THE MARGINAL PATH: FM RADIO AND THE REAL WORLD
by Dan Roach
FM
radio was never meant to be a mobile service.
From the start, it was intended to be a high-fidelity (monaural) medium
for listening in the home. That's one
reason why FM coverage is predicted, and measured officially, based on using a
horizontally-polarized antenna ten metres above the ground: that's about the
height of your average home rooftop mast-mounted antenna (or at least it was,
back when people used such things). And
that of course is why all FM transmissions were originally horizontally
polarized. That was the best way to get
to those horizontal rooftop antennas that everyone had.
Of
course, car radio development continued, to the point where a compact car radio
using a vertical whip antenna eventually was able to pick up those FM stations,
too. FM transmissions upgraded to
stereo. And somewhere along the way,
some smart apples starting designing circularly-polarized (CP) transmitting
antennas. These are able to transmit
both vertically- and horizontally-polarized waves, and not just willy-nilly
mind you, but orthogonally to one another.
As the vertical wave reaches a maximum, the horizontal is minimum, and vice versa.
To picture the plane wave sum travelling through space is to visualize
it spinning in a circle as it goes: one rotation per wavelength, hence the name
CP.
Well,
circular polarized waves have some interesting properties. Aside from being equally well received by
either a vertical or a horizontal antenna, when a CP wave is reflected by an
object, its direction of rotation is reversed.
This makes it possible to discriminate between incident and reflected
signals, a useful property that has never been fully utilized for reducing
multipath for FM reception (you'd need to use a CP receive antenna, and these
have never been made for consumer use).
Even
so, CP allows full power transmission to both vertical (car antenna) and
horizontal (home rooftop antenna) receivers.
Even more, CP seems to offer something for nothing, especially the way
that effective radiated power is calculated by Industry
Well,
maybe not, and certainly not always. One
of the biggest challenges for quality FM reception is the presence of
multipath: phase cancellation of an FM wave when out-of-phase signals partially
cancel out at a receive point. Generally
speaking, if there is an incident wave available, it will be so much stronger
than any reflections that these may be ignored.
But when there is no direct path, multiple reflections, travelling
different distances to the receive point, may largely cancel one another
out. The effect is that of a high-Q comb
filter, cancelling some frequencies and enhancing others. It is much more critical for stereo than it
is for monaural transmissions. The
result is distortion and a "picket-fencing" effect, even more
pronounced with mobile reception.
I
said "generally speaking," in the previous paragraph. Of course, we're dealing with a statistical
kind of thing, here, and there will still be locations, even where an incident
signal is available, that the reflections will overwhelm it. The key to improving reception quality inside
the service area isn't a power increase--we need to increase the ratio
of incident to reflected waves at the receive point.
How
can we do this? We don't have much
control over the path from transmitter to receiver, except to choose a
transmitter site that offers the most best paths for the most receivers (the
grammar is terrible, but you get the gist).
If we could just convince listeners to lug around directional receive
antennas and continuously adjust them for minimum multipath,
that would help, but let's get real for a moment… we can't do that
either. If they'd mount their antennas
ten metres or so above ground, that would help, too, but it doesn't seem likely
to happen again anytime soon. These
suggestions are right up there with CP receive antennas,
and diversity receivers… promising from a technical point of view, but
unrealistic from a practical viewpoint.
Next
month, all (or at least that small subset of "all" that exists inside
my skull) will be revealed!
Controversy!
Suspense! Pathos! Next month in Broadcast Dialogue!