How to build your own LPFM radio station.
Or tune for maximum smoke!
Although this guide is based on New Zealand LPFM conditions and law the technology remains the same. Please do not steal this but feel free to link to it. It is a work in progress so check back. You may also not agree with everything I have written so please feel free to submit corrections or additions. It is based on my experience of 15 years in the broadcast industry as well as building my own LPFM station.
Start by doing some research. The following websites make essential reading
1) Ministry of Economic Development Low Power FM Broadcasting short range devices
2) The Society of Low Power FM Broadcasters Incorporated
Before embarking on building a LPFM radio station you need to ensure there is space available for you on the band. In New Zealand the frequencies that can be used are 88.1, 88.2 88.3, 88.4, 106.7, 106.8, 106.9, 107, 107.1, 107.2. 107.3, 107.4, 107.5, 107.6 and 107.7. Additionally if you transmitter is further than 120km from Auckland’s Skytower you may use 88.5, 88.6 and 88.7. Have a good tune around and drive around the area. Try and choose a frequency 400kHz away from any station you can hear from your proposed transmitter site. Most LPFM broadcasters broadcast from their studio location due to difficulties of getting a stereo link between studio and transmitter. More on this later. Choosing a good transmitter site is probably the most important objective. (This is all likely to change under the new General User Radio License)
Lets assume you are Wayne Anderson , you live in Manurewa and wish is to start your own radio station called Radio Classy from your house in Halver Road. You have had a listen and have chosen 107.1 as your frequency. You plan on erecting an LPFM standard j-pole on the chimney. What sort of coverage would you expect?
In broadcasting we refer to two main signal strengths, 66dBuV/m and 54dBuV/m I could rave on here about what it means (and I might later) but in the mean time accept 66dBuV/m is deemed the primary coverage area. This is the area that high power radio station expects no interference in. I like to explain it as clock radio strength as in your station should be able to be received on a clock radio.
So back to Wayne Anderson and his Radio Classy, Putting in a 4m antenna height into the coverage map software we generate a map of the 54dBuV coverage Wayne would expect out of Radio Classy.
Click to see larger version
Now if Wayne Anderson took a leaf out of “Andy Archer’s” book and swung a 15 metre pole on his house his coverage would look more like this. (check with council bylaws before embarking on an erection)
Click to see larger version
This is NOT rocket science. Golden Rule #1 Height is Right. Radio Classy just got a whole lot classier. One could onl imagine what Wayne could achieve if he had a power pylon in his backyard. I am also guessing that as Wayne lives right opposite a police station that installing a power pylon in the backyard is not necessarily a good idea. I am also quite certain that some of your neighbours may not see the same beauty in a 10 metre pole as you do.
Golden Rule #2 What goes up, must come down. Swinging 10 metre poles on houses should be performed with care and thought. Try to ensure that it will not land on your neighbours Mercedes should a big storm rip it down,
Now the men from the ministry state The maximum peak radiated power must not exceed -3 dBW (500 mW) e.i.r.p. Now this is the type of stuff you expect from the men from the ministry. What the blazes is an e.i.r.p. It reminds me of Jeremy Clarkson trying to describe torque.
“Caution – Technical stuff follows”
The 500mW or -3dBW is the easy bit. 500mW or -3dBW is your good old half a watt. But shoving half a watt up a cable into an antenna does not necessarily equal -3dBW e.i.r.p. So what is an e.i.r.p. It stands for Effective Isotropically Radiated Power and is calculated as e.i.r.p = transmitter power – line losses + antenna gain where the gain of the antenna is measure in dBi (dBi is as compared to an isotropic radiator)
Okay, so lets deal with what we know. You have bought a transmitter of some dude on trademe and it is rated at 500mW. Good start. As we know from above 500mW (1mW= milliwat = 1/1000 of a Watt) so we have the first part sorted.
e.i.r.p. = -3dBW – line losses + antenna gain
So lets discuss line losses. Line loss is the loss in the cable between the transmitter and the air. The cable commonly used is 50 ohm and generally there are several varieties. I will deal with two. RG58 and RG213. or the thin stuff and the thick stuff. This is not TV cable. TV cable is 75 ohm not 50 ohm so keep that for your TV. Cables vary from manufacturer to manufacturer so check the specifications. RG58 has a typical loss per 100 meters of 23dB. 23 dB is a factor of 200 therefore if you put your 500mW into 100 maters of RG58 you would end up with 2.5mW out the spout. Look what that does to Radio Classy.
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RG213 (the thick stuff) by comparison has a typical loss of 6.3 dB per 100 meters. So back to the calculation.Radio Classy needs 10 meters of cable to get up the pole and a further 3 meters of cable to come in the window and to the transmitter. That is 13 meters. With RG58 that = 23dB/100 x 13 = 3dB Golden Rule #3 Understand the maths, a calculator can give you the wrong answer to more insignificant digits that anything else.
