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 Canada and the FCC.  These august bodies take the greater of horizontal or vertical radiation and ignore the other plane, so that you can effectively double the amount of RF that you transmit for a given power level by using CP.  And more RF is good, … right?


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!