NAB Engineering Handbook: Antenna System Moment Method Modeling

11/9/2017 8:33:00 AM

Cris Alexander is a well-known name in our field and composed Chapter 7.13 “Antenna System Moment Method Modeling.”


Radio Magazine: The 11th edition is a compendium of knowledge about radio and TV broadcasting gained over the last 100 years. It's overwhelming really. Who should be reading your chapter on the method of moments?

Cris Alexander: Engineers who are responsible for the care and feeding of AM directional antennas would benefit most from the chapter on moment method modeling of AM antennas, particularly those who have for years been running full and partial proofs and chasing drifting monitor point values. Any engineer who has been frustrated at constantly having to deal with reradiators such as wireless towers and power line supports would benefit from this as well.

Radio: Clearly the MoM topic isn't one for beginners in the field. What background does one need to really appreciate and understand what you've included in this chapter (7.13)?

Alexander: Moment-method modeling of AM directional arrays is not for novices, but thankfully it is not difficult to learn. Once users understand the basic principles of the method of moments and a few “rules” of the process, it does not take that long to become proficient in the art. There are several tutorials that I strongly recommend. First is Basic NEC with Broadcast Applications by J.L. Smith, P.E. (Focal Press, 2008). The other is the ARRL Antenna Modeling Course, available in paperback on Amazon. Finally, there is an SBE on-demand webinar, AM Antenna Computer Modeling, available at

Radio: Cris, what would you say is the most important takeaway from your chapter?

Alexander: The most important takeaway from my chapter is that licensees of most AM directional antennas do not have to live with the difficulties of proving the performance of their arrays with error-prone magnetic field measurements anymore, and neither do they have to worry about monitor points and their seasonal variations, and in most cases they do not have to worry about encroachment by reradiating structures. Licensing their AM directional arrays using the 2008 moment-method rules eliminates all those variables and insures that their patterns are correct. It also eliminates a lot of costs.

Radio: Would you care to speculate on how the chapter might be updated, ten years hence?

Alexander: In the almost ten years that we have had the moment-method AM proof rules in place, we have learned a great deal about modeling different kinds of arrays, what works and what doesn’t. We have learned that some of the requirements were over-cautious early on, and the FCC has responded to this at the request of the engineering community with policy changes and rule revisions. Going forward, as our knowledge and experience base continues to expand, we will undoubtedly find additional ways to further streamline the process and perhaps broaden its applicability. One thing that comes to mind is the use of moment-method modeling on directional arrays with skirted towers. At this point, we haven’t come up with a good way to do that, but as time goes on, perhaps we will.



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