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Author Topic: VHF / UHF penetration  (Read 6669 times)
LB5KE
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Posts: 141




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« on: December 28, 2011, 10:55:03 AM »

Lower frequencies is better at penetrating  than higher frequencies, however I have heard hams stating that UHF is better inside buildings and cities. So i did a test at my work which is a large concrete building. Outside i got about the same level of signal to another ham around 3 miles away. Inside the building 2 meter was hard to get trough at all, but 70cm worked just fine but at a drop form 5s to 4. The reason is probably the window at my office that are twice the wave length at 70cm.

The next experiment was that i went in to a room with out windows. I was surprised that i could still get through at 70cm, even if i was week. 2 meter didn't work at all. So why is this? Could it be because 90% of larger buildings are made out of steel reinforced concrete. The spacing on the steel mesh seems to be larger than 1/4 wave. And waves at 430 are getting through?

Is there a chance that UHF RF is better at penetrating steel? I have tried to put my cellular in a steel pot from the kitchen, and i am still able to phone it, i tried the same with a 27MHz and no response. (Aluminium foil blocks both completely).


All this tests are unscientific, and i could be wrong.
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AA4PB
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Posts: 12856




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« Reply #1 on: December 28, 2011, 11:16:32 AM »

Its all about the size of a wavelength vs. the size of openings that might pass the signal through. Neither VHF or UHF will penetrate solid steel or any other hightly conductive material. The cell phone gets out of the pot because the lid is not sealed electrically. Solder the lid to the pot and it won't get out. That's why you often find component shields inside equipment with covers soldered into place.

I did an experiment with a metal box with a hinged lid and snaps. A 1W VHF transmitter inside got out with the lid closed but when I closed the snaps to pull it down tight and make a good electrical contact all around you couldn't hear the transmitter with the receive sitting right beside it.
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K1CJS
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« Reply #2 on: December 28, 2011, 11:42:42 AM »

Lower frequencies is better at penetrating  than higher frequencies, however I have heard hams stating that UHF is better inside buildings and cities....

No, switch that around.  You've got it backwards.  Higher frequencies are better for penetration of buildings.  Lower frequencies are better for achieving greater distance communications.  That is why UHF could get out of your closed building while VHF couldn't.  Remember that the higher the frequency, the shorter the wavelength, and the smaller the 'hole' that is required to let that signal through.
« Last Edit: December 28, 2011, 11:44:50 AM by K1CJS » Logged
LB5KE
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Posts: 141




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« Reply #3 on: December 28, 2011, 11:53:39 AM »

The cell phone gets out of the pot because the lid is not sealed electrically. Solder the lid to the pot and it won't get out. That's why you often find component shields inside equipment with covers soldered into place.


So what i observe is that UHF is better to leak through than HF/VHF?  I had a real hard time stopping 70cm, except when i used  a space blanket :



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KA4POL
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Posts: 2000




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« Reply #4 on: December 28, 2011, 11:55:25 AM »

Anything can work as an antenna which removes energy from the electromagnetic field. Some materials are perfectly tuned by their dimensions and the characteristics. Other materials are less qualified. A tuned antenna will remove more energy, i.e. permit less RF to get away. Your building can be regarded as a Faraday cage. Some windows may be coated for reflectivity. This also attenuates RF. So obviously the metal mesh in the concrete is better 'tuned' to 2m. Your cell phone, working at much higher frequencies does not have as severe problems as 2m. If you rotate the antenna thus changing the polarization you might get better or worse reception.
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