Published 27 Oct 2014
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SO YOU WANT TO SET UP A LOW POWER FM STATION
What you need in New Zealand

Status at October 2014
Axino-Tech Consulting and Services

The Radio spectrum management (RSM) group of the Ministry of business, innovation and employment (MBIE) sets the specifications for your low power broadcasting operation. It will pay to read both the General User Licence and the 'Additional Information' documents they provide.

OVERVIEW

Anyone may set up a low power FM station (LPFM). These work under an unlicensed regime and are restricted to 1 watt radiated power. Setting up a station is not that difficult, but don't expect it to cost nothing. There are licence limitations although the licence itself is free. You need to decide on your best frequency based on the allowable ones and avoidance of whomever is already broadcasting in your area. You will need to establish a studio with playout equipment, then obtain a transmitter and antenna system. The ministry has fairly tight tolerances of accuracy of frequency, power output and unwanted emissions. These tolerances mean the equipment will cost some money. This guide discusses the necessary transmission hardware but not studio equipment.

FREQUENCIES

The RSM allocates 19 frequencies for low power FM broadcasting. These do not need a licence and are a free-for-all. The list is shown on the Ministry's LPFM General User Licence 2010. There are 8 frequencies available at the bottom edge of the band; 87.6 to 88.3MHz, and also 11 frequencies at the top end; 106.7 to 107.7MHz.

Some of the high end frequencies are not able to be used in approach zones to Wellington and Auckland airports. The ministry has maps of these exclusion zones. Apart from in the exclusion zones mentioned, you can choose any allowable frequency, but it will be prudent to avoid one that is already used in the same area. In fact if there is another station broadcasting in the same vicinity, it will be best to try and choose a frequency at least two channels away. You do not need to apply for a frequency.

The NZ LPFM Radio Station Network database lists all known frequencies in use, but they may not know them all. Users do not have to register, so there may be some stations operating which are not listed. Also, there may be some that have ceased operations but have not been removed from the list. Check it out but take with a grain of salt. Best way is to drive around your proposed coverage area and tune a radio to each allowed frequency to see if there is someone already operating. Sometimes an adjacent channel will 'splatter' into the channels each side. They may be operating too much frequency deviation, but in any case you probably can't do much about that, so avoid the channels each side of an operating station.

Remember the RSM doesn’t police LPFM interference to and from other LPFM operators, but please be socially responsible and avoid disrupting other broadcasters. Remember this: "When stuck in a lifeboat, it is best for all to paddle in the same direction". If you cause interference to a licensed service however, the ministry will be hot on your tail, possibly resulting in confiscation of equipment and monetary fines. The potential for you to cause interference will be reduced if you stay with the rules and obtain quality equipment.

EQUIPMENT

You will need a transmitter, an antenna, some coaxial cable to connect the transmitter to the antenna and of course, a source of programmes.

1. TRANSMITTER

You have to buy or otherwise obtain one. Most transmitters will be able to tune to any of the available frequencies, but better check that it does handle your frequency. You will generally not need more than 1 watt output power. The RSM allows a maximum of 1 watt radiated power. Because the common types of antenna have a small amount of power gain, the transmitter is usually going to be running less than 1 watt, unless the run of coaxial cable is very long. Some transmitters might be capable of more than 1 watt, which is OK because they can always be reduced in power below their maximum. If you are spending money, it could be prudent to get a transmitter of up to 5 watts as an example, and back it off as needed, because one day the ministry might revise their restrictions on power.

You might be able to buy a really cheap transmitter from Ebay, but take care with this. The RSM does have specifications for transmitters; which some of the "cheapies" don't meet. I am not saying avoid Ebay at all costs, but have someone check the specs of a model you are thinking of buying.

You will see LPFM equipment from the USA. Note they have a different definition of LPFM there and may be licensed up to a hundred watts. Their transmitters have to be 'type-certified' to FCC rules Part 15. Because their transmitters can be higher in power, they are more expensive.

The New Zealand LPFM transmitter general requirements are:

Output Power:

1 watt; perhaps buy one capable of up to 5 watts. Power output will be set to suit the combination of your antenna and coax cable, so that no more than 1 watt is radiated.

Frequencies:

Must cover the range of possible frequencies and keep the frequency accurate with a maximum error of 5kHz. The transmitter must automatically shut down (mute) if the set frequency is not correct for any reason. When switched on, most transmitters take a couple of seconds to internally lock and during that time, they must mute.

Stereo:

Preferably buy a stereo transmitter. Even if you decide to stay mono for better coverage, a stereo transmitter is more future-proof. That means getting a transmitter with built-in stereo encoder. You can always switch a stereo transmitter to mono mode. (If you buy a mono transmitter, and later want to go stereo, you can buy an external stereo encoder anyway.)

