Electrically Sheilded Antenna

[dmorris619]

I am trying to get an understanding of how to create an electrically shielded loop antenna. Is it possible to have a multi-turn electrically shielded antenna?

Also how does an electric field create interference with the loop antenna? Is it because the electric field is not constant throughout the antenna?

 

[JimmyAlz]

If your antenna is shielded it will not work as an antenna.

 

[berkeman]

If your antenna is shielded it will not work as an antenna.

It's a trick with loop antennas -- see my next post (in a couple minutes).

 

[berkeman]

I am trying to get an understanding of how to create an electrically shielded loop antenna. Is it possible to have a multi-turn electrically shielded antenna?

Also how does an electric field create interference with the loop antenna? Is it because the electric field is not constant throughout the antenna?

An electrically shielded loop probe or antenna is commonly a single turn, often made out of coax cable. You break the shield at one end of the loop (typically the end away from the feedpoint), so that you do not create a shorted turn around the loop (which would prevent most magnetic coupling to the loop, depending on the quality and construction of the shield.

I'm only familiar with single-loop shielded probes/antennas, but it seems like it should work for multi-turn loops as well. Just be sure to provide a single break in the shield at some point in the circumference, so that the incoming B-field can couple to the turns.

 

[berkeman]

Oh, and why use shielded loop antennas -- yes, typically it is because of nearby capacitively-coupled E-field noise. The example that I'm most familiar with is B-field probes for PCB EMI tracing. You want to be able to hold your small loop probe on the PCB and geometrically trace out RF currents that are getting out of your device and causing radiated EMI problems, but you only want to sense the B-fields generated by the RF PCB currents. There is plenty of capacitive pickup into the loop probe if it is not shielded...

 

[dmorris619]

I am designing a loop antenna for a senior design project and the customer asked what I thought about electrically shielding the antenna. Essentially I am so green to designing antennas that I cannot speak with confidence on whether or not I need to shield the antenna.

Heres what my antenna needs to be able to be capable of, detect magnetic fields from 60Hz to 100KHz, from a magnitude of .1 microtesla to 10 microtesla. At that level my customer and to the degree that I can be are concerned that the electrical noise from the surrounding environment(this will be near electrical cables carrying 120 volts ac) will drown the magnetic signal we are trying to read.

Do you think it is necessary to shield the loop antenna? Also the kicker is that the antenna diameter can't be more than 5.25 inches, which is why I'm asking about multi-turn loops.

 

[berkeman]

I am designing a loop antenna for a senior design project and the customer asked what I thought about electrically shielding the antenna. Essentially I am so green to designing antennas that I cannot speak with confidence on whether or not I need to shield the antenna.

Heres what my antenna needs to be able to be capable of, detect magnetic fields from 60Hz to 100KHz, from a magnitude of .1 microtesla to 10 microtesla. At that level my customer and to the degree that I can be are concerned that the electrical noise from the surrounding environment(this will be near electrical cables carrying 120 volts ac) will drown the magnetic signal we are trying to read.

Do you think it is necessary to shield the loop antenna? Also the kicker is that the antenna diameter can't be more than 5.25 inches, which is why I'm asking about multi-turn loops.

That's a very low frequency range, and problematic because it includes AC Mains frequencies (50/60Hz). What is this antenna supposed to pick up?

For low frequencies, a ferrite rod antenna is more typically used. Here is a recent PF thread where we discussed them:

https://www.physicsforums.com/showthread.php?t=450045

.

 

[JimmyAlz]

Alright!
Every day you learn something new :)

 

[dmorris619]

This antenna is supposed to just read the intensity of the magnetic field. Eventually it will be read by a microprocessor and then Fourier Transformed to give a frequency breakdown of the magnetic field.

It should pick up the magnetic field of the AC Mains.