Can Block FM but Not AM

[John1HitWonder]

Hi guys,

I'm working on a project involving radio frequencies. I want to use a steel mesh as a Faraday cage for a radio alarm clock inside the sealed mesh. Somehow, the FM frequencies are totally blocked but the AM frequencies come through the mesh loud and clear!

Can anyone please help me understand WHY this is happening? I know how to block the AM frequencies using other materials,but why won't my stainless steel mesh work? The openings are about the size of a pinhead, and the Faraday cage is sealed tight, so I know it's not the hole size or a leak.

I'm at my wit's end...this is the most frustrating thing I've ever encountered. Help!

Thanks!

 

[AB4BF]

Some radios use the power cord as the antenna. You'll have to shield the power cord somehow or use an isolation transformer or find another radio that works.

Thinking about this though, the FM usually uses the power cord and the AM uses a ferrite w/coil. Is it a completely battery powered radio?

On further thought, how many AM stations are you receiving? Could be the size of the cage is a close harmonic of a nearby AM signal and bleeds through. You'll have to experiment.

 

[John1HitWonder]

Barry, thanks for your input! The radio has battery power and a power cord. I used only the battery power and slipped the cord inside as well. The entire AM spectrum goes through the cage while all FM stations are completely blocked, so it can't be the harmonics. I also used another cage whose size is 6 inches different to see if it was the size of the cage, but that had no effect either.

Do you have any further ideas? My thoughts are either that the material I'm using is too thin, which "shouldn't" matter, or it's the wrong material specifically for AM frequencies. I don't know what else it could be. Anyone?

 

[jhooten]

Is the cage grounded? To a Real RF ground?

 

[John1HitWonder]

Not Caged

Thanks for your input guys! I posted another message earlier but it didn't get through.

The power cord is inside the cage and the radio is powered by batteries. I am receiving all AM frequencies through the cage. The cage is not grounded. It should not make a difference, however, because the FM waves ARE blocked.

Any thoughts?

Thanks again!

Joel

 

[rfradioconsult]

Without a ground connection you do not have a true Faraday cage. You reference an AM vs. FM signal, you need to think about how radio waves propagate at different frequencies, a signal at 1MHz propagates differently than one at 100 MHz? Study the basics and you will be enlightened.

 

[jhooten]

You can lead a Horse to water...

 

[AmpedForay]

Yeah, what rfradioconsult said. Without being grounded your "Faraday cage" is basically an antennae.

 

[n5ims]

The entire AM spectrum goes through the cage while all FM stations are completely blocked, so it can't be the harmonics.

It isn't harmonics, it's the frequency of the different stations. FM is much higher in the RF spectrum than the AM band is (about 100 times the frequency). AM is often listed in Kilohertz (about 530 - 1700 kHz) while FM is in Megahertz (about 88 - 108 MHz). Your cage appears to work correctly with high frequency signals, but not low frequency ones.

Think of it this way, assuming it's strong enough to handle the weight, a typical screen will prevent larger particles such as pebbles from passing through it but smaller ones, such as sand will flow through easily. Your VHF range FM signals are like the pebbles and are stopped at the cage, but the MW range AM signals flow through like the sand.

 

[AmpedForay]

It isn't harmonics, it's the frequency of the different stations. FM is much higher in the RF spectrum than the AM band is (about 100 times the frequency). AM is often listed in Kilohertz (about 530 - 1700 kHz) while FM is in Megahertz (about 88 - 108 MHz). Your cage appears to work correctly with high frequency signals, but not low frequency ones.

Think of it this way, assuming it's strong enough to handle the weight, a typical screen will prevent larger particles such as pebbles from passing through it but smaller ones, such as sand will flow through easily. Your VHF range FM signals are like the pebbles and are stopped at the cage, but the MW range AM signals flow through like the sand.

I believe the opposite is true. The higher frequencies have shorter wavelengths which pass through smaller openings. This is why a 2m reflector can be so large but a microwave dish has to be such a fine mesh. I believe it is more likely that an ungrounded piece of metal is acting as an antennae as all ungrounded metal objects do. And that it is due to it's size and construction that it "pulls in" the AM signals.

 

[AmpedForay]

Ground

Just try grounding the dog garn thing, then post back your results.