DEPOLARIZING A POLARIZED WORLD

When we start talking about circular and horizontal polarization for FM broadcast, a lot of broadcast technicians' hackles start to rise. Everybody seems to have an opinion to share, but there's very little hard evidence to support the various views. Multipath it turns out, despite being ubiquitous, is a very complicated subject, and what works for you doesn't necessarily work for me (this is a variation of the principle YMMV (your mileage may vary), which is itself a subset of the infamous Murphy's Law).

One of the arguments in favour of circular polarization is that you essentially get a free license to double your transmitter power. Another is that the vertical component of the circular transmission provides enhanced reception to vertical receivers (such as motor vehicles). I say that these two points are debatable at best…

I mentioned last month that the key to improving stereo reception quality inside the service area isn't necessarily a power increase--what we really need to do is increase the ratio of incident to reflected waves at the receive point.

While there's little hard data on multipath, there are a few items that have come to light:

1) When a circularly polarized signal is reflected by irregular objects (hills, trees, etc.), the polarization data is lost. The reflected signal exhibits random polarization.

2) Vertically polarized signals are reflected more than horizontally polarized signals. Trees in particular reflect more vertical than horizontal. This is one of the few facts regarding polarization propagation that you'll find in a textbook!

3) Vertical motor vehicle antennas, contrary to public opinion and common sense, receive horizontally polarized signals just as well as they receive vertical signals. Marvin Crouch of Tennaplex used to say that the asymmetrical grounded body of the car or truck caused pattern distortion so that the vertical whip antenna could no longer discriminate between vertical and horizontal. I don't know about all that, but the first part seems to be true.

What can we conclude from all this?

Circular polarization is not the panacea that we were told it would be. While in many cases it provides reception equivalent to horizontal polarization, there are several cases, particularly where the geography provides lots of undesired reflections, where CP signals are seriously degraded versus HP. This is most likely because of the increase in reflected signals caused by points 1) and 2) above.

I hope you've been noticing that I've been careful in this column and last month's to refer to stereo performance inside the primary service area. Where that extra CP power is useful is at the horizon, in extending coverage in the far field. And when we're dealing with mono transmissions, it may be an overstatement to say that "all reflections are good," but only just by a little. Again, the CP signal, with its extra wattage and enhanced reflections, can seriously extend the coverage of a mono signal -- and that goes double when the terrain is mountainous.

Receiver manufacturers have not been ignoring the multipath problem, either. There's both good and bad news. The bad news is that more and more receivers are using variations of blend. Blend circuitry senses marginal reception and surreptitiously fades the receiver to mono mode. While it does overcome some of the noise and distortion of multipath, an aggressive blend circuit can mean that your station is received in mono more than in stereo. Blend circuits invariably work stealthily, because if they blinked the stereo pilot light, consumers would soon catch on to their nefarious design and start to complain.

The good news is the promise of digital signal processing in receivers. Remember last month when I was bemoaning the fact that listeners wouldn't stand for lugging directional antennas around? Some of the new Motorola Symphony© designs manipulate phase and gain from diversity antennas, in effect to produce a multipath-minimizing directional antenna. Controlled by the microcomputer, these receivers constantly adjust the IF and baseband signals for minimum distortion. We keep hearing that the market will be flooded with these DSP-based receivers in the next few months, because in large quantities they're cheaper to produce than old-fashioned analog sets. Bring 'em on!