Antennas and radiant barriers

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Ryan:
Our zoning restricts me from putting an antenna on my roof, so I was thinking about putting one in the attic of our house. We have a radiant barrier on our roof decking and I was wondering if this creates a big problem for 2m and 440 work? Thanks!

Charles:

Say how does my new TV antenna look?


http://users.belgacom.net/hamradio/schemas/69KC8VWM.JPG

73 de Charles - KC8VWM

Bob Lewis:
If it's aluminum foil, signals aren't going to go through it very well. I have a "fall back" TV yagi in the attic and it's pretty poor.

Charles:
There are additional losses associated with many other ordinary construction materials and sometimes knowledge of their water content is a contributing factor. A newly built attic has more moisture content vs. an older built attic for example.

Metal usually exists in most attics in things like aluminum siding, aluminum roof vents, electrical wiring, light fixtures, wall stucco (which has an embedded metal screen), and foil backed radiant barriers, all things which can block and/or skew VHF/UHF signals in unusual and undesired patterns.
 
Typically any metal object that is larger than 8" will have some noticeable effects on VHF signals.

Another consideration is concrete and most bricks because they also have enough moderate water content to become a radio signal barrier. It's not just the water content alone but rather their overall thickness and density which contributes to this blocking effect and it is especially measurable when you increase operating frequency. At 2.4 ghz an ordinary brick in the wall is capable of completely blocking out RF signals.

Exterior wood is generally always wet inside, especially in north facing surfaces on most houses.  The amount of water varies with the age of the wood, weather and seasonal conditions in your area. Meaning New Mexico is great while New York State is poor in this area.  Dry asphalt shingles are mostly transparent to VHF/UHF signals, however the sheer number of metal roofing nails holding the shingles and wood sheathing in place are generally spaced at a minimum of every 3 inches or less apart depending on  building code standards in your area. I am suspecting this nail pattern would look very interesting on an x-ray image of your roof.
 
Similar to an X-ray image, your radio signals cannot  pass transparently through this "metal fence" of roofing nails any differently than an X-ray would pass through it.

The vapor barrier is often wet on one side or the other and may reflect or skew radiation patterns.  

The bottom line is that there is no scientific way to accurately predict all the potential signal losses occurring in your attic due to these various construction materials. One thing is for sure though, a radiant barrier is not the only consideration in this equation.

73 de Charles - KC8VWM

Jerry Montgomery:
Large objects like a wall that must be penetrated offer some attenuation but for most ham bands non-conductive things offer insignificant attenuation.  For example a cinder block has an attenuation of 1.6e-5 dB at 10 MHz, and 3e-4 dB at 100 MHz, hardly something you would notice.  Attenuation at 2 GHz is about 4 dB for a cinder block.  Attenuation of a brick at 2 GHz ranges from 2 to 4 dB. So for HF and VHF most nonconductive objects are for all practical purposes transparent.  If you have a cinder block wall filled with rebar, you can forget the cement and just figure the attenuation of the rebar, which can be high.

Conductive surfaces like the wire mesh in stucco and aluminum panels in heat barriers are a different story.  The size of the object that has to be penetrated makes a huge difference in what happens.  If these objects are small compared to the wavelength of the signal, they offer little attenuation.  The currents induced into small objects will be very small and any small signal that is induced in them tends to get re-radiated and little attenuation is noticed.  You may see some detuning of the resonant frequency of the antenna.  As the resonant frequency of the object starts to approach that of the signal, the object may act like a big reflector, and produce large amounts of attenuation mainly because the signal is reflected back toward the radiator instead of being re-radiated.

Radiant heat barriers are unpredictable because they come in 4 by 8 sheets and have a laminated surface of aluminum, and when they are applied it will be unknown if the aluminum on adjacent sheets will be touching.  If they don't touch, then you can probably get 30 MHz and below thru this.  If they do touch, you probably won't be so lucky.  Even when the sheets do not touch, 6 meters and above are likely to show significant attenuation.  So the answer to your question is yes, this causes significant problems for 2 meters and 70 cm.  Some people have tried connecting wires to these sheets and using them as antennas, not something I would recommend.  You don't need any arcing going on in your attic.

Also when you put an antenna into an attic, there are lots of wires and other conductive objects that may cause some unexpected results, especially when these objects happen to be resonant on the band you are using.

Jerry, K4SAV

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