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Confusion about range on watt.

 
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Confusion about range on watt.
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Joined: 08 Jan 2004
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Post Confusion about range on watt. Reply with quote
Hi

First of all I am in the planning phase on getting a LPFM license hopefully but here is something that confuses me.

Some people refer to getting far with 25watt amps. Some say even further with 100watt amps. But on the nlgcdc.org site they are talking of only 3.5 miles with 100watt.

I know the FCC is picky on where the transmitter can be placed but let's say it will be on top of a highrise that is at least 15 storys high. Technically what nlgcdc.org is saying with an LP-10 I can go as far as 1-2 miles and an LP-100 3.5miles.

Anyone has experience with the actual miles?

Many thanks -- Thomas
Thu Jan 08, 2004 7:32 am View user's profile Send private message
marek
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I'm pretty sure about one thing. The higher your antenna is the better coverage you get for your 100 watts. The same with the antenna. The better antenna, the better coverage. If you have your antenna on ground level your 100 W will not be much worth and the coverage will be a disaster. Put it on a hill or a high building and you'll see the difference. For better coordination I would recommend buying a map showing the terrain and study it. There you'll get much valuable information for where to place your transmission site. You won't regret it.
Thu Jan 08, 2004 10:26 am View user's profile Send private message ICQ Number
R_QRP
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Joined: 01 Oct 2003
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This is the most frequent question, and many newbies want a simple answer. There is no simple answer. The distance at which your signal can be heard will vary from listener to listener, depending on their equipment and location. In fact, many elements affect your range:

* the transmitter power
* the transmit antenna's height and effectiveness
* local terrain (hills, large buildings, etc.)
* interference from distant stations on the same channel
* splatter from local stations on adjacent channels
* the height and effectiveness of the listener's antenna
* the sensitivity of the listener's receiver
* amount of "static" at the listener's location

Predicting the range of an FM or TV signal

On FM and TV broadcast frequencies, antenna height puts an upper limit on your range, regardless of power levels. (It is true that diffraction can extend your range slightly, and signals sometimes travel greater distances when atmospheric conditions are just right, but we will ignore these factors for the time being.) The distance from your antenna to the radio horizon is determined by this formula: distance in miles = 1.415 times the square root of the antenna height in feet.

Now you see why FM and TV stations go to the expense of building antenna towers that are hundreds of feet high, or locate their antennas on mountain-sides that overlook the cities they want to serve.

Of course, the formula assumes that the terrain is relatively flat. If the transmitting antenna is located at the top of a hill, its range might be better, but this depends on the direction in which the antenna radiates its energy (i.e. its vertical radiation angle). If the antenna shoots a lot of energy up into the sky (as some types of antennas tend to do), placing it in a higher location will not help much.

73s de R_QRP
Thu Jan 08, 2004 5:25 pm View user's profile Send private message Visit poster's website
marek
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Especially interference can be an interesting issue. I have checked the frequency (107.9 MHz) I use and no one is supposed to use it within a distance of 400 km. Also there are no other stations transmitting on the nearest freqs. The closest one is on 107,6 MHz and it's 60 km away. Should be a perfect set up. Still when I took my tx off the air the station that is 400 km, on the other side of the Baltic Sea, goes in clearly with RDS!!! Now that is a great tx set up! It doesn't interfere whatsoever when my tx is transmitting, at least not so much that I have noticed, but still when the tx is off it can be noticed in different parts of the area.
What I try to say is that you have to be very careful when you choose your freq, set up of the site etc.
Of course you can try to calculate the coverage, but (!) this can still vary a lot just by moving the antenna one meter! I have set up transmitters on antenna towers experiencing a huge difference between placing the antenna on one side of the tower and the other and this was on a high location (103 m + 69 m). Once you've done the theoretical part set up the antenna, turn it on and take a drive with your car, return, do adjustments and take a similar drive again seing how the coverage changes (even if the adjustment is very little). In time you'll find the best set up... Also not to forget, geology, like what ground the tower is standing on is of some meaning too. The station 400 kms away from here stands on a surface of mountain ground while all others in that area are standing on regular ground, although that is high. The other ones can't be received here.
That's also a reason why I use both geological as well as topographical maps when choosing a transmitter site.
Anyway to make it short... test, test, test and test again and you'll find the best set up in the end.
Fri Jan 09, 2004 9:16 am View user's profile Send private message ICQ Number
DrSandi
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Joined: 06 Oct 2004
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Location: Olympia, WA USA

Post Low Power, Low Elevation FM coverage Reply with quote
Here are some tips on FM coverage for low power broadcasters.

Antenna height is vital. Your biggest coverage killer is buildings and vegetation. Get your antenna above as many trees, houses, fences, sheds, etc as possible. Solid objects suck down your signal faster than pizzas and brew at a frat party.

At LPFM antenna heights, remember that doubling your height above ground is equivalent to increasing power 4 times. Doubling antenna height is twice as good as doubling power. Always build on that mountain at the center of town!

As others have mentioned, the biggest enemy to a large coverage area is a busy channel. Find the quietest channel you can find. Not only is it friendlier to the neighbors on the band, but it's not going to stomp on your signal as much out at the edge of your coverage area.

Use a low loss antenna lead. I generally go for Belden 9913 or similar. It's about $1 a foot in most places. You only lose about 1.5 dB with 100 feet. That's 1/4 of your power if you use a full 100 feet. The skinny stuff is a lot cheaper, but it's like using a skinny garden hose... lots of resistance means less pressure coming out the other end.