Unwanted Emissions

The RSM has strict requirements for unwanted radio emissions. That means the transmitter is not allowed to emit frequencies other than the one it is designed to emit. There is a specification for this, which needs a bit of interpretation by a radio engineer, but the bottom line is that it will need to be checked by someone who knows what they are doing. Here is my Proforma unwanted emission mask. This example assumes you run exactly 1 watt radiated power and is here shown on 98MHz which is not a low power allocation, but your mask will be centred on your chosen frequency. The shape is what matters. If you buy a transmitter that doesn't meet this specification and it results in you interfering with a licensed service they can shut you down and fine you quite steeply. If you bought a transmitter and tested it, only to find it doesn’t meet the specifications, then an RF filter will be necessary. That can be designed and built by an RF engineer.

Who makes these transmitters?

There are a few suppliers making low power transmitters. These are the ones I have heard about, but I have not had experience with any of them, so cannot offer a recommendation. I took the following info from the respective websites. I don't guarantee these prices. You will almost certainly have to pay NZ GST on imported models.

Broadcast Warehouse: Model TX1 is 1 watt stereo currently costs US$745 + US$125 for shipping. Model TX5v2 is 5 watts stereo, more sophisticated design. US$1095 + US$125 shipping.

PCS-Electronics: CyberMaxFM+SE 25 watts maximum. EUR$700 + shipping. (About EUR$60). (Can be reduced in power) There may be cheaper options available. Email them.

Aareff Systems: 1 watt stereo for NZ$472.60 includes shipping. A version of the 1 watt including an audio processor is NZ$519.77. 12 watt version including audio processor is NZ$1179.

NRG This supplier went out of business, but the franchise was resumed by a N.Z agent. The NZ website advertises only a mono transmitter (PRO4) for NZ$229 + GST. However, emails have not been answered by the supplier, so it is unlikely that this product can actually be supplied.

Ramsey Their 1 watt basic model (mono only) is US$260 + shipping.

Nexus Their 5 watt stereo model is US$1095 + shipping.

Tennatron Industries Nelson Advertise a 1 watt stereo model for NZ$790 +GST.

Remember, there are many other suppliers around, but this list will give you an idea of the prices from the more common suppliers. As mentioned above, I do not endorse any of them, but could offer advice on whether one appears to meet specifications. I may be able to assist with testing your transmitter and possibly both in assembling a kit or repair, should that should be necessary.

Transmitter PCI cards

PCI transmitter cards which are fitted within your PC are available, but this type of transmitter is not commonly used. First, the transmitter cards may not have a good enough spec to satisfy RSM requirements. Secondly, they draw a reasonable amount of bus power to generate 1 watt or more and cooling is important. Thirdly, strong local RF fields within your PC can play havoc with the PC itself. Despite that, the solution is convenient if it can be made to work, especially considering the transmitter will take audio directly off the digital bus via the Windows mixer. One supplier offering these is pcs-electronics, who have their PCIMAX3000+ card (1 watt version) listed for EUR299.99 presently. There may be other suppliers but be very careful with specifications.

If you only wanted enough power to cover a house, or within a commercial premise, then the PCI card option running much less than 1 watt (typically only 20 milliwatts) looks attractive.

2. THE ANTENNA

There are only two types that are realistic for this purpose. The most common is the half-wave dipole. The other type is the quarter wave ground plane. They both perform very similarly and will give the same coverage, but have to be built for the frequency, so the frequency has to be decided first. Only two variants need exist, since the LPFM frequencies are either in the low end or high end group. One design will cover each group. The antenna must be mounted for vertical polarisation.

Tennatron Industries Nelson offer one for $174 +GST It's a bit pricey. There will be other sources around.

FM dipole pic An FM dipole antenna could possibly be built from an old analogue Band 1 TV antenna dipole if that is in good condition. These are the large antennas that used to receive TV1 in Wellington. With some work, one could be modified to be an FM broadcast dipole like the one in the picture.

The antenna has to be solid to take Wellington winds and the connections have to well weather-proofed. It must be mounted on a boom well away from the metal support pole. (At least 2 metres is recommended.) Each of the two types of antenna mentioned are omni-directional. That is, they radiate equally in all directions in the horizontal plane. If you mount a dipole too close to a metal pipe, the coverage might be reduced in the direction behind the pole and possibly increased in the direction away from the pole. However, you can't make use of this fact to get more coverage because the antenna gain has to be accounted for. 1 watt radiated power means that 1 watt must be the maximum in any direction.

Getting good coverage

Best coverage for the 1 watt radiated power will be achieved with height. The higher the antenna, the better; preferably at the top of a hill without any nearby obstructions. You cannot have unrealistic expectations for coverage. 1 watt is a limitation that means coverage will be a circle of 5km radius give or take. A person might receive your station at more than 10km away if they have a good receiving aerial pointed at your station and they are not behind a high hill.

Mono versus stereo

Broadcasting in mono will give a larger coverage area. However, you might prefer stereo, so then have to accept the limitation of range. In the beginning, FM was mono. When stereo came along, a system was widely adopted that gave stereo reception on stereo receivers but still gave mono reception on mono receivers. (GE/Zenith) The addition of stereo was not allowed to compromise mono reception. All the compromises are to stereo reception.

