can a metal enclosure that is completely surrounding a Tx antenna, act as a receiver (a receiving antenna)?

Thread Starter

genekuli

Joined Oct 21, 2018
103
If I put a metal enclosure around a Tx antenna, can the metal enclosure, since it is completely surrounding the antenna, act as a receiver (a receiving antenna) even if it is not of any specific dimension relating to the frequency of the TX antenna radiation within it?
 

Sensacell

Joined Jun 19, 2012
2,893
Mmmm No.

Not if it's a continuous conductive envelope.

Otherwise, RF would leak into every shielded compartment and the world would come to an end.
 

Thread Starter

genekuli

Joined Oct 21, 2018
103
Mmmm No.

Not if it's a continuous conductive envelope.

Otherwise, RF would leak into every shielded compartment and the world would come to an end.
the Tx radiating antenna would be inside the conductive, non grounded, enclosure (eg. cuboid). could the enclosure act as a receiver (a receiving antenna) for the RF radiation from within?
 

Thread Starter

genekuli

Joined Oct 21, 2018
103
I read the question as "If I put something in a metal box, can the metal box receive signals?"
to clarify, the Tx radiating antenna would be inside the conductive, non grounded, enclosure (eg. cuboid). could the enclosure act as a receiver (a receiving antenna) for the RF radiation from within?
 

Sensacell

Joined Jun 19, 2012
2,893
to clarify, the Tx radiating antenna would be inside the conductive, non grounded, enclosure (eg. cuboid). could the enclosure act as a receiver (a receiving antenna) for the RF radiation from within?
"Grounded" or not- is irrelevant- the inside of the box is its own electrical universe, completely contained to itself.

INSIDE the box- there is capacitance between the internal (and presumably floating) transmitter, the enclosure will intercept the radiation.
You could make a current flow through this capacitance by referencing (connecting) the box electrically to the TX - creating a loop for current flow.

The inside could be resonant at the TX frequency, which could create a current in the skin on the inside?
 

andrewmm

Joined Feb 25, 2011
1,757
I had a thought over night

The reason for the confusion,

Antennas are Typically designed and specified to work into a far field / open space.
with a defined "plane" of reference,

In testing , literally a large field is Typically used .

The nearest most of us get to antennas in a metal box get to are microwave ovens,
The microwave oven illustrates a lot of the problems your asking about in the multiple posts.
the "magnatron" is loosely coupled into the cavity,
The oven ends up with standing waves in it, hence food get hot in spots if you dont "stir" the waves or rotate the food.
If you put metal in the oven, and especially if it touches the sides, enough energy can be coupled back to the magnatron to blow it
If you run the oven with no load in it, i.e. no food, the reflections can blow the magnatron,

There is even a space craft idea, that uses a magnatron in a sealed box,
can't find link at moment, but its there.

QED, all the standard equations assume the radiation is absorbed,

Even near field equations are more difficult than far field,
I have been working with phased arrays, and the "easy" equations break down

The answer , is a 3D field simulator,
which basiclay works on Maxwell equations,

But , as your so far outside the "Typically" , you are into the world of , every thing matters, just how far from case are you, antenna properties, dialectric porperties, eta ll
 

Thread Starter

genekuli

Joined Oct 21, 2018
103
I had a thought over night

The reason for the confusion,

Antennas are Typically designed and specified to work into a far field / open space.
with a defined "plane" of reference,

In testing , literally a large field is Typically used .

The nearest most of us get to antennas in a metal box get to are microwave ovens,
The microwave oven illustrates a lot of the problems your asking about in the multiple posts.
the "magnatron" is loosely coupled into the cavity,
The oven ends up with standing waves in it, hence food get hot in spots if you dont "stir" the waves or rotate the food.
If you put metal in the oven, and especially if it touches the sides, enough energy can be coupled back to the magnatron to blow it
If you run the oven with no load in it, i.e. no food, the reflections can blow the magnatron,

There is even a space craft idea, that uses a magnatron in a sealed box,
can't find link at moment, but its there.

QED, all the standard equations assume the radiation is absorbed,

Even near field equations are more difficult than far field,
I have been working with phased arrays, and the "easy" equations break down

The answer , is a 3D field simulator,
which basiclay works on Maxwell equations,

But , as your so far outside the "Typically" , you are into the world of , every thing matters, just how far from case are you, antenna properties, dialectric porperties, eta ll
thanks so much for your thought out helpful answer. after the hundred answers i read and the info i have researched and your answer, it seems that inside the box, there would be a high band pass filter effect and perhaps even amplification at that narrow resonant band resulting in the box being able to act as an extremely efficient antenna but only for that narrow band. the issue would be matching the high impedance and finding a good feed spot. is that correct? or did i misunderstand it all?
 
Last edited:

alan01346

Joined Aug 13, 2020
30
How perfect is the box? The first thing that came to mind is the shielded loop antenns. Only about an inch of the loop is actually uncovered but the shield blocks ambient noise in a directional manner. You rotate and postion it for minimum noise, the response to signal is omnidirectional. I used to use one to listen to 75 meter AM phone in an apartment in a city with lots of noise sources around.
shloop.png

Another case which I didn't figure out for years later is that I was doing some audio recording with a relatively high impedance circuit but coax for all the lines, and sometimes I could hear the VOA transmitter coming through. The coax shield was acting as an antenna and causing current to flow on the inside conductor. Maybe neither are relevant.
 
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