What Materials Block AM Signals

I know this has been discussed before, but I can't seem to locate the posts.

The Talking House instructions state that if you are going to locate the wire antenna indoors, it should be in the center of the house, on the 2nd (or, presumably, higher if you have one) floor.  It should not be strung up in front of a window, as glass (they claim) blocks RF in the AM range.

So I'm wondering, is that really the case?  And what else would block (or at least diminish) AM signals (other than metal).  PVC?  Wood?  Gyprock?


Carl Blare's picture

Yes, I also recall some technical discussions about RF passing through materials but it's very difficult to relocate old posts.

On a related matter, it is odd that AM radio literally disappears when driving under a bridge or overpass.

The thing that makes it odd is that two sides of an underpass are wide open, so why doesn't the AM signal simply come from those directions?

I am guessing that it's the metal in bridges and overpasses that does the signal blocking, maybe the concrete also.

I use the metal in window frames as part of the AM antenna, which makes it radiate both indoors and out.

Using a radio receiver I notice that reception is better in the middle of rooms compared to the sides, and RF is reciprocal whether receiving or sending, but it would be difficult to place a transmitter/antenna in the center of most rooms because that's where human movement takes place.

Carl Blare

Mark's picture

Steel and concrete buildings other than houses, going under bridges as was pointed out but FM goes through this much better. The large wavelength of AM sees the steel grid used in buildings and underpasses as a filter with space to small to get through but the smaller FM waves go through.

Glass does not block AM because if you are in a building you go to a window to get reception. That is wrong info. that Talking house gives you. Even if you are in a metal building like a camper if you have the radio against the window you will get reception. The bigger the window the better.



ArtisanRadio's picture

Mark is correct, after a bit of research (I just didn't have the right search terms in google).  CLEAR glass is transparent to radio waves, as is wood, etc.

Here is a website that explains the basics really clearly:


Carl Blare's picture

That was a great read and I came upon something I had previously wondered about:

Example: a silvered mirror reflects about 95 % of light power and about 95% of radio power and absorbs about 5 % of light and radio power.

With so many of our transmitters located indoors they are subject to reflection from mirrors in the confined environment.

What might this do to the signal, either pro or con?

We wonder if an AM transmitting antenna placed against the center of a large mirror might enjoy a 95% increase in signal strength, probably directional into the space seen in the mirror?

For FM we might wonder whether mirrors add substantially to multi-pathing by putting reflections out-of-phase with the antenna signal.

Hey, I can experiment with mirrors, antennas, and a spectrum analyzer!

Carl Blare

wdcx's picture

Here is a guess Artisan. By locating the transmitter in the middle of the house and preferably on a second floor, the transmitter takes advantage of multiple long radiating grounds.

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Carl Blare's picture

It's a long time coming but I still plan to set up an indoor antenna experimental project, the first step being the sign on of KHZ AM 1640 from an AMT3000 SSTran transmitter.

Step two will be KHZ-FM at 88.9 MHz.

It is unlikely that an indoor shortwave experiment will take place, or, looking back in time, we already know what to expect. When we developed the Big Talker Shortwave Transmitter we had an indoor 17' horizontal antenna up above head level and there was massive hum due to inter-action with house electrics.

Returning to the matter of Medium Wave, the attic, if you have one, seems like a perfect out of the way place for indoor antennas.

Carl Blare

MICRO1700's picture

but didn't get around to it.

Having just received a 

Talking House transmitter

a few weeks ago,  I saw the

set-up instructions also and

was perplexed at the statement

about glass blocking radio waves.

Looking back on it - I think the aim

is to get the transmitter/3-meter-wire

to couple into the house wiring somehow.

WDCX has a great point there, too.

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