[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Standard vs. Augmented?

A more pertinent question is what the difference is between the theoretical
pattern and the standard pattern. I had thought that the standard pattern
_was_ the theoretical pattern, but I've been told that it's not, and I don't
know the difference.

In the pattern data in the FCC AM database for most stations, there is a
column called (if I recall) expected values. These seem to be the standard
values at each azimuth adjusted by some constant factor (I think 105% of the
corresponding standard value). I assume that the expected-values column is
intended to account for such effects as the seasonal variation in soil
conductivity. I believe that a 5% correction doesn't come close to
accounting for such variations.

Then there is the column of augmented values, which appears in the data on
many--but not all--AM patterns. Augmentations are established after the
proofs of performance have been completed following construction of the
array. After the engineers have done their best to bring the array into
specification, they give up and apply for one or more augmentations to
provide a description of the field vs azimuth that more closely approximates
the array's measured performance. In gneneral (though, again, not always)
the designers and the FCC bother with augmentations only in directions in
which the augmentations could make a difference--that is, in directions in
which the augmentation could affect protections to other stations. Each
augmentation has an azimuth, a magnitude, and a span. I've seen patterns
with upwards of 20 augmentations--not 20 azimuths with augmented values, but
20 augmentations each having a span of as little as 10 degrees and as many
as maybe 50 degrees. It's not uncommon for several augmentations to affect
the pattern's augmented values at particular azimuths. Each augmentation
follows a formula with a specific rate of drop off vs azimuth in the effect
of the augmentation. The effect is greatest at the augmentation's central
value and the drop off is based on the ratio to the augmentation's span of
the difference between the actual azimuth and the augmentation's central
azimuth. The whole scheme seems to me to be excessively complex--involving
the use of mathematics to describe an effect that probably would best be
described by tables of augmented values constructed from measurements.

There's another fudge factor that I don't understand at all. Many--but not
all--AM patterns have something called a Q-factor. It applies to entire
patterns. I haven't a clue about the origin or the effects of Q-factor.

Current ratios in towers are specified, as you figured out, with respect to
the current in a reference tower. Usually one tower is shown with a ratio of
100%, though I'm not sure that any tower _has_ to have a ratio of 100%. Nor
am I sure that the tower that is used as the current reference is always the
one that is used as the location reference. Most arrays (but again not all)
use one tower as a location reference, but some arrays (WMKI's is one) use
as the location reference a point at which there is no tower. Many complex
arrays use a combination--some tower locations are described with respect to
a reference tower and others are desribed with respect to some other tower
or towers.

Degrees are used in three ways in the array specifications. Azimuth is
specified in degrees of compass heading, where a circle is divided into 360
degrees and an azimuth of zero always corresponds to true north. (The number
of degrees in a circle and zero equalling true north may be the only parts
of array specifications that apply to all arrays ;>) The phase of the
current in any tower applies to the delay (as a portion of the
carrier-frequency cycle) in the current in one tower with respect to the
current in the reference tower. One cycle equals 360 degrees (something else
that's always true ;>) It seems to me that a specification of tower current
that did not include phase as well as magnitude would be meaningless.

Finally, as you correctly surmised, tower heights and spacings among
towers are specified in degrees, where one degree is 1/360 of
the station's wavelength--_almost_ always assuming that the velocity of
propagation is that of light in free space (299.8*10^6 m/sec). It turns out
that for some AM antennas (skirt-fed folded unipoles being the best example)
the velocity of propagation in the antenna is somewhat lower than that of
light in free space. This little-understood effect has apparently resulted
in at least one folded-unipole installation (WOLF, Syracuse) delivering
efficiency so much lower than the predicted value that the 5/8-wave tower,
which had a predicted efficiency of 440 mV/m/kW unattenuated at 1 km,
actually delivered less than the Class C minimum of 241 mV/m. That meant
that 1 kW into the supposedly super-efficient antenna was actually
equivalent to less than 300W! As we say in high tech, "oops!"

Dan Strassberg, dan.strassberg@att.net
617-558-4205, eFax 707-215-6367

----- Original Message -----
From: Bill O'Neill <billo@shoreham.net>
To: Boston Radio Interest <boston-radio-interest@bostonradio.org>
Sent: Saturday, October 06, 2001 2:38 PM
Subject: Standard vs. Augmented?

> Checking out different AMer in the database and some patterns are listed
> "augmented" and some as "standard."  Unashamed to admit how little I know
> these areas, here I go:
> - what's the difference?
> - What's "degrees" - something to do with tower height, right?
> - Some towers list phase and ratio and some just phase?
> - Orientation: to the reference tower, right?
> Bill O'Neill