One of the key technologies that enabled capacity increases in microwave backhaul links was the introduction of dual polarized antennas. Dual polarized antennas work thanks to a device called an ortho mode transducer (OMT), which separates vertically and horizontally polarized signals for simultaneous transmission and reception. A dual polarized antenna should include the OMT—it should not be seen as a separate component. That is not always the prevailing view in the industry, however.
Customers regularly call me, asking if there is an Andrew-brand integrated OMT they can buy. I have to respond by saying, “No, we don’t sell OMTs. We sell integrated dual polarized antennas, which include an OMT.” The answer I get is usually along the lines of, “Well, why can’t I just add an OMT to my existing antenna?” The customer basically is asking me why it is so important that the OMT be included as part of the dual polarized antenna, and not sold as a separate component. Here’s my answer—and it basically comes down to four parameters:
Return Loss (RL) – The dual polarized antenna system return loss as presented at the radio interface must be of sufficient level so as not to impede the attainment of the radio system transmission quality objectives, for example, Bit Error Rate (BER). The best practice is to match the return loss of the antenna with that of the OMT to ensure the requisite combined (integrated) performance is achieved.
Insertion Loss (IL) – Influenced primarily by the integrity of the antenna to OMT junction, IL must be of sufficient level so as not to significantly impact system gain. The best practice is to match the antenna to the OMT at the component design stage to guarantee a reliable interface.
Inter-port Isolation (IPI) – Two types of IPI are relevant to an OMT. The open circuit isolation will provide a first level indication of the functionality of the OMT. More importantly, however, is the short circuit IPI, which provides a much firmer indication of the microwave quality of the OMT. Once the OMT is integrated with the antenna, it will likely influence the radiated cross-polar discrimination (XPD), impacting the all-important measurement of radiated emission level close to the antenna line of sight direction. The best practice for achieving minimum contribution to XPD emission is to again match the OMT to the antenna design ensuring that target objectives are achieved at the integrated level.
Cross-Polar Discrimination (XPD) – Within the transmission path, there must be a sufficient level of discrimination between the vertical and horizontal polarizations to ensure that adjacent or co-channels maintain their individual integrity. Within the dual polarized antenna system, stable target levels can be achieved from well designed antennas and OMTs. The performance of the combination (integration) design must be confirmed, however, by measurements on a specialized antenna test range. It’s impossible to state the XPD of an OMT, since there is no such thing. There is only the XPD of a dual polarized antenna.
Based on these four key performance metrics, you can see why it is important to integrate the OMT into the antenna system from the start. Lack of control for these four factors is a serious issue as international antenna standards—such as those published by ETSI, Anatel, and ACMA—contain requirements based on these features for point to point antennas. In a future blog, I’ll review the problems in more detail and look at solutions to ensure optimum antenna, and network, performance.
So have you bought an OMT separate from a completely integrated dual polarized microwave antenna? I’m curious to know how many of our readers have tried this approach.
What made you do so? If you are comfortable discussing the topic, leave a comment below about your experiences.