RE: DXing Question

[Garrett Wollman <wollman>]

<<On Sun, 11 Nov 2001 01:11:50 -0500 (EST), Sven Franklyn Weil <sven@gordsven.com> said:

> What's the pancake effect?

When we talk about the power output of a VHF or UHF broadcast
facility, we usually use the ``effective radiated power''.  This is
not actually the power being radiated -- hence the ``effective'';
rather, it refers to the power which would have to be emitted by a
perfect point radiator in order to obtain the same field intensity at
a given location.

In the case of broadcast stations, this location is a point on the
unit circle, in the horizontal plane through, and centered on, the
(vertical) radiation center of the antenna.  If the antenna is
``directional'' (more about that later), the specific point chosen is
the one where the field is at its maximum.  If the antenna is
``non-directional'', then all points on the circle are deemed to
receive the same intensity -- even though for a real radiator this is
not the case.

Now it turns out that broadcast engineers mean something different by
``directional'' than physicists do.  To a physicist, a directional
source is anything which isn't a point source.  (Point sources radiate
uniformly in every plane, not just the horizontal one.)  This isn't so
useful for broadcasting, where any energy sent off towards outer space
-- or above the radio horizon generally -- is money wasted, since it
can't possibly serve any listeners.  So most -- but not all --
``non-directional'' antennas are designed to redirect energy which
would have gone directly up (or down!) to go outwards, instead: in
other words, in the horizontal plane rather than the vertical.  (A
physicist would consider this to be a directional radiator.)  Hence,
using an antenna which does this more strongly -- i.e., one with a
higher gain -- will allow a greater effective power with the same
amount of input energy.  By the same token, a higher-gain antenna
would also allow the same effective power to be achieved with less
energy input.  Either way, the station saves money.

This also makes facilities technically feasible which weren't before.
In the early days of commercial FM broadcasting, FM transmitters were
not available with more than a few kW output; they're still not
commonly sold above 20 kW, and those are significantly more expensive
to purchase and operate.  If your license calls for a 100-kW station,
the only way to get there was to build a high-gain antenna to make up
the difference.  (TV stations do the same thing; you don't see 5-MW
UHF transmitters.)

But there's a price to be paid: the more antenna gain you use, the
less of your signal actually reaches the ground in the vicinity of
your transmitter site.  If you envision the ``rays'' of the station's
signal being squeezed into that horizontal plane through the radiation
center of the antenna, so that you're left with a thin ``pancake'' of
RF that doesn't actually hit the ground near any of your intended
audience.  (Of course, if your antenna is low enough to the ground --
which the FCC and EPA discourage -- or far enough away, then this is
not a problem.)  The way broadcasters deal with this problem is
simple: by fiddling with the relative phasing of the antenna elements,
or by physically tilting the antenna a few degrees from true vertical,
it is possible to dip one edge of that ``pancake'' closer towards the
ground.  Obviously, this comes at the price of sending the other edge
above the horizon, but for high-power stations operating close in to
their desired service area, the price is worth the benefit of improved
local reception.  This is what we refer to as ``beam tilt'', and in
typical installations it amounts to between 1/4 and 2 degrees of arc.

Now, in the specific case of CKOI, we don't have much information to
go by, because the Canadians are not as nice about letting people see
their technical databases as the FCC here is.  But one thing we do no
is that CKOI is a super-power operation (allegedly the result of a
clerical error at the CRTC); CKOI is licensed to operate at 310 kW
from 217 m AAT.  (The maximum facility normally licensed is 100 kW.)
By contrast, most of the Mont-Royal stations operate with 36 kW from a
little higher, about 225 m or so AAT.  (I don't have the precise value
at hand.)  You can compare this with the NYC stations operating from
Empire with 6 kW from 415 m AAT.  (None of the Empire stations need to
be beam-tilted for the simple reason that they use very little antenna
gain and so don't suffer from the pancake effect in the first place.)

I hope that makes some amount of sense.

-GAWollman