CORRECTION * CORRECTION * CORRECTION
The Standards of Fairness in Forums requires that I set the record straight by recognizing that Rich did in fact post physical measurements demonstrating one of his points.
Rich did indeed post some 'relative' measurements, but that's not what is in question. The measurements were for a powerful, licensed station signal. The hypothesis here is that puny Part 15 signals may be affected more by obstructions than those he measured. This is derived from real world observations and measurements by real Part 15 practitioners. Rich's contention is that that hypothesis is invalid, and he points to the NEC software as proof, which is known not to be perfect, and to have issues in certain situations. He cannot point to the real world, as he does not do Part 15 broadcasting.
I do not intend to enter into a pissing contest as to who is more credible - those who are out there measuring real effects, or those who are running software simulations in the comforts of their homes. I believe that the NEC software is accurate in the general case, but I'll go back to my original statement. Part 15 signal strength levels may be a boundary condition or conditions for the mathematics behind the NEC simulations, and accurate (not relative) measurements need to be taken in the real world for the effect of obstructions on those signals.
Until such time as that happens, I agree that the discussion should be shelved. Since Rich doesn't do Part 15 broadcasting, I don't see why this is such an issue for him. Those of us that do, need to be aware of the potential effect of obstructions on Part 15 AM signals.
I knew I read a study having some relevance to this discussion, I just had to find it again, while this refers to TIS/HAR stations, it's still low power AM.. Here's a few choice excerpts from this 92 page document, it illustrates that lower power AM does not behave the same as high power stations, and that obstructions are detrimental to low power AM (I've quoted other things from this same study before). I tried to keep it short to avoid boring you off the page!. It's not that long:
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Improving Highway Advisory Radio Predictability and Performance
Requested by Ian Turnbull, Office of ITS Engineering and Support, Caltrans District 2
Revised August 5, 2011 Final
Highway Advisory Radio (HAR) stations, sometimes referred to as Travelers’ Information Stations (TIS)... Caltrans has deployed HAR stations across the state, but performance is often unpredictable.... This Preliminary Investigation is limited in scope to AM band HAR stations and is focused on design factors related to radio wave propagation; topography; environmental characteristics (geology, ground conductivity); relevant system components and configurations (antennae, grounding); and related factors affecting HAR performance.
Three different HAR Vendors shared relevant design considerations with us. Some common themes arose among these conversations. It was noted that considerations for HAR stations are often not the same as those for high-power commercial AM radio stations..
Interview subjects universally stressed the importance of system grounding.....
Interview subjects similarly addressed siting. Wide open spaces are ideal for AM radio wave transmission...
In response to a request for information about practice at the regional level, Charles Price in District 4 put us in touch with Michael Lee, chief of electrical systems hardware support for that district. Lee discussed broadcasting challenges related to geography, such as hills throughout the region as well as low HAR signal strength compared with commercial stations operating at nearby frequencies...
AASHTO Special Committee on Wireless Technology
We spoke to Bill Brown, radio manager for Virginia DOT and chair of AASHTO’s Special Committee on Wireless Technology... Brown suggested that we speak with Information Station Specialists, an HAR provider based in Zeeland, MI...
AASHTO’s Bill Brownlow, the association’s representative on the Special Committee on Wireless Technology, suggested that we contact Peter Moncure of HAR consultant RadioSoft, based in Toccoa, GA: "Peter Moncure is the vice president of RadioSoft, the firm AASHTO contracts with to perform our radio frequency coordination. Peter has more than 30 years experience in RF engineering and is recognized as an expert in radio propagation by the Federal Communications Commission. Peter also served as a member of the Telecommunications Industry Association (TIA) working group defining standards for the modeling of radio wave propagation and as AASHTO’s technical representative to the Land Mobile Communications Council’s Spectrum Committee.”
Information Station Specialists
We spoke with Information Station Specialists, a company based in Zeeland, MI, with 30-plus years of experience designing HAR stations and installing HAR equipment, including the design of some of Caltrans’ systems. Bill Baker provided information on the company’s approach to maximizing HAR performance through installation planning.
Baker said that for any HAR system, the first step is to define the geographic parameters... For best coverage, the immediate location should be free of objects that exceed 25 feet (about 2 stories.) This includes tall buildings, trees, terrain features, lighting, power and communication poles and towers, overpasses and highway signs. For example, Baker said that a critical factor for an AM radio siting is openness of location. Coverage in an open valley can be as good as a high location (unlike FM, where height of antenna installation is the primary concern). It is important to avoid crowding by structural and environmental features. It is possible, but not preferable, to install an HAR station near a structure, and performance will suffer.
RadioSoft
RadioSoft’s Peter Moncure had the opportunity to review the AASHTO survey questions and responded via email: In a nutshell, low-power AM without a suitable counterpoise is so highly determined by very local ground conductivity that good prediction is impossible. There is considerable variation in specific antenna installations as well. We followed up with a telephone interview with Moncure. He described how generally the design guidelines and calculations associated with large commercial AM stations don’t apply to small-scale AM HAR stations. Moncure discussed with us some of the fundamental differences in approaching the design and deployment of HAR stations: The variation in the grounding systems and the small radius of operation for HAR stations result in propagation characteristics that can vary significantly, as much as ± 15 decibels, which makes performance hard to predict.
Rich Powers thanks that sums it all up get your antenna away from obstructions and you'll perform much better. For some this means Get It High As Possible.
Also if you have or know someone who has the Original Manual and ATU for the Talking House/iAM transmitters there is very interesting reading in the user manual which also states the higher you put the ATU/Antenna the further your signal will travel. It also talks about ground and proper location of the transmitter and time should be taken.
It is even suggested to do a temporary Install to find the best place for your transmitter as testing and comparisons need to be made before the permanent Install.
The Topping
Rich Powers totally relevant text from HAR/TIS studies and reports puts the perfect topper on this discussion as it wraps up: "The variation in the grounding systems and the small radius of operation for HAR stations result in propagation characteristics that can vary significantly, as much as ± 15 decibels, which makes performance hard to predict."
It is also NOT trivial that our members all shared feelings, beliefs and intuitions that are the same as the HAR/TIS report.
Speaking as your resident "peer reviewer", this conversation has been enlightening.
If only one point was to be made from the above quoted text in regard to this threads discussion it's Peter Moncure of RadioSoft reputation and what he had said..
"...Peter has more than 30 years experience in RF engineering and is recognized as an expert in radio propagation by the Federal Communications Commission.... He described how generally the design guidelines and calculations associated with large commercial AM stations don’t apply to small-scale AM HAR stations..."
Talking House/iAM transmitters there is very interesting reading in the user manual which also states the higher you put the ATU/Antenna the further your signal will travel.
That is basically what any part 15 am transmitter manual will tell you... However, it is undeniable that in most cases the reason is simply because the antenna system is being extended in length..
Nevertheless, I still believe that height makes a difference with part 15 AM. Only real world test performed and compared while at the same time eliminating any possibilities of the length of the lead playing a part in it can confirm if height makes a difference or not - but theory, charts and graphs aren't going to cut it alone.
Some only believe what they read and theorize, while others also factor in what they experience first hand. If the two don't mesh, then they have to decide on believing the expected results of theory or their own real world experience... If they have no real world experience in the matter, nor wish to experiment to gain that experience, then they have no option but to believe in theory only.