- Posts
as found on the Hobbybroadcaster website (accessible from the main page).
I approached this missive, written by the webmaster who is a self-professed Broadcast Engineer with over 45 years experience, with great interest. Anyone who has worked in the Part 15 broadcasting arena for any length of time recognizes that there’s a great deal of misinformation floating around.
The first sentence was promising, in that it stated that “radio waves work under the realm of science.” Unfortunately, that was pretty much the last science, and engineering, that appeared in the article.
Instead, what followed was some bragging about the author’s qualifications (while knocking other’s), direct quotations from the FCC rules and finally, some hand waving about potential ranges that you can obtain in the AM and FM bands. The author’s well known bias towards Part 15 FM was shown by his using minimum ranges for FM (under poor conditions) and the absolute maximum, possible ranges for AM (under ideal conditions). The article ended, naturally, with a conclusion that Part 15 AM was the preferred solution.
In the scientific world, we call this coming up with a solution in search of a problem. I ask – a solution for what?
If you don’t know what problem it is you’re trying to solve, any solution you can come up with is meaningless.
There was no attempt made to delineate all the various factors that go into determining Part 15 ranges on either band, including, but definitely not limited to, the specifications of the radios being used to listen, the surrounding geography, potential interference including buildings, background noise, etc. There was also no attempt at discussing the many other (and maybe more important) factors involved in what a broadcaster might be trying to do, such as types of programming, desired sound quality, budget, etc.
Unfortunately, under the guise of setting the record straight, the document introduces yet more misinformation into the Part 15 world. I won’t speculate on the author’s motivation.
The only, and I repeat, only, way to go about determining how to proceed in Part 15 broadcasting is to lay out what you’re trying to do in great detail, along with completely describing your surrounding environment. You also have to know what you’re willing to spend.
At that point, and only then, science and engineering can be used to determine the best way for you to proceed.
There is no one or preferred solution. It’s never that easy.
I can agree with some of the article. I’d really like to take a good look at it.
It Does depend on the receiver and that is why I choose the Tecsun Radios as a reference. 2 friends of mine get 2 Miles on AM.
I’m always amused when someone tries to pitch FM as a great way to do Part 15. Obviously the audio is superior to AM, I’ll give you that.
But range is always limited by science. A maximum allowed field strength is only going to travel so far.
Then comes all the FM practitioners arguments. “But under ideal conditions. But with a quality receiver. If they put up an antenna. If you put bit up high enough. With a temperature inversion.” etc. So, maybe if all the stars align a legal transmitter might get more than 200-300 feet. But I can tell you, after all my testing – with a calibrated Potomac Field Intensity Meter, and the use of a variety of receiving radios from those in cars, to Tecsuns, to vintage quality stereo components, boom boxes and cheap portable transistor radios, and on down the line. A LEGAL FM Part 15 transmitter operating at the legal maximum is going to go 250-300 feet. I’ve yet to see anyone but me set up a transmitter, with the FIM and set exact legal output, then roam around an open field with various radios to see what actual, measured coverage can be. This was done where I could physically stand there in the field and SEE the transmitter a couple hundred feet away from me. Zero obstructions.
Put that transmitter in a building, or put the receiver in a building and you lose it. At this field strength you have very little if any building penetration.
Saying “My coverage is xxx to a good car radio” is silly too. No one is going to stop their life and go sit in the car to listen to any station.
See, with FM, in reality, there are very few, if any variables. You are allowed 250 uV/m at 3 meters period. A better transmitting antenna doesn’t help. If you put one on that is more efficient, matched better, maybe even directional, and you get more range, it’s because that improved antenna increased your field strength. You can’t claim “but, with the right atmospheric conditions…” because there aren’t a whole lot of conditions that can happen within a couple hundred feet.
And thus will kick off another round of “But FM is the only way, and I can get a legal mile… or half mile… or my station is so great the masses will go out and buy good radios and put up outside antennas to hear it…”
Let me know the next time you put in dozens of hours with an actual, real Field Intensity Meter, and walked all over a wide open field with a 100 foot tape measure marking out coverage plots, and let me know how that works for you. Saying “But I got half a mile solid with my legal 123XYZ transmitter” let me know how you knew it was legal. Out of all the FM Part 15 transmitters I tested ONE was legal. That was the C. Crane. And the second you clip on a hunk of wire for a longer antenna, or open it up and crank the output adjustment pot for more zing, it’s not legal any longer either.
