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Wavefront Z and TEM cells

 
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pelican2
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Joined: 21 Jun 2011
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Location: Santa Barbara California

PostPosted: Tue Jun 21, 2011 5:15 pm    Post subject: Wavefront Z and TEM cells Reply with quote

I'm looking to make a very small indoor test chamber for small DF ( direction finding ) antennas, running from maybe 50 to 500 or 1000 MHz.

Main goal is to eliminate reflections from nearby objects, ( furniture, lights etc ) and provide a repeatable reading of DF direction, with DF antennas placed inside this chamber.

Reflections inside chamber should be reduced 10 db or more, size = maybe 3 x 3 x 5 feet. ( h x w x l )

TEM cells look promising, but I am thinking of something similar without a sceptum plate, and using a 2-dimensional sheet "dummy load" at one end of the chamber, made of an array of resistors to closely simulate 377 ohms / square of wavefront impedance.

background info : Refer to this article about wavefront Z expression :

http://www.hallikainen.org/rw/insite/WaveZ.pdf

The idea is to provide a top and bottom metal plate ( twinlead conductors to form the T-line ) and a 377 ohms/square sheet load ( "dummy load" ) at one end of chamber. Taper the other end to a point and connect the two conductors to a co-ax fitting to inject a drive signal. ( and add a shunt R to bring drivepoint Z down to 50 ohms )

This basically would yield an RF "wind tunnel"

The vertical side walls of this chamber are the question... I think they also should be made of sheet conductors with Rs = 377 ohms / square, but I can't intuitively reason this out... Obviously the walls should NOT be metal, that would yield a waveguide, and should NOT be "open" which would permit ingress of external signals...

TEM cell is still an option, but this method seems like it should work.

Comments = ?
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Schubert
Antenna Wizard


Joined: 08 Apr 2009
Posts: 161

PostPosted: Thu Jun 23, 2011 4:49 am    Post subject: Reply with quote

It is extraordinarily difficult to get repeatable measurements in TEM cells. What exactly do you want to measure with the cell? Antenna efficiency, or ?

Basically, you will end up with a resonant cavity. To get an idea how it behaves, consider running a test antenna and the probe antenna cables out into a VNA and measuring S12. You will see a lot of rapid variation.

Why do you want 377 Ohms/square? To match the intrinsic impedance of free space? Not sure if the ratio of E/H for plane waves in free space (which will 100% not be the case in a small near field cell) matched to the ratio of V/I will yield what you are looking for.

Note: if antennas radiated better when V/I=377 into free space, antennas would be designed to be 377 ohms. They aren't, by the way. People connect intrinsic impedance of a medium and impedance of an antenna with the concept of impedance matching and assume they should be the same.
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pelican2
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Joined: 21 Jun 2011
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PostPosted: Thu Jun 23, 2011 2:48 pm    Post subject: Reply with quote

The main goal is to test receiveing antennas used by radio direction finders. Specifically, to create an RF "wind tunnel" so that wavefronts are traveling in a specific direction, with minimal "cross winds" or "head winds" caused by wall reflections. Absolute measurements are not required, just repeatable conditions with minimal reflections from walls... that is why the 377 ohms per square for the walls.

As for the meaning of 377 ohms per square, refer to this article, cited in the first post :

http://www.hallikainen.org/rw/insite/WaveZ.pdf

Antennas do not have drivepoint Z = 377 ohms because they do not generate collimated plane wavefronts in one specific direction... they generate spherical wavefronts with different field strengths in different directions. ( non - isotropic )

If a vertical dipole radiated ONLY in directions N,E,S and W, ( and ONLY in the horizontal plane ) then the drivepoint Z would be 1/4 x 377 = 94.25 ohms, since the antenna is driving 4 separate 377 ohm paths sinmultaneously. ( like four 377 ohm resistors in parallel )

When the REAL radiation of a dipole is examined ( in ALL directions, to various degrees ) and the power in all the wavefront directions are "summed together" and compared against the drivepoint voltage, the equivalent resistance ( at the drivepoint of the antenna ) equals the "textbook" value of ( I recall ) 73.6 ohms.
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R. Fry
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Joined: 06 Jun 2011
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Location: Illinois USA

PostPosted: Thu Jun 23, 2011 10:21 pm    Post subject: "Z" of Free Space vs. Antenna Feedpoint Reply with quote

The impedance of free space is ~377 ohms regardless of the feedpoint impedance of the antenna system radiating that power into free space (and regardless of its radiation pattern).

The ratio of the current and voltage for the power applied to every antenna is related to the value of the feedpoint impedance of that antenna configuration, but the far-field E and H components of the power it radiates always is related to 377 ohms.

For example the peak, free-space E field of an electrically short dipole radiating 1 watt in a given direction is no different than that of any other antenna configuration radiating 1 watt in that direction -- even though the feedpoint impedances of those antennas may be greatly different.
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pelican2
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Joined: 21 Jun 2011
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Location: Santa Barbara California

PostPosted: Thu Jun 23, 2011 10:44 pm    Post subject: Reply with quote

All very true... and I already knew those facts... but none of it is really on topic. I was suggesting a geometry for a small RF chamber that was similar to TEM cells, but significantly different... something without a sceptum... and soliciting comments on that.
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