# Loop antennas

I am transmitting an electromagnetic field from a loop antenna of 0.28 radius to one with 0.216 radius. I know the antenna factor of the receiving antenna but when I add it to the measured field strength, it does not equal my calculated transmit field strength. Should the shorter antenna be centered in relation to the transmit loop or should it be level with the transmit antenna at the top of the loops, or the base of the antenna?

berkeman
Mentor
I am transmitting an electromagnetic field from a loop antenna of 0.28 radius to one with 0.216 radius. I know the antenna factor of the receiving antenna but when I add it to the measured field strength, it does not equal my calculated transmit field strength. Should the shorter antenna be centered in relation to the transmit loop or should it be level with the transmit antenna at the top of the loops, or the base of the antenna?

Welcome to the PF. How far apart are the antennas? You need to be in the far field in order to use the equations you are referring to.

In the far field, the antennas should be coplanar for the best transfer function.

Are you transmitting around 170MHz? How much power are you transmitting? What country are you in, and what licensed bands are near your transmit frequency?

The loops are 1 meter apart. The testing frequency range is from 1kHz to 30MHz. Both of the antenna are 1-turn loops. I am using the setup described in IEEE-291-1991. It worked fine when my receiving antenna had the same radius as my transmit loop, but now that I am testing a smaller antenna, the results are questionable. I assume it would be best to use a transmit loop with the same sized loop, but that is too expensive and impractical.

berkeman
Mentor
The loops are 1 meter apart. The testing frequency range is from 1kHz to 30MHz. Both of the antenna are 1-turn loops. I am using the setup described in IEEE-291-1991. It worked fine when my receiving antenna had the same radius as my transmit loop, but now that I am testing a smaller antenna, the results are questionable. I assume it would be best to use a transmit loop with the same sized loop, but that is too expensive and impractical.

With the loop sizes you described, the separation you are using, and the frequency range that you list, those are not antennas. They are just coupled inductors.

What are you intending to do, exactly?

berkeman
Mentor
Hmm, I just read through that IEEE spec that you cited:

http://www.scribd.com/doc/11355231/IEEE-Std-2911991 [Broken]

And it does say to use small loops and short, coaxial separation. Yuck. Are you using a shielded receiving loop?

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What am I doing? Well, I am trying to validate loop antenna calibrations using a transmit loop antenna with a 0.28 radius to test the antenna factor of receive loop antennas of various radii. It works fine when they are similar in size but I'm having difficulty if the receive loop is significantly larger or smaller than the transmit loop. I'm trying to find any information that would guide me about the proper placement of the receive loop, particularly if it is much smaller than the transmit loop. And yes, they are all shielded loops.

vk6kro
I found the following link. Excellent article.
http://www.djmelectronics.com/articles/emc-antenna-parameters-p3.html [Broken]
This covers antenna factor and a lot more.

I had to wonder if you are feeding the transmit antenna symmetrically?
For example, if you fed it directly with a coaxial cable, one end of the loop would be effectively earthed so the radiation would probably not be symmetrical.
If you rotate your coil by 180 degrees and get different results, the radiation is not symmetrical.

The reference above refers to using the test setup in an anechoic chamber to avoid reflections.

You might find the following site useful too. Not that it will fix your present problem, but it is a great calculator for large single turn loop inductance.
http://www.qsl.net/in3otd/ind1calc.html
The inductance of your large loop would be nearly 50% greater than that of your small loop.
1.48 uH vs 1.04 uH.

I did a check on the self resonant frequencies of your loops. The big one would resonate about 180 MHz and the small one would resonate at about 280 MHz, assuming a tube diameter of about 10 mm. So, this is sufficiently high in both cases to not matter.
This was done with Eznec, an antenna modelling program.

Does the shielding on your loops have a gap somewhere around the circumference? I think they are supposed to have this to avoid circulating currents in the shield.

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berkeman
Mentor
What am I doing? Well, I am trying to validate loop antenna calibrations using a transmit loop antenna with a 0.28 radius to test the antenna factor of receive loop antennas of various radii. It works fine when they are similar in size but I'm having difficulty if the receive loop is significantly larger or smaller than the transmit loop. I'm trying to find any information that would guide me about the proper placement of the receive loop, particularly if it is much smaller than the transmit loop. And yes, they are all shielded loops.

Could you please post the antenna equations that you are trying to validate? Could you please also comment on electrically small "antennas" placed 1 m apart?

The loops have a gap at the top. BTW, I am not using an anechoic chamber. They are placed in a large open area with at least 3 meters of empty space behind each of them. The current applied to the transmit loop is being monitored with a current probe.

vk6kro
I wonder what sort of a result you are getting?
Do you get a steady rise in signal strength with increasing frequency?

Normally, when transmitting, the last thing you want to do is shield your antenna.
Would you consider trying it with two loops wound on opposite ends of a plastic drum? No shielding and just two equal diameter loops of heavy wire?

You didn't say if you were feeding the transmit loop symmetrically.

berkeman
Mentor
Normally, when transmitting, the last thing you want to do is shield your antenna.

Yeah, the shielded loop antenna is a strange beast. But you pretty much have to use it when you are in the near field like this, to prevent E-field coupling. The shielded loop antenna is a B-field sensor (like they are used for sniffer probes in PCB EMI debug, for example).

The shield is broken at one point, somewhere around the circumference of the loop. This keeps it from being a shorted turn and excluding B-field from cutting the loop itself.

vk6kro
It is pretty alien. The original question started "I am transmitting an electromagnetic field from a loop antenna " but then the antenna is shielded so that it doesn't have any capacity coupling between the loops.

Can it be "electromagnetic" without any electric component? I don't know. Don't think so.
I think your first reaction that this is just magnetic coupling was about right.

I do know that a loop like that will parallel resonate at HF with quite a small capacitance across the feed point. If this happened, it would give a very non linear signal strength vs frequency graph.
That shielding could easily provide this capacitance if it wasn't very carefully grounded.

berkeman
Mentor
Good points. I wonder if it could work better with the TX antenna not shielded. But then again, he's still in the near field. I'm not real sure what that IEEE spec is intended for, but I didn't read it very closely either. Certainly all the antenna calibrations I've done have been in the far field; you too probably.

vk6kro