Bias tee circuit and calibration board for base station antenna

A bias tee circuit, comprising:

• a direct current (DC) port;
• a radio frequency (RF) port;
• a combined port;
• an impedance transformer that is configured to pass DC signals received at the combined port to the DC port while substantially blocking RF signals received at the combined port from passing to the DC port and that is configured to pass RF signals received at the combined port to the RF port while substantially blocking DC signals received at the combined port from passing to the RF port, wherein the impedance transformer includes a closed loop that connects the combined port to both the DC port and to the RF port;
• a combining path that connects the combined port to the closed loop;
• an RF path that connects the RF port to the closed loop;
• and
• a DC path that connects the DC port to the closed loop,
• wherein the closed loop includes a first conductor that is connected between a first intersection of the combining path and the closed loop and a second intersection of the DC path and the closed loop, a second conductor that is connected between the first intersection of the combining path and the closed loop and a third intersection of the RF path and the closed loop, and a third conductor that is connected between the second intersection of the DC path and the closed loop and the third intersection of the RF path and the closed loop.