FM Radio Transmission Distance


by SLiM6y
Tags: distance, radio, transmission
SLiM6y
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#1
Jan11-08, 09:10 AM
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I have a simple question - but I somehow doubt there is a simple answer.

1) I would like to start a small, non-commercial, home transmitted FM Radio Station - mostly for fun and as a hobby. I have seen an 18W transmitter for a reasonable price. I live on relatively flat land in the country side with no buildings around.

What is the best case scenario for distance the 18W transmitter will transmit?

Is there a simple formula to use to calculate the transmission distances?

I do not know the size of the antennae - but does increasing antennae size increase transmission distances or again is it not as simple as that?

Thanks in advance if you have the answers. I am looking forward to knowing more!

Cheers

Slim
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mgb_phys
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#2
Jan11-08, 09:35 AM
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It's a combination of the transmitter and receiver antennae size and the sensitivity of the receiver. Voyager's transmitter isn't much more than 18Watts and that can be received from quite a large distance!

Are there limits on the power of a transmitter at that frequency in your location?
berkeman
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#3
Jan11-08, 11:11 AM
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You will generally need a license to transmit on most frequencies. The radio spectrum is controlled by the FCC in the United States, and by other government agencies in other countries. The radio spectrum is considered a shared resource, so there are rules and regulations for how it is used.

What is this 18W transceiver that you have seen? It doesn't sound like a HAM rig -- is it GMRS or CB or some other general band radio? If it transmits in the commercial FM band, then you will need a license from the FCC to use it. You can transmit a very low power signal in the commercial FM band, but when you meet those limits, you only get about a 20m-30m range (that's why you're allowed to transmit at such low power -- you don't interfere with anybody else who is trying to listen to a regular broadcast).

Even though you live in the countryside, you still cannot be broadcasting in a licensed band without a license. Your signal can carry a long way, depending on atmospherics and other factors, and can cause harmful interference with other radio signals. The FCC takes a dim view of this (as do HAM radio operators and others skilled in using radios), and there is a good chance that you will get a knock on your door one day, with a funny looking vehicle sitting in your driveway (what my son would call a "nerdmobile").

Having said all that, there are still some ways that you can learn more about radio and do some experimenting. One way is to build a low power FM transmitter kit, to learn more about how transmitters work. You use your regular FM receiver to receive the transmissions from a kit transmitter. You can get a kit at Radio Shack, or online at a number of places:

FMST-100 down the page on the right: http://www.transeltech.com/kits/kits1.html

Another option would be for you to check into local HAM radio clubs, to get a feel for what kinds of things are going on in your area. Morse code is no longer required for getting your HAM radio license, and there are several easy ways to get licensed and start transmitting on voice channels. Your local clubs can show you your options. You won't generally be allowed to transmit music on HAM channels, however (quiz question for the HAMs -- what is the one exception to this rule?). There are some pretty cool things in HAM radio, however, like there are bands where you are allowed to transmit amateur TV.

And on your distance question, with the right atmospheric conditions (varies with sunspot activity), and a good multiple-element Yagi at a reasonable height, you can reach most of the way around the world using skip with medium power like 18W in the lower HF frequency bands (like 10m wavelength, etc.). UHF bands (400MHz+ or so) are mainly line-of sight, and VHF bands (150MHz or so) will bend a bit with the Earth's contour, but generally will not make it over big hills.

SLiM6y
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#4
Jan11-08, 03:15 PM
P: 10

FM Radio Transmission Distance


Thanks you two...

Firstly, I am in Australia and very aware of the laws - I release it is possible I could be some sort of Pirate Radio - but, I believe there is a lower frequency that is capable of being used without license (I am still checking into that).

The system I have looked at has these specifications listed:

Specification:

Frequency range: 88MHz ~ 108 MHz
Tuning Step: 100kHz
Transmit Power: 18 watts
Stability of Frequency: 10ppm?-10?~+50??
Ripple or harmonic waves: <= -60dB
Modulate Frequency error:<=75KHz?100%?
Freq. Response: 100~15000Hz
Antenna Connector: BNC type
Audio Input Connector: 3.5mm headphone connector
Power Supply: 12V DC (The current load of power supply should over 4A)

Unfortunately I don't understand a lot of these descriptives - but I understand what it does :)

I really appreciate all your help with this, I am actually thinking of using it as a school resource as well (a school radio station - this will encourage understanding the physics of transmission for us all hehe).

Anyways, I look forward to learning more.

Thanks.
berkeman
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#5
Jan11-08, 03:29 PM
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Well, in the US, those are exactly the specs for a licensed broadcast FM band transmitter. The 18W power output would be sufficient for a local area station. You can probably reliably hit about a 10km radius on flat ground, with a vertical monopole mounted 10m up a tower.

You should check into getting a license for one of the frequencies (assuming that the Aussie frequencies match up with that US transmitter). It's probably pretty inexpensive for your situation. Let us know what you find out about the license possibilities.
SLiM6y
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#6
Jan11-08, 03:40 PM
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Thanks again berkman...

10km is exactly how far I'd like to try and reach that covers my two main townships.

