Post
by Albert H » Sun Aug 02, 2020 9:53 pm
Having done this lark for about 40 years, I can assure you that mixed polarisation and circular polarisation are just worthless fads for FM broadcasting. When I was engineering a major market FM flamethrower station in the western USA, they had a couple of huge towers with these "turnstile" aerials that they'd bought from Europe.
They'd been told that these were the canine's gonads as far as getting signal into the listeners' receivers. One site ran 850kW and the other ran 450kW. The signal was present almost everywhere, but suffered badly from multipath distortion, and was generally weak. Stereo was usually horrible.
After a week of driving 'round the service area with a test receiver, measuring field strength and multipath, and looking for "radio shadows" behind hills, big buildings and where the local topology screwed with the signal, I came up with a plan....
I got in touch with an aerial company in the UK, and had a heap of gamma-matched dipoles and phasing harnesses supplied. We took the station off the air for 16 hours overnight Sunday to Monday, and put the bigger site back on the air at 6am Monday morning. The lower powered site stayed off to allow us to do the aerial work during daylight hours (the rental on the floodlighting was really expensive!).
By 8am on the Monday, the switchboard had logged a couple of hundred calls from listeners commenting on how much better reception was, how much lower the distortion was, how it was better in their cars, and that they could cleanly receive it in places where it had previously been inaudible. By 9am, I had the MD of the station on the phone asking is we needed the secondary site at all, since the coverage from the primary was so exceptional!
I spent the next few days doing the same routes that I had done before, logging the field strength and the multipath. Field strength was up by around 30% in most areas, and multipath was acceptably low. The coverage was improved, with many of the "shadows" now gone, with weakened (but useable) signals there.
We sold the secondary site to another station about two months later, and re-filed with the FCC as a vertically polarised high power FM. This actually reduced the bills in several ways - not least when I had a dodgy PA valve at the site, and the power was reduced to ~500kW and we found that there was no significant reduction in field strength in the target area.....
Reading the original FCC Filings and the Operational Notes from the original "Chief Engineer", it became apparent that he had gone down the mixed polarisation route because it was trendy - "that's what all the mass-market stations are doing". There was no real reason for it whatsoever. >95% of the listeners used aerials with vertical polarisation (telescopic rods, car aerials and so on) and very few still used the old horizontally polarised, building-mounted aerials at all!
Last edited by
Albert H on Sun Aug 02, 2020 10:03 pm, edited 1 time in total.
"Why is my rig humming?"
"Because it doesn't know the words!" 
