Unexpected antenna impedance difference

[kellogs]

Hello,

Homemade BLUE bowtie antenna vs factory made BLACK whole board with the same bowtie antenna.
Over 300 - 330 MHz range I am seeing some ~20 change in reactance with the home made antenna and ~50 ohm with the factory made. Also the resistance is 7-8 times greater for the factory made antenna and varies much more over the frequency range.

Sames:
- placement of the antenna
- calibration place, cables, balun and others related (*)

Diffs:
- calibration range: 300-330 for home made vs. 235-395 for factory made
- (*) connection to home made antenna through SMA connectors vs. direct soldering for the factory made antenna;
- calibrated with standards for home made vs. with wire / nothing / two 100 ohm resistors measured at 50.5 ohms with DMM

Is it normal to see such large measurement differences ?

Thank you

 

[merlin]

Looks and sounds perfectly normal given the two are not alike. I presume the first image is factory, the lengths of coax then into a combiner matching to 50 ohm. (A phased array perhaps)
Your home made counterpart is configured as a dipole, which would have near 300 ohm impedance.
Likely a 6:1 balun would correct that.

 

[kellogs]

Interesting, what makes them not alike to such extent ?
That small green board is a 1:1 balun and it has been used with both antennas. Also these are both electrically short dipoles with expected radiation resistance in the 1 ohm range and high reactance. Getting the expected measurements for the blue home made, and totally not for the black factory made.