AM signal penetration

[collegeDJ86]

Hello all. I have a basic understanding of wavelengths and such from physics classes I have taken. If I remember correctly, longer wavelengths are more likely to pass through "solid" objects such as walls (hence why you can hear thumping bass in an adjacent room without necessarily hearing a guitar riff). So, that being said, why is it that AM signals don't travel well through things like overpasses, while FM seems to penetrate okay?

Also, why do electrical wires affect AM signals worse than FM signals?

Thanks in advance for the education provided.

 

[Les]

> So, that being said, why is it that
> AM signals don't travel well through things like overpasses,
> while FM seems to penetrate okay?

Lots of steel in those overpasses. All of it grounded.
Look up "Farraday Cage".

In tunnels, you're below ground (and/or water) which has
sufficient conductivity to ground out the signals (AM/FM, etc.)
Only some ELF (extremely low frequency) makes much headway
through earth or water. That's creepy stuff. If you haven't
looked into it, consider doing so if only for savage amusement.

> Also, why do electrical wires affect AM signals worse than
> FM signals?

Electricity travels around wire, not just in it. Strong
fields beneath high voltage lines. Mostly 60-Hz but not
pure, somewhat more broad and very powerful. FM, being
based on altering frequency, not amplitude, is less prone
to the noise.<P ID="signature">______________
Special Prosecutor: Synonym for future 'Congressional Candidate".</P>

 

[tpt]

The lengths of wire between insulators can approach AM wavelengths, which can have varying effects:

Sometimes it seems like the signal is "sucked out," sometimes the signal intesnified or distorted. In areas with many hi-powered AM signals, this can cause many interesting effects. Not that FM signals arn't also affected by power lines. Bad insulators, arcing disconnects, etc. can create a cloud of grit that destroys all but the strongest signals. In this cabeled-world you don't see it much anymore, but the "sparklies" on low band VHF channels were often caused by arcing from high-voltage transmission towers. Short lengths of wires are wrapped around the end of the long cables and used to tie them to insulators and the next wire. As they corrode, they beging arc in dry weather. They are just the right length to transmit this static on TV channels.

Now, why can you hear FM under overpasses? Probably because the shorter FM wavelengths bend and reflect around such obstacles, leaving enough signal for your receiver to detect the signal. The much longer AM wavelengths don't act this way. Go into a long tunnel, of course, and everything is lost.

 

[Andy_Brown]

Impress the physics geeks

> why is it that
> AM signals don't travel well through things like overpasses,
> while FM seems to penetrate okay?

The short answer to this question is to say
that at US AM band frequencies, boundary conditions can not be satisfied.


<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>

 

[pianoplayer88key]

> Now, why can you hear FM under overpasses? Probably because
> the shorter FM wavelengths bend and reflect around such
> obstacles, leaving enough signal for your receiver to detect
> the signal. The much longer AM wavelengths don't act this
> way. Go into a long tunnel, of course, and everything is
> lost.
>

An interesting thing I noticed, coming into L.A. (near Burbank or so) on my way home to San Diego from Sacramento, I was listening to KDIS, and getting a pretty solid signal (this was just after they had switched to their nighttime pattern). We went under a bridge, and while there, KDIS disappeared, and a mexican station replaced it. When out from under the bridge, it was back to KDIS, with no trace of the mex.
What could have caused that? Different skywave/groundwave angles maybe?

 

[rbrucecarter]

There is a place on US 75 in Richardson where local 620 KMKI disappears, and is replaced by the PHoenix 620. I got an ID the other day. It is under a power line, and works 24/7. I always heard it, and wondered what talk station it was. The power line must be continuous all the way to Arizona for that to happen. And somehow the 620 signal from Phoenix travels down it and even makes it through transformers somehow.

 

[CW]

> There is a place on US 75 in Richardson where local 620 KMKI
> disappears, and is replaced by the PHoenix 620. I got an ID
> the other day. It is under a power line, and works 24/7. I
> always heard it, and wondered what talk station it was. The
> power line must be continuous all the way to Arizona for
> that to happen. And somehow the 620 signal from Phoenix
> travels down it and even makes it through transformers
> somehow.

Not the power line.....Texas's power grid for the most part is NOT connected to anything to the west (Most of the Texas grid is totally isolated...well those parts of ERCOT...SE Texas and NE Texas is part of the Eastern Interconnect and I think a section in western Panhandle is part of the Western Interconnect)...
BUT TXU service area is part of ERCOT....

The 620 in Phoenix is KTAR, 5KW omni day, 5kw DA-N with a NULL toward DFW!

 

[Ai4i]

Very often, a bridge will take out a station from one direction but leave another one in tact. It is, in effect, directionalizing your car radio antenna. We have a horizontally polarized pirate station in my area which seems impervious to bridges while much stronger licensed vertical stations wipe out.
Before you ask, before the nineteen thirties, most or all AM stations were horizontal and car radio antennae consisted of horizontal wires under the running boards. The receive antenna polarity must match the transmit polarity. Problem is that at these frequencies, vertical produces a much better groundwave whereas horizontal produces a harmful short skywave.
<P ID="signature">______________
Proud 2 B a pioneering satellite radio subs¢riber
Ai4i is always on the trailing edge of technology
______________</P>

 

[rbrucecarter]

> The 620 in Phoenix is KTAR, 5KW omni day, 5kw DA-N with a
> NULL toward DFW!

It certainly makes for an interesting DX logging. I heard the calls again this morning. There is no mistake.

The power lines in Richardson must take a north / south turn somewhere and present a large profile for KTAR. It goes to show what a lonewire antenna, even inductively coupled, is capable of.

That null is very effective. There is no trace of KTAR in Lubbock, KMKI still owns the frequency. I tried from Carlsbad, NM, and got a weak mix of both KMKI and KTAR. It took a loop to help the receiver.

 

[PhilJSmith67]

> Very often, a bridge will take out a station from one
> direction but leave another one in tact. It is, in effect,
> directionalizing your car radio antenna. We have a
> horizontally polarized pirate station in my area which seems
> impervious to bridges while much stronger licensed vertical
> stations wipe out.
> Before you ask, before the nineteen thirties, most or all AM
> stations were horizontal and car radio antennae consisted of
> horizontal wires under the running boards. The receive
> antenna polarity must match the transmit polarity. Problem
> is that at these frequencies, vertical produces a much
> better groundwave whereas horizontal produces a harmful
> short skywave.

This prompts me to ask... Do any Mexican AMs use horizontal polarization?

I ask because in Puerto Vallarta, I experienced something quite odd with some of the AM stations, both indoors and outside.

On a Sony AM/FM handheld, most stations were strongest with the radio in the "normal position" (with the internal ferrite bar horizontal), and the width of the radio perpendicular to the station. This was also the case with several distant California and other U.S. stations that I received at night. However, there were some Mexican AMs for which the signal was strongest with the radio positioned "on end" with the ferrite bar vertical. In this position, there was no way to null out the station by direction of course, nor was there any way to "peak" the station. Every distant U.S. station was eliminated, as well.

In any other part of the U.S. and Canada, this radio operates exactly as I would normally expect, sitting upright with the ferrite bar horizontal. That includes oceanside locations.

I not only noticed this in Puerto Vallarta, but back in the early 80s with a different receiver in Acapulco, where I was able to leverage this phenomenon to null out a local and pull in a very weak WLS Chicago.