Building penetration of FM translators

[Schroedingers Cat]

Magnum Dynalab is a Canadian company. They marketed their Sleuth products quite a bit to the CBC, and reviewed their performance in that application. They used their products to pull in distant FM stations for off air rebroadcast on satellite stations and translators. Maybe that was why they had a 600 ohm output on their tuner, to rebroadcast directly off air into a control board.

 

[Greg Strickland]

Magnum Dynalab and Fanfare FM tuners have been, and are used for off-air monitoring, in addition to feeding other radio stations for rebroadcast. Traditionally that would have been via 600 ohm balanced output. Today the Magnum Dynalab model used for off air monitoring might be the MD 90, coming in at about $1,500.

In earlier days a stand-alone FM station might have had two Magnum Dynalab tuners. One in the rack tuned to the station for off-air monitoring in the studios and house monitor, and another tuner in engineering for the engineers to tune around the dial and compare audio. I knew of stations that put up a second FM antenna pointing towards another station (in another direction ) simply to compare audio.

Something to consider is the stand-alone FM Tuner segment is a very low volume business. Selling them to only broadcasters might not support more than two people. Thus the motivation to sell audiophile market FM Tuners at a much higher price point, and greater dollar profit per unit, in order to generate enough profit to stay in business.

 

[Kmagrill]

That is odd indeed. I wonder if they could explain exactly what was happening. Reflections are proportional to the incident field. The only other thing could be if the reflecting surfaces are non linear, like the "old wiring and plumbing in older areas" you used to hear about affecting, and even detecting, AM BC signals. Could part of it be receiver induced, like IF Beat and RITOIE?

Were the different ERPs at the same HAAT and same antennas and numbers of bays? I've also heard that diplexing antennas with different vertical patterns due to inter bay phasing can cause unwanted beam tilt.

So, I don't personally know about the Alaska station. The one that I personally witnessed was a single station using an ERI 5 bay full wave spaced antenna with the bottom bay about 130 feet above the ground on a plateau overlooking the mid-Willamette valley in Oregon, between Eugene and Salem. The coverage was full of problems starting a few miles from the site and getting progressively worse the further out one got. By the time we got to Salem, about 30 miles away, it was pretty awful. Everyone always just assumed that it was the effect of the terrain. Then one day, there was a major transmitter failure that was not economically repairable. Not having a backup transmitter, the engineer put the 50W exciter into the antenna while a new transmitter was ordered and for about 4 weeks we all listened in amazement to that 50W exciter. The coverage got clear all the way to Salem and could easily be heard to the southern outskirts of Portland. I can't answer about building penetration close by, but I could pick it up in my apartment in West Salem where it wasn't listenable before.

I had a similar experience at a site overlooking Portland. The original 4 bay ERI antenna didn't perform as well as we had hoped. But then, the antenna was destroyed in a tower collapse. We braced the remaining tower and installed a temporary 6 bay antenna while waiting for the new tower to be constructed. The emergency antenna (cut down from an abandoned 10 bay ERI ring-stub type) performed far better than the standard four bay at less than half the power. Of course, there are too many variables in this story (6 bays vs 4, ring-stub vs rototiller, 100kW ERP vs 35kW ERP) to know why the temporary worked better, but my guess is that the ground reflections are the big factor. Back in those days, we weren't using very many half wave spaced antennas yet and nobody had even heard of a 0.8wave spaced antenna. Perhaps those would have made a big difference.

I realize that the RF should scale, but it's not linear which is why reducing power by 50% only results in about a 10% decrease in coverage. I have never really tried to analyze why this occasionally happens, but I've talked to several engineers from mountainous areas that have observed the same things. I just keep it in mind when coverage fails to meet expectations and there are buildings or land masses very near the antennas.

 

[Schroedingers Cat]

Now that I think about it, it could be that the different bays follow different signal paths, and the phase shifts on reflection result in cancellation rather than reinforcement from different bays. You would have to know all the antenna configurations. If the exciter was just feeding the top bay, and that bay was line of sight, and the rest were blocked, that lower ERP might be better.

I know of a situation where I advised a Section 73.215 station upgrade I came up with, to choose a 40 kW ERP at a higher HAAT to protect the short spaced stations. Somebody bought the station, and the new owners wanted it to be 50 kW at a lower HAAT. This resulted in a very noticeable signal reduction toward a high elevation area with significant shadowing in one direction, toward the cochannel short spaced station, that I drove along frequently. It resulted in much greater distances with capturing and switching between the two, vs. a sharp one time change from one to the other cochannel station. Peter Moncure's FMR programs showed it plain as day, the terrain shadowing resulting from the lower HAAT, well before Longley Rice programs were more widely available. Radio people, particularly the less technical ones, are often obsessed with being at maximum ERP for their Class, though often not being the best option signal wise. I'm sure many of you have seen this type of situation.