I have written to the ACMA (Australian Communications and Media Authority) so I will hear back from them soon.

The antennae would probably be about 4 - 6m up on my shed rooftop.

If I got a 50W transmitter does that dramatically improve my signal distance?
berkeman
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#7
Jan11-08, 03:57 PM
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The power increase will give you a corresponding range increase. Think of it in terms of the area of a finite height cylinder, to a first approximation (with an omnidirectional vertical antenna with a low launch angle, like a monopole with tuned radials at the bottom to lower the lobe, or a vertical dipole). The power per unit area ratios with the power, and the area of the finite height cylinder ratios with the square of the radius out from the antenna. So if you quadruple the output power, you can go out to twice the radius and get the same power per unit area. What the receiver really picks up is the antenna receive terminal voltage, however, and the power ratios with the square of the voltage. So the relationship between output power and range ends up being basically linear (again, in the first approximation).

To get the same receive V at twice the R, you need double the power at the transmitter. Alternately, if you have a particular geographic situation, you can shape the horizontal radiation pattern to match the geography, and gain extra range without more transmit power. You use an antenna with gain, or an antenna array, to change your omnidirectional radiation pattern into a lobed pattern, aimed at your target receivers. Like for your 2-town situation, you could put the transmitter and antenna between the cities, and make your pattern mostly aim at the cities. You won't go as far in the orthogonal directions out into the countryside, but your signal for the cities will be stronger.

BTW, here's a fun wikipedia article about antennas: http://en.wikipedia.org/wiki/Antennas
SLiM6y
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#8
Jan11-08, 04:52 PM
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It appears the law is quite simple...

I have till February 6 to apply for one of three frequencies (cost of application AUD$300) then from there I can get a license from $250 (to a maximum of $2000) depending on residential area.

As I live quite rural I think I should be able to get the $250.

A maximum of 10W is permitted for broadcast.

So I could get away with it cheap enough by the looks of things.

I might become a radio station (legal) afterall!

A new question though:

Assuming someone (not me of course) decided to start a pirate radio station and used the airwaves illegally. Let's assume no one ever complained about it. Could the broadcasting agency locate you?

Let's say someone did complain... How then do the broadcasting agencies locate you. Do they really have a nerdmobile with a satalite dish trying to home in on a broadcasting frequency strength?
berkeman
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#9
Jan11-08, 05:15 PM
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Sounds great, SLiM6y! Very cool that you can get a low-cost license in your area.

As for pirate stations that are unlikely to be complained about, there are two considerations. First, you may end up with some random person finally complaining about it, and then there might be a hunt to find the transmitter. This is more likely in an urban environment, and less likely in a friendly outback environment, and obviously depends a bit on how offensive the content of the broadcasts might be to some people.

The far more serious problem comes when there is some insideous side effect of the transmission, like a harmonic interfering with emergency transmissions of police, fire and EMS. Even if the transmitter is within the FM broadcast band and has no spurrious harmonics, if the antenna were relatively hidden (or not obvious) and an emergency vehicle drove by, there might be the potential for interfering with their radio (via overloading, intermodulation, and other obnoxious stuff).

As for the hunters, the government often enlists the help of HAM radio operators to hunt down offending transmitters. We affectionately call these "T-Hunts", and we actually practice them fairly often. Here is a typical group local to me in the Silicon Valley area:

http://www.thunt.org/

The equipment used varies all the way from just your hand-held HAM radio (using a technique called "body fading"), all the way up to some HAM vehicles that are bristling with multiple phased-array antennas and steerable beams. That would be your nerdmobile, but incredibly useful for T-Hunts. The equipment inside those vehicles can rival some electronics labs -- cool stuff.

I actually went on a practice T-Hunt at a recent HAM radio convention, and learned a lot. I expected the transmitter that we were hunting to be a box with an antenna hidden somewhere.... But in the end, it turned out to be hidden inside of a little stuffed bird that looked like it was sleeping on a tree limb about 10 feet off the ground. That took a good long while to hunt down!
SLiM6y
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#10
Jan11-08, 05:28 PM
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HAHA... That's cool berkman.

Well, my plan isn't to be a pirate radio station - but a fully legit one. I have now under a month to get my application in!

It will be very interesting either way.

I appreciate your knowledge on this subject. And even at the least, I plan on incorporating transmission etc into lessons during the 'waves' part of the physics course at my local school. I think the students will gain a lot out of it and seeing an actual transmitter will also encourage them greatly.

Well, once again, thanks for all your knowledge. It's very much appreciated!
berkeman
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Jan11-08, 05:57 PM
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Glad to help -- sounds like it will be a lot of fun for all.

BTW, if you check around to find some local HAM radio operators in your area, I'm sure that they would be glad to lend a hand in setting up your FM station. They are very handy with cables and power supplies and antennas, etc. And they have the equipment (like SWR meters) to be sure that you are getting all that you want out of the setup.

