Highly selective band pass filter vs circulator? (IMD)

[Plasmateur]

I have a particular project in which I will be exciting two loop antennas within very close proximity to one another. The frequencies I expect to excite these antennas at are around 60MHz, and there will be a difference of about 1MHz in frequency with respect to one another.

I expect there to be some kind of induction causing intermodulation distortion within these antennas. In order to minimize this I wanted to use a highly selective band pass filter on each antenna. However I don't believe such a filter commercially exists for such a narrow range between the two antennas.

What if I were to use a circulator before each loop antenna? Would the induced signal from one antenna simply pass through the other port of the circulator, acting as a highly selective band pass filter?

[David Hess]

Most of the intermodulation will be produced in the amplifier so filtering will help.  Circulators will help but do not have much directivity.  6 meter cavity filters would work if they can be tuned to the operating frequencies but they are pretty big.  Helical resonators would work, are much smaller, and can be home made.

[jbb]

 Can you adjust the locations of the 2 coils to reduce their coupling?  That can get you 20 dB or more.

Either twisting one 90 degrees or offset half a radius laterally. Depends on your requirements.

[dmills]

Very high Q 1/4 wave stub lines can be useful for this, think large bore heliax or copper water pipe with an air spaced cap at the top to tune the notch and the bottom shorted with a copper disk.

You will really want access to a VNA to tune this sort of thing, but it used to be fairly standard on colocated band II PMR sites.

73 Dan.