FM Broadcasting

If you are in the US, you might want to get ahold of the FCC...

Here, you can't just make your own radio station, there are a list of rules and regulations a mile long, and growing.

Hmm, a nice dipole array on a 50 foot tower fed with about 50watts would cover it. Put the tower on the highest hill you can find, and put the transmitter in a shack at the foot of the tower. Supply mains power and a land line to send the audio to it.

I'm assuming you intend to broadcast in the VHF FM band (88-108Mhz).

In most civilised countries there are regulatory bodies to be involved with. With whom you will need to hand over fistfulls of currency both over and under the table. And thats before you think about a catchy station logo....

FM stations are usually measured by effective radiated power, which is a function of transmitter power and antenna gain. The station I'm associated with is allowed 50 Kilowatts of ERP, supplied by a 10 Kilowatt transmitter into an antenna with a small amount of gain mainly from vertical plane directionality.

The second limit on distance is the height of the transmitter and receiver above the ground, the higher the better.

Broadcast authorities will usually specify the allowed ERP and any horizontal directional variations in power to allow a station to cover its designated listening area and no more.

Our 50 Kilowatt signal can be received in some directions over 100 Km away.

Peter.

RVZ717,Tobugrynbak, plbplb thank you very much for all the information.
well this is for an assignment I'm working on so it doesn't matter about the regulatory bodies involved. i need the technical side involved.
what about the antenna?? and what are the other factors and technologies i have to consider??

It's unlikely that anyone would know what "RVZ717,Tobugrynbak, plbplb" is. I haven't a clue. Whereever you are referencing to, you need to check with your communication law/regulations.

Topography is a likely issue that you need to address primarily for elevation of whatever antenna. Certain antennas can be purchased, but what is your budget? We can't guess here, although some do that only serves to add confusion.

As this is a "Theoretical" radio station, I'll throw in some points of consideration. Depending on what the topography of the area (in your 100km radius) in question is, it will definitely determine the propagation qualities of the signal.

As the FM broadcast band is above the MUF (Maximum Usable Frequency for skipping a signal on the ionosphere) it tends to require good line of sight for best reception. Though it will reflect and bounce around a bit but sometimes to its detriment.

As an example,

Adelaide South Australia is a very flat city with a single hill range to its north east. A transmission tower placed on this ridge line will effectively cover the entire city and suburbs with minimal effort. A courier company I did work for had one 25Watt vhf (75Mhz) base station which had good coverage of the city and suburbs.

Where as in Sydney NSW (80km by 90km is the metropolitan area)it is a city of rolling hills within the basin of the mountains to the west. To adequately cover Sydney metropolitan area you need a constellation of two way base stations, or in the case of broadcast quality FM radio, a large tower, on the tallest hill not already developed with housing and several if not 100's of Kilowatts.

As this is a theoretical exercise you could develop your own best case environment of a tall hill surrounded all round by gently sloping ground rolling away into the distance. With low rise commercial and residential development. As long as its ideal you won't need to provide environmental impact statements for the development of the tower on the hill, or deal with NIMBY's (not in my back yard) complaining that the tower looks ugly and spoils the natural ambiance. Also for consideration is the effect of radiation in the the immediate area, a lot of people get concerned about having your radio signal lighting up the fillings in their backteeth. Thinking ideal in this situation, 100ft tower, a ganged dipole array of 8 vertically polarised elements, about 5 to 10Kw worth of RF should be sufficient to provide good coverage

As one of the other respondents pointed out your radio station would be granted an operating license with very strict guidelines as to its power output and frequency by the relevant authorities. In Australia a commercial broadcast license is acquired by auction. You bid for the privilege of applying for it. Then you have to prove you are capable of funding it. Then comes the arguments with local authorities for where you want to put it. The BS you have to deal with is a much greater challenge than the actual effort in adding RF to the ether.

There are a few "community" licenses around but they are extremely restricted in the performance of both the transmitter and revenue generation of the station. The process for gaining a community license is even more political than for a commercial license.

Hope that helps you find some more questions..

well im having more doubts about the type of antenna to be used. the type of polarization... it was said that an antenna of vertically polarized can be used for this purpose.. but

http://books.google.com/books?id=DSHSqWQXm3oC&pg=PA1563&dq=FM+broadcasting+antenna#PPA1563,M1

please help me out here

My understanding is that by using a vertical polarised antenna you will have the better omnidirectional propagation, especially to "mobile" targets ie cars. Horizontal polarised transmitting antenna will provide better propagation to fixed targets ie homes at the "boundary" of your 100Km limit. As long as those homes have a horizontally polarised recieving (typically a Yagi) antenna on the roof.

The joy of using phased stacked array dipoles is that you can do both. That is you can have some dipoles vertically polarised and some horizontally polarised.

Antennas and rf propogation are still a black art even in this century but you won't go to hell by indulging in it unless you hang onto an antenna that is arc'd up.

what about the antenna height ?? this too is a major concern

The rule of thumb is the higher the better, hence siting the tower on a hill, once you get over a certain height the physical size of the tower may become an issue. You could create a signal shadow under the tower because of the height.

If you go really tall then you will get significant losses of signal on the cable between the antenna and transmitter. That depends on what cable you run. I.E. if you run RG58 (which is cheap usually used in mobile installations) you will lose a lot of signal over a run of 30 metres. Where as LDF550, a run of 30 metres the loss is negligible in comparison. Its also a lot dearer, the connectors alone are over a $100 an end (woe betide the apprentice that stuffs one of those...).

If you are looking to actually do this there are various companies around the world who do this stuff on a consultancy basis, you would be best advised to employ them.

This is where like most things in life, fun cost money, how hard do you want to laugh...

As most FM broadcast listeners use vertically polarised antennas it makes sense that the TX station should also use an omnidirectional vertically polarised antenna but it would be far better to use a single antenna that has some genuine gain over a dipole than to use an array of stacked dipoles that may have none at all.

The afformentioned Dipole antennas are really just reference antennas ie: Their performance is normally used as a fixed reference point from which to guage the performance of other antennas. Gain over a Dipole is measured in "dBd" and an antenna with 2dBd will "put out" double the power than a dipole with the same input power in Watts.

Single vertical Colinear antennas can have gains of around 8-10dBd but they are not as easy to build as dipoles and suitable materials for their construction become much more expensive, especially when they have to take umpteen KWs of power.

A good alternative would be the "Slim Jim" antenna, which is both cheap and simple to build yet has around 3dBd gain. Its a very popular antenna for the 2m amateur radio band (144-146mhz) and with proper consideration of potential flashover problems when dealing with xx KW inputs, construction should still be a breeze...Like dipoles they can be arrayed and co-phased for even greater gains. It would only need to be about 2.25m (7' 4 1/2") long for the 3m FM broadcast band and they can be made from easily available copper plumbing tubing.

Alf (G7VGG)

Someone help me on this,

I know we use moltiple Dipole Antennas in FM Radio Broadcasting dipole just to increase GAIN,why cant we use a single antenna with higher GAIN?and if each dipole antenna has a GAIN of 2.15dBi,what will be TOTAL GAIN for 8 antennas mounted for transmission?can someone give me formular for this?