An example of an integrated dual-polarized antenna

 

In my previous blog post, I explained why CommScope does not sell ortho mode transducers (OMTs) separately from its Andrew microwave antennas. In this blog, I’ll examine in more detail the problems doing so would cause and also look into some possible solutions. So let’s go back to a typical customer response when I state that we do not sell separate OMTs…

“C’mon, does it really matter? I mean, will it really affect my network?”

“Yes it does,” is my answer, and here is why:

Problem 1:  Flange Tolerances

The waveguide flange connection at the antenna to OMT interface can couple a microwave signal from one polarization to the other unless carefully engineered and carefully paired for matched tolerance (with, for example, respect to concentricity). This can manifest as radiated cross polar interference (XPD) particularly in the all important emission region close to the antenna line of sight direction, or as degraded inter-port isolation (IPI). Such cross-coupling would not necessarily show on a simple open circuit IPI test. A short circuit IPI test at the antenna integration level, however, will indicate a potential problem.

Solution: Make test range measurements using a specialized facility to verify the adequacy of the integrated design. This is the preferred method of design qualification.

Problem 2:  Misunderstanding XPD

In today’s market there are a number of companies making and selling only OMTs. Unfortunately many of them don't really know what XPD is (in terms of depolarized radiated energy on a link). What they do understand is inter port isolation (IPI). They measure IPI (at say 30 dB) and declare the XPD as being 30 dB. This is not the same thing. IPI is not a measure of XPD, which can only be performed on a proper antenna test range. It is a bench test used in production to show major issues in the unit (not as a guide to XPD performance). X-pol energy can be generated in the OMT and go undetected unless…

Solution:  Far field cross polarization patterns are measured on a specialized test range. Unfortunately if the OMT manufacturer does not make the antenna itself, it is very unlikely that a 3rd party design team will have a far field test range to utilize.

Problem 3:  A Good OMT and a Good Antenna Don’t Necessarily Mean Both Will Work Together

Interaction between an apparently good OMT and an apparently good antenna can be less than desired. Like how VSWR matching relies on the phase and magnitude of the reflecting (embedded) elements, XPD can manifest in a similar way. Any good microwave engineer will tell you the theory about how the overall match of a circuit is dependent on the interaction of all elements. The same is true for XPD.

Solution:Design and test the OMT and antenna together – not independently.

Problem 4:  Will the Operator Know About XPD?

In short, no they won’t – well, not at the start, and specifically not with poor XPD, which basically means potential interference. This potential interference will require cross polarization interference cancellation capability (XPIC) in the radio to correct for unnecessary antenna-OMT induced cross polarization. XPIC should be used, however, for link induced cross-polar impediments, thus antenna-OMT induced cross polarization lowers the availability of high quality transmission capacity time. And that means that the operator who cannot test the antenna once it is installed will likely doubt the performance capability of the radio, and perhaps even move to another vendor. Is that a risk worth taking in order to procure separate OMTs?

Solution: Buy the OMT and antenna together – not independently.

Problem 5:  Integrity of the OMT to Antenna Mechanical Interface

The return loss, insertion loss, IPI and XPD will all be influenced by the integrity of the OMT to antenna mechanical interface. Moisture ingress due to a poorly designed junction can lead to significant degradation in these parameters. Not usually evident at the time of commissioning, performance degradation can eventually lead to system failure. A correctly designed and assessed junction, supported by due diligence verification, will mitigate against this potential issue. Remember even if the integrated OMT has the “same” antenna interface as the radio would have, there is no way a 3rd party manufacturer can know the mechanical loads with which the antenna interface has been designed to cope. In other words, the OMT and antenna must be designed together.

Solution: Buy the OMT and antenna together – not independently.

Bottom Line: In order to achieve and maintain superior transmission quality objectives from a dual polarized radio system, the antenna system (comprised of a microwave antenna integrated with a dedicated OMT) must meet minimum performance metrics of return loss, insertion loss, inter-port isolation, and cross-polar discrimination. In order to minimize the risks associated with the use of multi-sourced components, CommScope recommends that the integrated antenna-OMT system is qualified as a single entity, so that these key performance indicators can be confirmed. The best practice is to “one stop shop” from a reputable antenna supplier.

So what do you think?

Does my argument convince you about the importance of buying integrated OMT-antennas as one unit? Let me know what you think in the comments section below.

About the Author

Derren Oliver

Derren Oliver is director of business development for the Microwave Systems unit at CommScope. He has nearly 20 years of experience in the telecoms industry, beginning in test and measurement for both wireless and wireline networking products at Hewlett Packard, Agilent Technologies and Ixia before joining CommScope in 2010. He holds a bachelor degree with first class honors in electrical and electronic engineering from the United Kingdom’s Heriot-Watt University and an MBA from Edinburgh Business School.

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Comments

1 comment for "Does Buying a Separate Microwave Antenna OMT Really Matter?"
Shelly V. Marks

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.

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