Is it 'two antennas', or is it 'a transformer' ?

If the impedance at the feed point of the antenna is 50 Ohms, then you should be able to connected it directly to the 50 Ohm output of the signal generator. Placing a fixed attenuator of 3 to 10 dB inbetween the antenna and signal generator could also be done if you are a little unsure of how close the antenna is really to 50 Ohms, and this will reduce any effects arising from a bad match issue.

Normally after tuning one antenna, placing a second antenna near the first antenna lowers it's resonance frequency. That's for normal antennas. An array of half wave length patches have a size that's 10-20% smaller than a single patch.
For electrically small antennas, usually they don't affect each other much due to their inefficiencies.
That doesn't answer your question for these tiny magnetic antennas, I wish I had the answer.
One thing to do is measure S21 from one antenna to another, although the cable you attach to these small antennas will likely change their properties (a common low frequency headache). If the coupling is really low (-30 dB), then they really won't affect each other.
kch

Thanks to all,

To VE1BLL: Well, It´s two antennas, no transformer.

The antennas I work with are 2 - 20 cm in diameter, which is only a fraction of the wavelength at 13.56 MHz (3.53m)

If the signal generator has a different output than 50 ohm, do I still have to construct a matching circuit?

And do the presence of another antenna always change the resonance frequency?
The reason I want to test this is with a very basic megnetic field simulation program I saw that placing two loop antennas close to each other with reversed polarity will yield a stronger magnetic field. However, with the simulator I couldn´t change the frequency (maybe it showed only a static situation) Does this sound reasonalbe, or would it be a waste of time even to test?

best regards
doneirik

If the two coil 'antennas' are sufficiently close together that the 2nd significantly affect the first, then - congratulations - you've just built an RF transformer.

Try monitoring the current in the first while shorting out the second. If the short propagates back to the source, then you can't pretend that it's not a transformer.

VE1BLL is correct.  There is no such thing as two antenna's close together.  You can not cheat physics.  You will have coupling, intended or otherwise.  Why do you have 2 apertures close?  What effect are you intending to acheive?

Hi again,

About the effect I was hoping for:

If I place two loop antennas close together (i.e. 2 50x50mm antennas about 5 - 10 cm apart) with reversed polarity, I was hoping that the effect of the coupling would be that the magnetic field would be "stable" at a longer distance from the antennas before it starts to fall off with 60dB per decade. In practice, with my RFID application this would yield a longer operating range.

doneirik

If I understand what you're saying, and you plan to feed both antennas out of phase, then the resultant parallel feed impedance will be very low. You'll have created a distributed, magnetically coupled, 'short circuit'.