Maximum Expected Operating Voltage (MEOV) is a relic of the earlier days of directional antenna design and proofing. It died about 30 years ago. I'll cover some history since the concept formed a very important part of current antenna design work and led to the development of the standard pattern. The term also became very important in the professional life of the consulting engineer whose directional antenna system failed to operate as planned. It can also be important to today's engineer who might encounter the term in an old proof and be puzzled by its meaning and application.
Fig. 1. Typical DA polar plot of a theoretical pattern with MEOV protection
The value of MEOV proved to be immeasurable to consulting engineers who had the foresight to use it. Some directional antenna patterns proposed to the FCC had specified zero radiation in a pattern null. The FCC's engineers very wisely, in my opinion, eventually said it is almost impossible — and maybe impossible — to reduce radiation nulls completely to zero in view of the environment in which the antenna system has to operate. This led to development of the standard pattern, which basically imposes a built-in minimum radiation of about 6mV.
After the first excitement over the Commission's approval of the first directional AM pattern, other engineers began to offer mathematically calculated directional AM patterns. Some of these relied solely on mathematics in presenting theoretical antenna designs. These theoretical antennas required actual construction in order to be tested and prove that the radiation field would do exactly what the design engineers said it would. Sometimes, much to the embarrassment of the designer, it was found impossible to achieve the promised radiation pattern. Occasionally large changes had to be made in the already-constructed antenna systems in order to satisfy the Commission's requirements for licensing.
The Commission's procedure for licensing requires the antenna pattern and radiation obtained in the proof of performance measurements to always be within the limits of the proposed antenna pattern. The usual problem delaying license approval was probably the presence of unexpected measured radiation that was greater than the proposed and approved, radiation value in the pattern nulls. The solution quickly became apparent. It seems that troublesome, larger-than-acceptable, measured null values could be acceptable if they were covered in the application by an acceptable caveat.
The approved pattern was generally the theoretical pattern, if this pattern could be easily obtained. In the higher-level areas, a small deviation in the null region could probably be written into the pattern so it passed the pattern proof. In many cases it seemed impossible to get closer than plus 3 percent to 5 percent of the theoretical value shown in the application. Sometimes filing an amended Form 301 could satisfy the condition and allow licensing to proceed. Unfortunately this procedure would involve another application showing the obtainable allowable radiation, presumably more legal fees, additional delay and professional embarrassment for the consulting engineer.
The new application would show a new value in the offending null that amounted to a few percent. Some very bright person conceived the idea of filing an original application, showing the desired theoretical pattern with a broken line outside this pattern, at the potentially offending null showing the maximum expected operating value in that area.