Loop antennas

[f95toli]

This is work-related so normaly I would not ask a question like this here, but I since I can't seem to find what I am looking for and this is an EE question...

I am using a small loop-antenna (with a diameter of about 1 cm) made from a piece of standard semi-rigid coax, I have shaped the inner conductor as a circular loop and soldered the end to the outer conductor of the coax, I.E, it is more or less a standard magnetic field probe but I am using it as an antenna.

Here are my problems. I want to use this antenna in the frequency range 1-40 GHz, obviously a simple antenna like this is not very broadband so I want to charachterize it so that I can tell at which frequencies it is radiating efficiently using a network analyzer.
The main problem is that I don't know much about antenna theory (I am a physicist, not an engineer) so my question is how to do this?
Of course I should do a S11 measurement but what to look at? Only the variation of the real part (which if I understand correctly correspond to the radiated power)? Or something else?

Second question, I have seen some examples where people have terminated the antenna in a 50 ohm SMD resistor (the inner conductor is then soldered to the resistor and then the resitor is solder to the outer conductor), I have tried this and it does remove the "peaks" in the S11 measurement but the question is if I am really making the antenna more broadband? Isn't this what is known as a "traveling wave" antenna?

Final question: Does anyone know where I can find theoretical expressions for the impedance of a loop-antenna (as a function of frequency)?

BTW, the antenna works (I have made two antennas and measured S12), it is just that I would like to understand what I am doing.

I should point out that I am only intersted in the B-field component of the field (indcuctive coupling) and that I am mainly using it is the near field region.

 

[TuxDave]

Ok... this really isn't my specialty so I can only answer a couple parts.

1) You could just have the source at a constant amplitude and cycle up the frequency. On the receiver side, note the amplitude and phase differences. That should be enough to characterize the antenna over a set distance.
2) The 50 ohm resistor is supposed to impedance match the antenna. Those peaks that you're observing is probably due to reflections of the source back to itself or something. Both the transmitter and receiver needs to have input/output impedances that match the 50ohm resistive nature of the antenna.
3) No idea...

 

[f95toli]

The problem is that I do not have a well-charachterized reciever, that is why I have to base everything on S11 measurements, if I look at the reflected power I see a lot of dips and I understand that they correspond to frequencies where the antenna is radiating efficiently; but I want to relate for example a 10 dB reduction of the S11 spectra to the amount of power which is actually radiated.

My idea was that if I look at the real part that should be related to the radiated power (the imaginary part should correspond to the reactive part of the impedance), but I am not sure wheter or not this is correct.

I understand that the 50 ohm resistor will reduce back-reflections, however I do not really care about that (my microwave source is not that sensitive), I am mainly intersted in how the 50 ohm resistor affects the radiated power, and as I mentioned the resistor DOES affect the antenna pattern in the case of the "traveling wave" antenna.

There are a LOT of books about antennas but most (all?) of them deal with low-frequency (<3 GHz) antennas in the far-field region radiating at a fixed frequency, and loop-antennas are not that common.
I am hopping to find an expression for the impedance and descibtion of the near-field properties of this type on antenna somewhere.

To be honest I don't want to spend more time than I have to on this (I could probably build a numerical model but that would take a couple of weeks for me, I am not used to modelling antennas) and I am hoping that someone familiar with the field can point me in the right direction.

 

[ScottMac]

The American Radio Relay League (ARRL - www.arrl.org) used to have some pretty decent antenna books (theory and practical). I have some in boxes here somewhere, I'll try to find 'em and take a look.

Good Luck

Scott