power combiner s21

Hi,
Does anybody know if it is possible to implement 2 inputs RF power combiner with less then 3db insertion loss on each forward path?
For example I have two identical inphase CW signals (2dbm each) at the inputs of the combiner (one per input). I want to see the same signal with output level of 5dbm minus some insertion loss which is usually in the order of 0.2-0.4 db.
Frequencies of interest are 1-2GHz.

Thanks!
Olxx

IF the two signals are indeed in-phase, then they will combine just as you wish, with only a few tenths of a dB loss.

IF the two signals are out of phase by a fixed phase shift, then you can either get the signals to combine at the output, or just heat up the isolation resistor, or some combination of those two, depending on the actual phase difference.

IF the two signals are non-coherent, or not even the same frequency, then they will combine, but with the 3.3 or so dB of loss.

I think you are confused about the 3 dB loss. A typical Wilkinson combiner/divder will do what you wish. If port 1 is your input, and 2 and 3 your outputs; then S21= -3 dB, however S12=-xdB only beacause you did not drive port 3 when measuring. The power is dissipated in the internal resistor. You can't measure 3 port devices such as baluns and combiners with 2 port measurements unless you do some mathematical tricks to account for the third port not being driven.

Thanks to all of you for the clarification.
I'll be more specific in the functionality I want to achieve.

There are two signals coming on the same path: wanted and interferer. Characteristics of the interferer are well known. I'd like to cancel interferer by combining it with it its own replica shifted by 180 degrees. At the same time I don't want to introduce 3 db of attenuation to the wanted signal.

As far as I understand using combiner I will cancell interferer but wanted signal will be attenuated by >3 db anyways.

Is there any way around?

Thanks,
Olxx

If the interferer is less than the wanted signal by a decent amount, say 10 dB, you can use a directional coupler. Feed the cancellation signal into the -10 dB coupled port. Your loss in the wanted signal will then only be 0.1 dB. This is how it's done in feed-foward amplifiers.