Cris Alexander, CPBE, AMD, DRB
Crawford Broadcasting Company
It’s hard to believe that another Colorado summer is already just about in the books. I trust that each of you completed your outdoor projects while we had the good (and hot!) weather. Now, think snow!
New Array Protection Rules
Last month, the FCC enacted a new set of rules, most of which are contained in Part 1, for protection of broadcast directional antenna arrays from pattern disturbances caused by nearby antenna support structures. This has been a long time in the works and is, in fact, part if MM Docket 93-177 (if that tells you anything about the origins).
My company was part of the “AM Directional Antenna Performance Verification Coalition” that was largely responsible for driving the drafting of the moment-method proof rules that were enacted a few years ago. The Coalition has remained in existence from that time since, and one of the things we were pushing for was a new set of omnibus rules that would protect broadcast directional antennas from all nearby antenna structures, not just wireless, cellular and other broadcast towers. As of August 14, those new rules became a reality, and they were placed in Part 1 to make them applicable to all services authorized by the FCC.
Here are the new rules in a nutshell:
- Construction of any tower taller than 60 electrical degrees at the AM frequency constructed within one wavelength of a non-directional AM station will require notification and study. Predicted or resulting pattern disturbances of more than 2 dB will require detuning.
- Construction of any tower taller than 36 electrical degrees at the AM frequency constructed within the lesser of 10 wavelengths or 3 km of the AM station will require notification and study. Predicted or resulting pattern disturbances in excess of the licensed standard pattern will require detuning.
- Modification of towers within the above-specified distances that would change the height of the tower by more than five electrical degrees at the AM frequency, or for detuned/insulated towers, the addition or replacement of one or more antennas or transmission lines, will require notification and study.
- Antenna supporting structures on buildings within the distance screens need not make notification or be studied unless the antenna support alone exceeds the 60- or 37-degree threshold for non-directional and directional antennas, respectively.
- Moment-method analysis shall be employed to determine the expected pattern disturbance.
- Pre- and post-construction measurements of monitor points can be used in lieu of moment-method analysis for stations licensed using a conventional proof.
One of the provisions that Crawford Broadcasting Company pushed for in these new rules was a way of dealing with existing pattern disturbances from nearby antenna supporting structures that were not required to notify, study and detune under the old rules. This encompassed just about everything except wireless and cellular, including two-way, paging, amateur radio and public safety.
In short, the county sheriff could put up a tower across the road from an AM antenna for its dispatch communications and not have to do a thing with respect to protecting the AM array. This happened more than you might think, and broadcasters were in most cases left holding the bag, either having to pay out of pocket to detune the other tower, readjust/re-proof their array or operate under an STA with parameters at variance for as long as the other tower was in existence. Crawford has one such situation on Mt. Scott in Portland, with a county tower right across the road that really wrecked the back side of our station’s directional pattern.
The FCC heard us and included a provision in the new rules that within a one-year window following the enactment of the rules, an AM licensee can submit a showing that its directional pattern has been disturbed by a tower construction or modification that occurred prior to the enactment of the rules. If the showing proves a pattern disturbance in excess of 2 dB for ND or standard pattern for DA, the FCC will order the tower owner to install detuning apparatus.
There is a whole section in the new rules covering installation of antennas on AM towers, both directional and non-directional. These are mostly common sense. On an ND radiator, if the base resistance changes by more than 2%, the AM licensee must file a 302-AM specifying the new resistance and base current. If installing on a tower in a conventionally-proofed directional array, a partial proof will be required. If installing on a tower in a moment-method proofed array, the base impedance of the tower must be measured and if it departs by more than 2 ohms and 4% from the modeled resistance and reactance values, a new moment-method proof will be required.
So what’s the bottom line? What can broadcasters expect as a result of the new rules? First, a lot of commonplace short monopoles, building tops and other support structures used for cellular/wireless won’t even come into play because they are below the 60/37-degree threshold, and the wavelength-based distance screen will take care of many more. Since more licensees are brought under the jurisdiction of the protection rules, the additional structures may somewhat offset those eliminated by the new height/distance screens.
The structures that are within the screen are likely to really cause pattern issues, and those will have to be studied. My advice is to pay attention to them. When you get the letter from the entity hired by the tower owner/licensee to do the study stating that the construction has been studied and determined to be no factor, respond quickly with a request for a copy of the analysis. Look at the model yourself and see.
Remember that site in Portland I mentioned? Here is a peek at the standard (red), undisturbed (green) and disturbed (blue) patterns, the latter two from moment-method models. Note that the forward nulls on the pattern disturbed by the county tower across the street (blue) are actually rotated forward by quite a bit, a new set of rear nulls has developed and the whole back lobe has filled out, exceeding the standard pattern. This reflects what we have seen in the field, and it’s a good illustration of why measuring just the monitor points is many times not good enough to detect serious pattern disturbances.
If you have news to share with the Rocky Mountain radio engineering community, drop me an email at email@example.com.