

 

[Kelly A]

There is an urban legend around here about one of the local college carrier circuit AM's
hooking their antenna into the local streetcar system and being heard thousands of miles away.
Never found anyone to independently confirm that.

Back in the day, carrier current AM stations were pretty common. Many college campuses which couldn't afford, or weren't interested in licensing an actual radio station, used to feed the college dorms electrical systems with RF. The distance a carrier current station travels depends on several factors: The input power of the station, continuity of the power system it travels on, and how many transformers the RF has to pass through (or not).

Hearing a carrier current transmission thousands of miles away would be unlikely.

 

[Kelly A]

I have noticed an AM reception sweet spot in my daily commute and am trying to understand the
science of just how this may be working.

I have noticed sitting in traffic inside that space between the concrete tunnel and the bridge
that my ability to receive AM stations on my car radio goes off the chart. All of the Class D
stations in town that broadcast less than 100 w. after dark and can barely be heard anywhere
else will suddenly start booming through the speakers as if the stick was next-door. The same
is true with the weakest or most distant station during daylight hours.

Railroad tracks are excellent ground. Since AM stations have a groundwave component, you're probably hearing the enhanced groundwave when close to the tracks.

 

[kenglish]

About 70 years ago, the high school in Mt. Pleasant, Utah had a carrier-current station, courtesy of my ex-boss. He climbed up the big transmission tower that fed the whole town, fastened a capacitor to one of the high voltage lines, and fed it from his home-brew transmitter in the schoolhouse. He says it got out pretty well.