Most people ask about the reduction in coverage of stereo compared to mono. There is a reduction, but it is not related to a lower RF signal strength. You still broadcast your 1 watt whether in stereo or mono. It is about a reduced signal to noise ratio in stereo.

There are two main reasons why the signal to noise ratio is compromised in stereo. First, to make room for the stereo coded information, the audio left and right channels are effectively halved. Then a small allowance has to be made for the stereo pilot tone. So, the upshot is the left and right audio channels are each modulated at 45% of the level that a mono audio signal would be. So you lose 7dB of signal to noise ratio from that alone. More significantly, the coded stereo information has to suffer more noise because it is placed on a sub-carrier which is above audibility and is subject to a lower modulation index resulting in a poorer signal to noise ratio. Now that is a simplified description of the process, but you can't do much about it anyway. There are variables involved in estimating the total reduction of signal/noise ratio and hence 'range' which go from as little as 15dB up to 40dB. In that worse case your stereo range would only be 1% of your mono range based on a nominal signal to noise ratio. Things are not that bad however, and this is where the design of receivers comes into play. Receivers have the ability to switch to mono automatically. A basic receiver might just switch to mono based on signal strength. That isn’t the best way. Better receivers slowly 'blend' to mono, which is to seamlessly and gradually reduce the stereo separation. So, when the signal is good, the receiver is fully stereo and when the signal is poor, it is in mono. At intermediate points, you get some stereo separation. Most car receivers do 'blend'.

Because of the power restrictions applied to LPFM broadcasts, a better result will be gained from being in mono, but if you do go stereo, your nearby listeners will get good stereo. An acceptable if slightly noisy stereo signal will be available to a greater radius, then further out, a mono signal will be received. For line of sight; that is no obstructions, stereo should be obtainable up to 5km away, if the listener uses a reasonable aerial. In mono, more than 10km should be possible under ideal conditions.

3. COAXIAL CABLE

Coax is what connects your transmitter to the antenna. Coax cables come in various diameters and two different impedances. Your SKY dish and TV antenna will use the 75 ohm impedance cable. Radio transmitters are mostly designed for the 50 ohm impedance types. That means you should get some 50 ohm coax.

The thicker the coax, the lower the signal loss. If you have a short length of coax; say 10 metres or less, then you might as well buy the thinnest and cheapest. Apart from having lower signal loss, thicker coax also has the advantage of being robust so will stand up to the weather better. New Zealand ultraviolet levels play havoc with the plastic used for coax cables, so look for the types with good UV protection.

Common 50 ohm types are:

I prefer the RG213/U for its solidarity in most situations, however any of them can be made to work. Remember that if you have a long run of coax and use the thin sort, you might need more than 1 watt out of your transmitter because the coax will lose some signal.

Who sells coax? There are various suppliers around, but sometimes you can only buy a 100 metre drum, which is usually too much. If your run is over about 40 metres, the drum is probably more cost-effective and you can probably sell the remainder on TradeMe. As an example, I found this site Soundguy.co.nz sells a 10 metre length of RG58/U already with BNC connectors fitted for $48.

How much loss do you get in coax?

For any coax type, loss is proportional to length. 10 metres of RG58 will lose about 1.7dB of signal. 10 metres of RG213 will lose about 0.65dB of signal. You have to know your cable loss to work out your maximum transmitter power. So, here is an example.

Your antenna is a dipole. It has 2.15dB of gain (wrt isotropic). You have 20 metres of RG213 cable. The loss of that 20 metres will be 1.3dB. Therefore the nett gain of your antenna/coax 'system' is 2.15-1.3=0.85dB gain. That 0.85dB gain as a power ratio is 1.22. So, to get 1 watt radiated power, your transmitter will have to be set for 1/1.22 or 820 milli-watts output.

If you have a very long run of thin coax, the loss of the coax will be greater than the 'gain' of the dipole, so you have to use more than 1 watt power out of the transmitter. This is a good reason to buy a transmitter with somewhat more than 1 watt maximum output power.

Connectors for the coax cable:

The common sorts are "BNC", "N-type" and "PL259". The thin coax types usually can be fitted with either type of connector, but the thicker RG213 coax will be either N-type or PL259. The connector usually has to suit the transmitter, but it is not a big deal to get adaptors to match up the various connector types. You might see 'F' connectors used. These are the type used on SKY installations. I would say they are unsuitable for transmitting purposes, but they do exist. If you have a transmitter with these, get an adaptor to turn it into BNC or N type. N-type connectors with RG213/U coax cable are my preference.

4. SOURCE OF PROGRAMMES

This I don’t know much about. The simplest method I have seen is to simply connect a CD player to the transmitter and let rip. However most people will use a computer with appropriate software and a microphone. This is up to you, but there are four things to remember:

I am not familiar with 'studio' setups but can assist with setting up levels and making sure the transmitter is operating correctly. I cannot condone illegal operations and that includes running more than the allowed output power.

APPENDIX: TABLE OF TRANSMITTER SPECIFICATIONS

* Assumes 1 watt EIRP. Further specifications limit bandwidth to 256kHz. See RSM GURL.

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