Also, HAMs are generally happy to show off their equipment, and help others to get on the air. You don't need a license to transmit on the HAM bands, as long as a licensed operator is there with you while you talk on the radio. It's pretty neat to see kids get to talk on the radio, sometimes to other continents if the skip is working right. That would make for a pretty fun demo for your class. The HAMs will often be able to bring their mobile rigs to wherever you are (school in the case of your class), and set up anything from a small hand-held walkie-talkie size radio, to a large transmitter with a temporary tower erected. Might be pretty interesting for the kids, especially if it's done on one of the world-wide contest days or other drill days.

Have fun!
berkeman
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#12
Jan11-08, 06:00 PM
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Quote Quote by berkeman View Post
But in the end, it turned out to be hidden inside of a little stuffed bird that looked like it was sleeping on a tree limb about 10 feet off the ground. That took a good long while to hunt down!
BTW, I do have to say that I went on this T-Hunt with one of my Assistant Emergency Coordinators, and she found the transmitter about 10 minutes before I did. She was extremely happy about that part!
Ouabache
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Jan11-08, 10:25 PM
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Quote Quote by berkeman View Post
BTW, I do have to say that I went on this T-Hunt with one of my Assistant Emergency Coordinators, and she found the transmitter about 10 minutes before I did. She was extremely happy about that part!
Were you using a 4-el phased array on your foxhunt-mobile? I learned about the usefulness of attenuators during RDF expeditions. I picked up a commercial grade one for $5 from box under a table, at a hamfest one time. I replaced the burned out components and it works like a charm. It comes in handy when you get in close to a transmitter and the signal appears to be coming from everywhere. You can then, knock down the signal by switching in varying amounts of attenuation, to the point where your directional antenna works again.

SLiM6Y You mention you're allowed a maximum of 10W for your (FM) broadcast station. You may want to find out how much peak envelope power (or PEP), you're allowed. In other words if you transmit 10W and your antenna has a gain of 3dB, your PEP is close to 20W. An antenna with 6dB of gain, your PEP goes up to 40W. I've seen some collinear antenna arrays with 12db of gain. In this example that would translate to 160W (PEP).
berkeman
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Jan14-08, 11:48 AM
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Quote Quote by Ouabache View Post
Were you using a 4-el phased array on your foxhunt-mobile? I learned about the usefulness of attenuators during RDF expeditions. I picked up a commercial grade one for $5 from box under a table, at a hamfest one time. I replaced the burned out components and it works like a charm. It comes in handy when you get in close to a transmitter and the signal appears to be coming from everywhere. You can then, knock down the signal by switching in varying amounts of attenuation, to the point where your directional antenna works again.
Hi Ouabache! I was hoping that you would see this thread. We were just using our handheld 2m transceivers and using body fade. Other T-Hunters in the group had all kinds of directional antennas and attenuator boxes, but my AEC and I were bare bones. It was a walking hunt starting at the convention center, and we had to deal with multipath reflections off of other buildings and chain-link fences, etc.

The closest we got to attenuators was when I bummed a paper clip off of person at a hotel front desk, and I cut it into various length small sections for my AEC and me to use for way-off-resonance antenna stubs for our HTs. When we got near the fox (or bird in this case), we just used the open SMA connectors for our antennas, with no antenna at all.

A directional antenna and switchable attenuation would have made that hunt a lot easier! But it was still fun, and we learned a lot about how 2m radio propagates in areas like that.
rcgldr
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#15
Jan17-08, 02:33 PM
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From what I understand, the FM frequency range you quoted is line of site, so curvature of the Earth is an issue. Distance is a function of how high the transmitting antenna is, combined with sufficient power to receive at the desired range.
drewski22785
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#16
May25-08, 07:33 PM
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I am in need of help with the same question of this original post. I am trying to find out how to calculate the estimated broadcast range of both FM and AM radio stations. I have all of the information that I can imagine necessary to calculate this considering I am pulling some of the data directly from the FCC website. Can anyone help me in learning how to calculate the broadcast range of both FM and AM radio stations? Thank you!
berkeman
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May27-08, 11:15 AM
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Quote Quote by drewski22785 View Post
I am in need of help with the same question of this original post. I am trying to find out how to calculate the estimated broadcast range of both FM and AM radio stations. I have all of the information that I can imagine necessary to calculate this considering I am pulling some of the data directly from the FCC website. Can anyone help me in learning how to calculate the broadcast range of both FM and AM radio stations? Thank you!
Ignoring multipath issues, you should be able to calculate it based on transmit power, TX antenna gain in azimuth, power decrease with distance (what is it?), and receive sensitivity of the RX device. What equations have you been starting to work with so far?
drewski22785
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#18
May27-08, 04:32 PM
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I have not been working with any formulas to this point. I did go through my physics book from college (i took the class about 3 years ago) and found information on calculating wave lengths and such but didnt think they were completely relevant. For starters, the calculations were not specific on radio waves and more directed toward light waves or general wave length calculation. They as well did not take into account factors including line of sight, penetration, transmission power and other areas I felt important. At this point I believe i am lacking any relevant formulas or calculations. I can assure I do have the calculus and physics background to be able to use the necessary formulas. Thank you for your reply!


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