Remember, if you feel any part of the cable heating up after constant transmitting, that's a problem spot that's turning your RF into heat. Nobody can tune in heat on their radio. If the heat is spread evenly along the entire cable, you need to step up to a lower resistance cable. If it's hot at the connectors, clean the connectors. If that doesn't fix it, replace the connectors with something that has silver or I am a spammer and this is my stupid link.

If the cable gets hot in one spot or a series of equally distant spots, you almost certainly have an antenna mismatch. That's a VSWR problem. You probably need a balun, or a least a 3 or 4 turn loop of coax near the antenna.

If it's not the cable, then your antenna may need tuning or repair. Find a Ham with a VSWR meter and start tweaking your antenna system until it's happy. Low VSWR means a longer life transmitter and less wasted signal for better coverage.

Those hot spots in your cable are caused by the RF not being able to escape efficiently through the antenna. The easier you make the RF path, the more signal radiates into space instead of heating your transmitter space.

Remember that broadcast range is very dependent on the quality of the receiver as well as the technical aspects of the transmitting facility.

Original equipment car radios are generally the best device for long distance reception at a low cost. They will usually pick up a station that has no competition out to the 35 dBu signal level. That's way, way past the protected area for licensed stations. That's about 158 km (98 mi) for a full power, Class C station, 100 kw at 2000 meters over flat ground. Or 90 km (54 mi) for a Class A station, 6 kw at 100 meters. Or 27 km (16 mi) for an LPFM, 100 Watts at 30 meters.

Boom Box radios are a little below car radios. They're usually not quite as sensitive. They're also less selective, which means they're going to pick up the adjacent channel stations much stronger than a car radio. This reduces the coverage of a low power station on the first or second channel adjacent to a strong local or semi-local station.

Clock radios are generally pretty lame. You'll be lucky to pick up a full power station at the edge of its protected (60 dBu) contour on many of them.

Then there's the Walkman style radios. If you're not within the city grade coverage (70 dBu) zone of a station, you might not hear it at all. Not recommended for listening to LPFM except at the transmitter site.

Replacement car radios are generally a piece of crap radio added thoughtlessly to a killer CD player and sound system. Don't expect much distant listening on AM or FM band with these things.

So here's the story of my strongest LPFM, KYAO-LP. It's licensed to Ocean Shores, WA. 100 watts at 30 meters (98 feet) The antenna is on a metal mast, with multiple bungee cords holding it to the tree. The top of the antenna barely peeks out above the crown of the 95' Sitka Spruce that houses it.

Ocean Shores is a 6 mile by 1 1/2 mile sand spit. Average height above sea level, 3 meters, (10 feet). The station is MONO to keep multipath to a minimum. There is NOTHING on this channel except KYAO. Being way the hell away from civilization gave us our pick of totally clear FM channels.

Auto radio coverage is GREAT on KYA. The area is very flat, being mostly at sea level. The station can be heard in some places as far as 34 km (20 mi) inland if there's no hill blocking it. It's kind of fuzzy, but still listenable in spots. The signal is pretty solid out to about 13 km (8 mi) and then runs into more and more trouble as you get further away.

Our biggest problem is that right downtown (such as it is) where the hotels are, we have a great signal in cars, but I've tried tuning it in on those crappy little hotel clock radios and it's barely listenable, and then only if I move it around to a sweet spot in the room. This is only about 1 1/2 miles (2.5 km) from the transmitter. But the ridge of trees where our transmitter lives extends toward downtown, so we have a LOT of vegetation in our way in that direction (north). That's one reason I can say from experience just how badly trees and such will degrade an FM. signal.

The FCC prediction maps shows a city grade signal in Downtown Ocean Shores, but the reality is that there's a much weaker signal because of factors that are not included in the very flaky FCC calculation method.

A mobile home less than 2 miles away, near downtown, cannot tune the station in on a boom box. But then, it's a pretty effective RF shield.

The absolute furthest I've ever picked up KYAO well enough to listen to it on my car radio is about 45 km (27 miles), on a hill near Raymond, WA. It's a fluke, but it works.

We use HEAVY processing. It makes a huge difference in overcoming the background noise in the fringe area. With a louder signal, the radio is turned down, which means the apparent noise is much further in the background. Sound isn't as GOOD close in, but since 100 watts means almost ALL listening is FRINGE listening, we made the sacrifice of purity in exchange for good sound in the car on the highway to Hoquiam.

My conclusion is that LPFM is GREAT for in car listening. And since over half of U.S. radio listening is done in cars, this isn't a bad thing.


Dr. Sandi
Wed Oct 13, 2004 7:43 am View user's profile Send private message Visit poster's website
marek
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You definately need to get the antenna higher up or even recoordinate it. Try Radio Mobile to calculate where the best position would be and then try to recoordinate it.
Wed Oct 13, 2004 10:53 pm View user's profile Send private message ICQ Number
revspalding
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Joined: 30 Nov 2005
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Post RF power /range is a squared function Reply with quote
The power required to maintain the same signal to noise ratio (signal strength) over a longer path, is related to the square of the power....

In other words, to transmit a signal that has the same signal to noise ratio over a distance that's three times the reference distance, you need to increase the power by a factor of three squared, or 9.

working the equation with a known power level increase yields the following:

two times the power gives you the square root of two, or (1.414), times the distance.

four times the power gives you the square root of four, or (2), times the distance.

25 times the power gives you the square root of 25, or (5), times the distance.
Wed Nov 30, 2005 9:52 pm View user's profile Send private message
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