What material blocks RF? - AVS Forum
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post #1 of 15 Old 09-30-2010, 12:34 PM - Thread Starter
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For an installation that is not outdoors. Would it posible to use multiple flat antennas, like an RCA Ant751 and separate them with galvanized steel or aluminum or something, rather than the four foot spacing that is typically recommended? Would the material immediately reflect the signal back and create something similar to a feedback loop?
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post #2 of 15 Old 09-30-2010, 07:53 PM
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> What material blocks RF?

Instead of "blocking", you should be thinking of absorbing the signal.
Sheet metal that is well grounded (to earth) might work.
If you cannot ground it, then try to research the material(s) used for stealth technology (e.g. B-2 bomber).

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post #3 of 15 Old 10-01-2010, 09:45 AM
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In general, the denser the material, the more RF it blocks. Metal is usually the worst culprit in construction. Brick and wood follow.

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post #4 of 15 Old 10-01-2010, 10:28 AM
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Quote:
Originally Posted by Steve_Weggus View Post

Would it posible to use multiple flat antennas?

What problem are you trying to solve?
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post #5 of 15 Old 10-01-2010, 02:51 PM
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TV & FM antennas are designed to operate in free space. Any metal objects near-by make life difficult for the antenna. Note: the mounting mast is part of the antenna design.

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post #6 of 15 Old 10-02-2010, 05:04 AM
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Yes, please explain what you are trying to do.
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post #7 of 15 Old 10-02-2010, 12:16 PM
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Radio waves are a form of light. Think of how light is affected by every day objects, and you will get some idea of what is happening in the radio spectrum. Remember that the color of light is just different wavelengths of light, and objects can affect some colors different than others.

The main difference is that radio waves are macroscopic (in the range of meters). This means that visible shapes of objects can affect radio waves in interesting ways. A mesh of wire with holes smaller than the wavelength of the radio waves will appear to be solid to the radio waves. An array of parallel wires would appear as a polarizing screen to radio waves, passing waves that are perpendicular to the wires and reflecting those that are parallel.

Good conductors will reflect radio waves (and light, this is why metal is shiny). Poor conductors will absorb radio waves, and insulators will be transparent (this is why you can see through glass).
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post #8 of 15 Old 10-03-2010, 08:46 AM - Thread Starter
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Thanks for your answers. As to what I was trying to do - I was trying to figure out an approach for a dense array of small antennas. I really was thinking about indoor antennas and trying to get gain thru multiple antennas rather than a long boom.

sometimes I fall victim to too much coffee and a fertile mind.
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post #9 of 15 Old 10-03-2010, 09:24 AM
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If you are just trying to "stack" a pair of matching antennas, in order to get more gain/directionality, they can be placed ALMOST touching each other horizontally. For a very directional specific channel, or to null out a reflection, they need to be further apart, since you are trying to capture the wavefront (like an electronic "bubble") at two widely spaced points, at the exact same moment.

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post #10 of 15 Old 10-03-2010, 09:41 AM
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Quote:
Originally Posted by Steve_Weggus View Post

Trying to get gain thru multiple antennas rather than a long boom.

When stacking antennas for extra gain the gain varies on the distance between them and not how isolated they are.
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post #11 of 15 Old 10-03-2010, 10:08 AM - Thread Starter
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Ken,

Thanks for your response. An RCA Ant751 is a pretty good small antenna. Its reflector is a rod, rather than a grid so its height is like 2 or 3 inches. Would it be possible to gang several of these, basically one atop the other. Connect all of the baluns to a combiner (or a splitter combiner) and have increased gain and not still produce good results (ie no crazy feedback that ruins everything).

There is the RCA Ant751 and there are also small UHF log -periodic antennas.
Either could be stacked several times within fifteen or 18 vertical inches.
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post #12 of 15 Old 10-03-2010, 12:54 PM
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If you are stacking antennas, the cabling is vary important. The length of the cable between each antenna and the joiner must be exactly the same, and they must all be wired in the same phase. Get one wired backwards, and you effectively remove two antennas from the array.
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post #13 of 15 Old 10-03-2010, 01:22 PM - Thread Starter
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Again - Thanks

I remember that physical length of the connecting coax needs to be the same.
Please, what determines the phasing of the antenna?
Is it the installation /orientation of the balun?
Can the phasing be checked with an inexpensive piece of test equipment?
(I looked at a balun that laying nearby and it looks totally symetrical.)

Thanks
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post #14 of 15 Old 10-03-2010, 01:52 PM
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The best way to check the polarity of the balun is with some sort of a signal strength meter. You'd have the two antennas aimed at a signal source (something like an analog channel in the low-VHF band would be ideal, but some signal in that general range would work...maybe a VHF two-way radio transmitter nearby), with the cables cut to matching lengths and feeding a splitter/combiner. With the baluns installed one way, check the signal strength. Then, flip one of the baluns over, and see if the signal goes up, or down. You want the position that adds, giving the higher signal. A radio scanner with a signal strength meter would work. A friendly Ham radio operator (with test gear) is even better.

The meter on a Digital TV is not good, since it reads lots more than just signal strength.

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post #15 of 15 Old 10-04-2010, 01:50 PM
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Don't trust the inputs of the combiner to be wired the same way either. You need to do the phase test with the combiner you intend to use in the final construct, and swapping inputs may affect the result, so make sure you test with the antennas connected to the inputs they will be connected to in the final result.
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