TIB
<p>Tim, whenever we talk about range, the elephant in the room is ALWAYS the quality of the receiver – sensitivity and selectivity.</p><p>There can be a wide variation in both beween receivers. If I recall, you used a Tecsun portable for your testing, which typically has a sensitivity of 4-5uv, unknown selectivity.</p><p>Some car receivers I’ve owned and used have had a sensivity of well under 1uv (after market Kenwood and Alpine) and excellent selectivity. These types of receivers will obviously have much better range (particularly if they use whip antennas mounted to the car body) than portables.</p><p>To say that no one will listen to Part 15 radio in a car is just plain obfuscation. There are plenty of specific applications (such as church parking lot radio) in which people can and will do so. In my case on Bowen Island, the listening audience was a ferry lineup, filled with cars.</p><p>The main point of my post was that you choose your operating parameters on the specific application. You CAN use Part 15 FM for some applications, given the increased range that can be achieved with car radios.</p><p>I don’t mean to cast aspersions on your testing endeavors. I highly doubt, however, that you drove a car with a known radio sensitivity into the field you were traipsing around and compared ranges with your Tecsun.</p>
I agree the radio is a big factor. AM or FM. The GE super or Ccrane EP Pro or Sangean’s better models will do a hell of a lot better than the average boom box junk will.
Yes with AM you are not limited by field strength as Tim said but still a radio designed for long range reception(more selective and sensitive) will do much better with the set up you have.
The smaller a signal a receiver will hear the better. For AM long range reception the best I’ve ever seen in a portable that rivals cars if you can find it is a Realistic(Radioshack) 12-656, with a FET tuned RF stage.
Talking about range without talking about the radio is totally meaningless.
Just to give an example, when Artisan Radio started up on Bowen Island, we located the transmitter on a building rooftop in a storefront/studio/residential complex. This complex was located on a hillside, with direct line of sight to our target listening area (the ferry lineup). This lineup was located on the main street leading down from Artisan Square, through Snug Cove (Bowen Island’s ‘downtown’) to the ferry docks. One lane on this road was for traffic, one for the lineup.
As the crow flies, it was about 300-350 meters down the hill to the crossroads where Snug Cove proper began. The road through Snug Cove was maybe 300 meters in length.
Artisan Radio broadcast on a BETS-certified MS-100. Now, I don’t have a FIM, so I have no idea of the actual field strength of the signal. However, that means nothing in the following comparisons, as the signal remained constant throughout.
Residents within Artisan Square were able to receive our signal clearly on reasonable portable radios (within about a 50-75 meter radius). It’s unknown how much further you could receive said signal, as reception outside the square on a portable radio was never attempted – our target listening audience was the cars in the ferry lineup after all. It was sufficient that the store/studio owners & residents could receive it on readily available radios.
Now, in my vehicle, I was able to receive Artisan Radio clearly on the 300 meter road stretch to the crossroads. Passing into Snug Cove, the signal was still reasonably strong for another 100 meters or so. After that, fence boarding began due to building obstruction until reaching the ferry dock, at which point the signal could again be received clearly, but weakly (about 700 meters or so range). You could continue to receive the signal on the ferry as it left the docks for a little while, until it left the bay (maybe another 200 meters or so).
For a while, I thought that that kind of range was the norm. At least, until I got a ride from someone who tuned into the station. Boy, was I disappointed. On their car radio, obviously much less sensitive than mine, the signal faded at the crossroads. Fence boarding occurred down the 300 meter stretch to the crossroads. There were plenty of areas where you could still hear the station if you were parked in the lineup, but reception was certainly spotty.
After that, I listened in on other car radios as well. Most were better than that one but still did not obtain nearly the range I did. A few equaled mine.
What I learned is that one size, at least in terms of radios, does not fit all. Unless you know the sensitivity (which includes the antenna/grounding system) of the radio, talking about range means nothing. All I could say about Artisan Radio is that it was possible to receive the station up to 1km away with the right radio. Most could receive it up to 300 meters away. Some could receive it more than that.
So to say that you’re only going to get 200-250 feet on a Part 15 certified radio is also meaningless. If you’re getting that on a portable radio, no matter how good, you’re going to get more on a car radio, particularly if it has a whip antenna and a metallic body (i.e. grounding). How much more will depend on the car radio, and in various studies it has been shown that sensitivity can vary from well under 1uv (some as low as 0.5uv) to that of a good portable (4-5uv). Most will be in the 1-2uv range, but even that is significantly ‘deafer’ than 0.5uv and will affect range significantly.
Applications that Part 15 FM can be used in include anything that deals with car radios, including drive-ins, parking lot churchs, shopping center retail advertising, etc. etc.
Again, as I’ve been saying all along, it depends on your application.
- You must be logged in to reply to this topic.
