- July 5, 2018 at 8:11 pm #104782
Total posts : 496
There’s been plenty of talk about Part 15 & BETS-1 FM range in these Forums. Some would have us believe that unless you have expensive test equipment, and can measure field strength, such talk has little value.
The simple truth, however, is that most of us don’t have that expensive equipment. And the first question any of us ask is ‘what kind of range will I get?’. We ask when we first get into broadcasting, and we continue to ask as we attempt to figure out how to improve said range, even when we’re the only listener.
So there is value in talking about range. Particularly when dealing with real world situations.
Now, we all know that the sensitivity of a receiver is probably the most significant factor in discussing FM range. Your range with a highly sensitive receiver & good receiving antenna will be much greater than a deaf receiver. Other factors include obstructions, height, and to a lesser but very real extent, weather (I find that my FM signal goes a little further, all other factors being equal, in cooler, crisper days; hot, humid days reduces range).
But if we keep all these factors constant but one, say antenna height, then there are real benefits to looking at differences in range based on differences in that one factor.
It is admitted that listening is an imprecise measurement, but one can certainly tell a strong, noise free signal from one that is dropping out, or has noise; one can also tell when a signal is not heard at all. It’s better than nothing, and for most of us, all we have.
I recently conducted an experiment with a Canadian certified (RSS210, theoretically equivalent in field strength to BETS-1) at different heights. In one, the antenna was mounted at ground level in front of a window. In the second, the antenna was on the 2nd story, about 8 feet higher, again in front of a window. The area was suburban, consisting of mostly 1 & 2 story wood frame houses, so getting the antenna higher certainly was increasing line of sight, and minimizing obstacles. All other factors, including the receiver, were kept constant.
To keep things simple, I attempted to classify several different types of signal; strong, in which no noise was heard, fringe, in which there was noise & dropouts, but the signal was listenable (in my opinion), and then effectively gone (you might be able to tell it’s there, but wouldn’t want to listen to it).
With the antenna at ground level, in most directions, my strong signal was anywhere between 150-200 meters out with my car radio. The fringe signal area was relatively small and added maybe another 25-50 meters at most.
Now, here’s where it gets very interesting. Just raising the antenna up that 8 feet or so increased the strong signal range to between 250-350 meters (the differences due to direction). The fringe area signal was now much larger as well, with anywhere between 50-100 meters added.
Before anyone pipes up and says that a transmitter being certified doesn’t mean that it is compliant, I say, that is true. The certification is something that I can point to as a broadcaster, to show that I am attempting to be legal. I definitely want to be.
In this experiment, however, I’m not looking for absolute measures, just relative. There’s absolutely no doubt that raising the antenna on my transmitter even that 8 feet or so greatly increased both my strong signal and fringe signal ranges.
In the world of FM micro broadcasting, elevation can be your friend.
- July 6, 2018 at 7:07 am #104825
Total posts : 187
Just to note that on VHF frequencies and above, it takes more than just a line-of-sight (LOS) path for minimum signal loss along that path. Clearance must be provided around the LOS path, as well.
In some cases, the signal on the LOS path can be greatly reduced or even canceled by reflections from surfaces around the LOS path. The graphics below show why this can occur.
Fresnel Clearances for a 98.1 MHz LOS Path 0.5 km in Length
- July 6, 2018 at 4:24 pm #104841
Total posts : 488
Artisan’s real world experiment with FM range with the same transmitter, same power, same antenna, same receiver, just different transmitter location, verifies that an article I read about getting the most range with FM is accurate(can’t locate the article now).
The article said that from ground level to two stories up(16 feet) can be the same as if you doubled the power of your transmitter. So if you are at a basement window and then go to the second floor of a house it is so much better.
So even though in theory all transmitters operating in exact compliance with BETS-1 or part 15 should have exactly the same range but in fact it’s location location location! as Artisan has shown.
- July 7, 2018 at 3:38 am #104887
Total posts : 187
RE: … in theory all transmitters operating in exact compliance with BETS-1 or part 15 should have exactly the same range …
Sorry, but no, that does not agree with theory.
… in fact it’s location location location! as Artisan has shown.
Artisan posted his (predictable) experience with elevating a VHF transmit system.
I posted the reasons for his experience — which have been known and utilized for many decades.
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