Re: AM band question

[Dan Strassberg <dan.strassberg@worldnet.att.net>]

At 07:48 PM 5/30/99 -0400, you wrote:
>In relatively simple language, what is the principle by which AM
>stations on the low end (higher meters) can have a larger footprint
>than a high end counterpart with the same (or much greater) power?
>And I also plead ignorance to which height AM towers are the most
>efficient for coverage at a given frequency.  For instance, I'm
>somewhat familiar with the installation for WCAP (980 Lowell) which
>has 4 towers at 256' HAGL (2 sticks day, 3 night).  Is HAGL determined
>by frequency and what other alternatives are there (and I assume the
>pattern/phasing is notwithstanding)?
>
>Is there 'Radio Engineering for Dummies' or something like that which
>speaks to stuff like this in dull-normal ranges of intellectual
>functioning format?
>
I addressed the question of optimal tower height in my last posting, which I
wrote before I read this one. As my previous answer suggested, there is a
difference of opinion among radio engineers about the optimal height of
conventional AM radiators. Several stations with legendary signals use
towers that are just half wave (180 degrees). WABC is one. Few people
suggest that conventional towers should be any taller than 5/8 wave (225
degrees). I explained the reason for that in my previous post. A lot of
former Class I stations have towers between 190 and 210 degrees. The effect
of going from an antenna a bit shorter than half wave, which produces the
minimum efficiency allowed (without a waiver) for a Class I (now Class A) AM
(362.2 mV/m/kW @ 1 km), to an antenna that is 225 degrees (and produces a
field of 432 mV/m/kW @ 1 km), is the equivalent of about a 40% increase in
antenna input power. The penalty the station pays for the increased tower
height (besides higher tower-construction cost and maintenance) is fading
within its normally protected groundwave service area. Many engineers argue
that the small increase in service at midday is not worth the cost when you
think about all of the issues.

For Class B stations (the former classes II and III), of which WCAP is one,
the required efficiency is only 282 mV/m/kW at 1 km. It is actually possible
to achieve that efficiency with towers of conventional design that are
slightly shorter than 60 degrees. That height can be even further reduced by
top loading. Indeed, in searching the FCC database, I note that one of the
towers at KIEV Glendale CA has a physical height of only 45 degrees but, due
to top loading, has an electrical height of 67 degrees. I know of no other
AM tower whose electrical height is 50% greater than its physical height.
Local stations with top-loaded towers include WNTN, WRCA, and WKOX. All of
these use so-called umbrella loads. In such a load, they guy wires run
through insulators near the top and a circumferential wire runs parallel to
the ground just above the insulators. Most broadcast engineers regard top
loading as a last resort, to be used only when environmental considerations
or local zoning preclude the use of taller towers.

You will note that all of these discussions state tower heights in degrees.
One degree is 1/360 of a wavelength. You get the wavelength in meters by
dividing 299.8 by the frequency in MHz. For WCAP, 299.8/0.98 = 305.6 m. To
convert to feet, you multiply by 3.28. For 980 kHz the result is 1003.4'.
One degree at 980 is thus equal to 1003.4/360 = 2.787'. WCAP's 256' towers
are just over 90 degrees, although to be strctly correct, you need to
subtract the height of the base insulators and the height above ground of
any concrete footings beneath them from the 256' HAGL. The important thing
to note is that, when converting frequency to wavelength, frequency is in
the denominator. What that means is that a 300' tower, which is 1/2
wavelength at 1500 kHz, is only about 1/6 wavelength at 540 kHz.

As for coverage vs power, the rule of thumb I've heard is that, if
everything else is equal (of course it never is), you need 20 times the
power at twice the frequency to get the same 0.5 mV/m coverage. I'm sure it
isn't that simple. Over salt water, you don't need much more power to get
the same coverage at the higher frequency. So obviously, the rule of thumb
makes some assumptions about the soil coductivity. I suspect that, as soil
conductivity gets worse, the 20-times-at-twice-the-frequency rule might
become 50 times.

A good place to get more info on this stuff is from the National Radio Club,
an organization devoted to AM DX, from which you can order many papers on
the subject. I also recently bought a nice illustrated book on AM antennas.
The book was self-published by a fellow out in Denver who flew around the
country protographing radio towers. I can't recall the fellow's name at the
moment (Patrick <something>) but I can look it up if you're interested. The
author is not an engineer and I've found several errors in the text, but if
you can overlook those, it's a nice illustrated treatment of AM transmitting
antennas. 

- -------------------------------
Dan Strassberg (Note: Address is CASE SENSITIVE!)
ALL _LOWER_ CASE!!!--> dan.strassberg@worldnet.att.net
(617) 558-4205; Fax (617) 928-4205

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