Do you mix antennas from different companies/ manufacturers/ brands?

I have a Fatshark antenna on my Attitude FPV googles and an unknown antenna on my quad.

My video transmitter is only 250mw and I only get about less than 100 meters of range , is it a sign of lacking power in VTX power?
Or could that be the mixed antennas?

I just ordered a 600mw transmitter and a matched antenna pair to fix that, but still would like to know what you guys think. 

I don't think it's your VTX power, 250mw is plenty. A lot of guys here use 25mw and still getting more than 300 meters of range with proper setup.

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If one antenna is LHCP (left hand circularly polarized) and the other antenna is RHCP (right hand circular polarization), this can cause the degraded reception. You might want to remove the cover shield and check it.

Normally with ImmersionRC antennas, White plastic case is LHCP, black is RHCP.

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another thing to check is your antenna connector gender, and SMA/RPSMA. If you have 1 SMA Female with a RP-SMA Male, you can plug them together without knowing any difference, yet you get bad signal as if there is no antenna at all!