We now have e.i.r.p = – 3dBW – 3 dB + antenna gain
Now for the antenna gain. The men from the ministry require you to measure your power against an isotropic radiator. An Isotropic radiator is mathematically the simplest antenna of all. It radiates all power delivered to it in all directions. Brilliant as it is, it can’t actually be built. That said, it is useful to measure against. All antennas gain is measured in dB either against an isotropic radiator dBi or a dipole dBd. So to get back to the formula, we need to add your antenna gain in dBi. A simple dipole os rated as 0dBd or 2.14dBi. So let us assume that you are using a dipole let us add 2.14dBi to the formula.
e.i.r.p = -3dBW -3dB + 2.14dBi = -3.86 dB = 0.412 Watts.
Look,we are now lower than our allowed power level. (And before you start running the scientific calculator to convert dBW to Watts this little website does it for you )
As you can see calculating transmit power is not as easy as you may think. In this case swapping the antenna cable to RG213 will result in
e.i.r.p = -3dBw – (6.3/100 x 13)dB + 2.14 = -0.041dB = .991Watts.
Holy smoke Batman, our 500mW transmitter is now putting out nearly 1 Watt
Before you get carried away we have to match a 50 Ohm transmitter into a dipole and get all the power up the spout. I have never been one for dipoles preferring the folded dipole. The reason I prefer the folded dipole is it’s impedance is 300 ohm and simple coaxial balun is all it takes to match it 50 ohms. Balun stands for BALanced UNbalanced. It takes an unbalanced feed, the coaxial cable, and changes it to a balanced feed where two wires feed the signal 180 degrees out of phase from each other.
Information on making a coaxial balun can be found here. The main objective is to calculate a 1/2 wavelength of coax to make the balun with. Just as a coaxial cable has specification for loss it also has one called velocity factor. Radio waves travel in free space at the speed of light which is approximately 300 million meters per second. Wavelength is the distance in free space a sine wave will complete one 360 degree cycle. Therefore Radio Classy on 107.1MHz (107,100,000 cycles per second will have a wavelength of 300,000,000 divided by 107,100,000 meters = 2.8 meters. Therefore a half wavelength would be 1.4 meters. But wait, velocity factor, Imagine you are riding your bike happily along the footpath and suddenly you ride on to the grass verge. All of a sudden you can’t ride as fast. The velocity factor of the grass has slowed you down. The same thing happens when a radio wave tries to run through coax, it slows down. RG58 has a velocity factor of 0.66. That means instead of hurtling through space at 300,000,000 meters per second it dawdles along at a mere 198,000,000 meters per second. The wavelength in coax of Radio Classy is 1.85 meters, This means a 1/2 wavelength is 0.95 meters which is the length of coax we need to build a balun for Radio Classy.
Universal truth: water wrecks the performance of your coax. Ensure that all your outside terminations are well sealed from water. A role of self amalgamating tape is a must.
Wikipedia decribes SWR as “In telecommunications standing wave ratio (SWR is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line”. it goes on to say The most common case for measuring and examining SWR is when installing and tuning transmitting antennas. Fortunately we can toss the maths aside because some clever people have invented the SWR meter. An SWR meter is a meter that you place between your transmitter and the feed to your antenna. The meter will show both forward and reflected power.In an ideal world all the power goes out and none comes back. The mother of all SWR meters is the Bird 43. Unfortunatly the mother comes at a price, $331 US plus $79 US for the 5B element.
So we now have the Radio Classy 15 metre antenna bolted to the side of Wayne Andersons house. A folded dipole has been mounted on the pole, the 1/2 wave piece of coax has been used as a balun and the feed line is ready to plug into your transmitter.
One of my engineers came up with a principle I named after him. We were discussing firewalls and he explained the three elements cheap – easy and secure. His principle states, if you want secure and easy its not cheap, if you want cheap and secure it is not easy and if you want cheap and easy its not secure.
Now let us replace secure with quality, if you want quality and easy its not cheap, if you want cheap and easy its not quality and if you want quality and cheap, its not easy. Lets call this the Charles’ Principle
Taking this a step further you have to realise that a cheap transmitter is probably just that. I am the kind of guy that works with the quality and cheap is not easy so I have decided to run with NRG Kits My sacrifice to the quality gods while saving my wallet was to spend hours soldering. To add insult to injury I also built two for the Waiheke Community Radio Trust.
Be wary of the cheap transmitters off eBay. The United States use 75uS pre-emphasis and New Zealand uses 50uS. Check that the pre-emphasis is correct before purchasing. Pre-emphasis is a bit like turning up the treble in the transitter and turning it down in the radio to produce the net effect being less hiss. If you run a 75uS transmitter in New Zealand your station will sound tinny. Wikipedia entry on pre-emphasis can be found here.