[BC] FCC Approves proposed AM MoM Rules

[dynotherm at earthlink.net]

This is turning into a very enlightening exchange so I'm
going to violate Barry's trimming rule to keep everything
together in one place, with apologies to those on the digest 
because this is one of the most important topics for AM
since the release of the IBOC rules.

Phil Alexander

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>From: Craig Healy <bubba at dukes-of-hazzard.com>
>
CH > It must be rather difficult to create the cell tower model.
>> > The antenna and line top load effect could be very complicated.
>>
PA> It is more tedious than complicated. VERY tedious if the cell
>> is on a self-supporter like a Rohn because these must be modeled
>> as the full geometric lattice while for uniform cross-sectional
>> guyed towers and poles a simple cylinder or tapered cylinder
>> is a suitable substitute.
>
CH>And if every line and antenna is connected correctly...  It may be tedious,
>but I'd also have to question how the accuracy could be verified.  However,
>it's not a deal breaker, I suppose.

PA- It is not that bad if you discount the drudgery of making an accurate
model, which is a first class PITA in the case you suggest. For verification
I would suggest an optical transit.

CH > If the tower has a detuning
>> > network in place that also can affect things significantly.
>>
PA> Not really. The first approximation is with no detuning to see
>> the maximum extent of the problem, and then you can suppress it
>> by reducing the current flow to near zero - actually by an analog
>> of the near field reduction of the detune.
>
CH>When stations are close in proximity, then the detuning network might not be
>for that same station.  I can think of a couple of places in the
>Boston-Providence area where a given cell structure can affect more than one
>station.  Brockton, MA 1410/1460 and Providence, RI 630/920 are a couple of
>examples that come to mind.

PA- Again, not a problem because you can insert the structure in the model and
then adjust the extent to which it participates in the interaction. Then
it become a matter of getting the same degree of detuning as you have
assumed in the model. I like to look at detuning in terms of near field
suppression, i.e., near field detuned x dB below structure's re-rad without 
detuning.

CH > Again, I probably am not clear on this but I don't see how any
>> > existing software could begin to accurately model real life.
>>
PA> How do you suppose they design stealth aircraft and stealth ships
>> which we do have although they may not be quite as stealthy as a
>> stealth bomber.
>
CH >I would think through modeling as well as computer design.  I know someone
>who is in the military who may well be able to provide a rough answer to
>that question.  I seriously doubt it's just theory and not physical testing,
>and lots of it.

PA- Modeling and CAD go hand in hand, and NEC-4 modeling software is the key.
NEC-4 is not theory. It is scientific simulation of proven fact, essentially
based on the practical application of Maxwell's equations using the principles
of the method of moments used in civil engineering for decades. Very heavy
use of NEC modeling has been done by the US Navy at San Diego.

CH >Given the huge numbers of things that can affect a pattern, plus wildly
>variable ground conductivity, I don't think I have a lot of faith in a
>software end-all solution.  If I were to see a number of designs done both
>in the traditional and computer-generated way, then it would clarify things.

PA- The new rule will include FIM verification points.

What the new rule will allow is proof by modeling which will give exact
operating parameters for the array. Thus, the antenna phase monitor
becomes the key and the accuracy of the monitor must be certified at
two year intervals. I think anyone who has used modeling to build or
rebuild an array will tell the same story, and that is that once the
model developed operating parameters are locked in the array will be
compliant without "tweaking" provided there are no near field problems
(array contributing reradiators). Note that there are two methods of
dealing with re-rad. First is the usual detuning, but it is also
possible to include the reradiators in the model and "tweak" the
operating parameters IN THE MODEL to achieve design compliance with
the CP constraints and adjust the array to those. In some cases the
solution will be a combination of both tactics.
>
CH > I remember a fellow
>> > around here who designed a lot of arrays, and many could never
>> > really meet the paper design results.
>>
PA >Other than environment issues (a/k/a re-rad structures and power
>> lines) plus the inability of some to understand the idea of field
>> measurement point quality, i.e. ratio of direct vs. re-rad wave,
>> there is no real reason for this because most of these issues can
>> be resolved except for dense power lines which is a major reason
>> for the new rule.
>
CH> Don't get me wrong, if this actually works - and can be PROVEN - then I'm
>all for it.  I just would want some hard evidence that it's as good as is
>hoped.

PA- All I can say is that the coalition (AM DA Performance Verification
Coalition) included a group of AM consultants that reads like a Who's
Who of AM DA's: Carl T. Jones; Cavell, Mertz; Communications Technologies;
duTriel, Lundin & Rackley; Hammett & Edison; Hatfield & Dawson; Sellmeyer
Engineering and several others. There were 53 comments etc. in the current
part of the record. About this, the Commission said, "We are persuaded by 
the arguments of the majority of commenters who conclude that moment 
method techniques offer an efficient and reliable means of verifying the 
performance of AM directional antennas."

You can find the Docket record here: (watch the wrap)
<http://gullfoss2.fcc.gov/cgi-bin/websql/prod/ecfs/comsrch_v2.hts?ws_mode=retrieve_list&id_proceeding=93-177&start=1&end=160&first_time=N>

and the Second Report and Order here:
<http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-08-228A1.pdf>

The Notice includes a Second Further Notice of Proposed Rule Making.
The purpose of this RM deals with the matter of tower standards for
cell and similar towers that may affect AM DA's.

Phil Alexander, CSRE, AMD