by Dan Roach


First, a couple of additional comments from those "in the know" about multipath and CP.  As I anticipated, technical people far and wide have strong opinions about whether to use circular or horizontal polarization for FM transmission.  Bob Calder of Victoria mentions an instance where he can see a CP FM transmitting antenna, and still can't get decent stereo reception from it because of excessive multipath.  In this case, the transmitting antenna is fairly high gain, and quite high up, so it's quite likely that his reception point is somewhat below the main lobe of the transmit antenna.  Since the gain is so high, it's quite likely that reflections from objects in line with the main beam are unusually strong.  The fact that the signal is CP is perhaps adding to the problem, by providing additional reflections.  (Back to my old saw about the ratio of incident to reflected signals).  Bob mentions A/B tests he's been part of where HP has been demonstrably superior (in relatively hilly terrain).


I also had an interesting discussion with Dave Newberry of CBC Vancouver.  Dave mentioned an instance on the Prairies where a change in facilities from CP to HP has resulted in reception complaints.  His information raises the possibility that I might have to modify my (oft-repeated) statement about car radios not being able to differentiate between HP and VP signals.  Dave's theory is that car radios are usually getting sufficient signal from HP signals because there are lots of reflections around.  The reflections randomize the transmitted polarization sufficiently to provide VP for the receive antenna.  In a flat prairie location, with fewer sources of reflections, the car radio reception of an HP signal seems to be impaired compared to a CP signal.


Of course, if this is true, the fact that a car radio is getting mostly reflected signals from an HP source would lead one to expect that the multiple reflections would show more multipath problems than a single incident signal from a CP or a VP source.  We would expect that car radio reception of HP signals would be somewhat better in mountainous terrain than flat terrain, but that there would be more apparent multipath around that hilly terrain than from a CP source.  I can't say that I have seen, or rather heard, this expected byproduct of Dave's theory.  But still, it's food for thought.


The main lesson I've learned over the years from listening to FM and to learned transmitter folk, is that when it comes to FM propagation, there's a lot of folklore and anecdotal information, and very little printed word or official documentation.  The little you can find in textbooks must be considered critically.  You'll read, for instance, in many texts that FM propagation is not affected by seasonal variations.  Try and tell that to listeners in the BC Okanagan, where multipath is known to come and go in many regions with the change of the seasons.  Is this due to changes in vegetation, or in the moisture content of trees?  The Okanagan isn't exactly known for either abundant vegetation or moisture (on hillsides outside of irrigation zones at least), regardless of the season.  But the phenomenon exists.


Another "textbook" fact is that skywave, skip propagation and "ducting" don't occur at FM frequencies.  Yet we read every year about all kinds of intermittent co-channel interference along the U.S. Atlantic seaboard and the Caribbean, with distant stations' signals suddenly appearing hundreds, sometimes thousands of kilometers from where they belong, and interfering with local FM radio.  They disappear just as suddenly.


The new computerized FM measurement sets from Audemat, that can make thousands of comparative measurements between several stations as they are moved around in the coverage area, forming a database with a connected GPS receiver, may be able to shed some light on these mysteries in the years to come!  And yet, I suspect that there will be just as many new questions….


Next month, a story of cutting edge technical innovation that gave Canada's biggest radio station its call letters!