Community Radio Frequently Asked Questions and Answers



How do I get into radio in the UK without becoming a pirate?

The UK Radio Authority (set up as a result of the Broadcasting Act 1990 to license and regulate all independent radio services) can explain the legal route to establishing a radio station. Their website can provide further information.


How far will my signal go?

FM is for all practical reasons line of sight. Your signal should go as far as the horizon (given enough transmitter power to get there). In real terms the background noise and other stations signals will prevent reaching the horizon. Generally if you can see somewhere from your transmitter sight (where the antenna is mounted) you should get there. How strong you are when you get there is down to the transmitter power (assuming you are broadcasting on a clear frequency).

1 WATT
1 watt usually gives a coverage area of a few miles, more if a high mast or mast on a tall building or hill is used
 
20 WATTS
20 watts will give a strong signal for over 5-10 miles when operated in a typical environment, (flat land, 50 foot mast, no hills or built-up buildings).  With a poorly chosen transmitter site (bottom of a hill) you may only get out a few miles. With a 50 foot mast located on a hill overlooking the intended coverage zone, you could expect the signal to be reasonable at distances exceeding 25 miles.
 
100 WATTS
When a stronger signal is required, 100 watts is better than 20 (obviously). The extra power will allow the signal to penetrate more effectively. Once again 100 watts on a 15 foot mast (flat land scenario) may only be effective for 5 miles. Raise that antenna height to150 foot and the coverage could be increased to 50 miles.

CONCLUSION

The most important thing to how far you can go is how high you can get your transmitters antenna and how high the land you can get the mast on.

  1. ALWAYS GO FOR THE TALLEST MAST YOU CAN.
  2. ALWAYS TRY AND GET THE MAST ON THE TALLEST BUILDING YOU CAN.
  3. ALWAYS TRY AND USE A BUILDING ON THE HIGHEST LAND POINT YOU CAN FIND.
  4. USE THE LARGEST TRANSMITTER POWER THAT YOUR BUDGET WILL ALLOW

Under typical geographical conditions (flat land, 50 foot mast) to cover the following distances the transmitters shown are required.

up to 2 miles-------------1 watt
up to 10 miles----------20 watts
up to 25 miles---------100 watts
more than 25 miles----300 watts
more than 35 miles----500 watts

The above ratings are only for comparison, for example 40 miles can be achieved with a good transmitter site with only 20 watts of power. The mileage shown is for a reasonable signal on a car stereo, if the receiver has a good directional antenna then the transmitter may be heard at a greater distance.

Posted by info from Broadcast Warehouse on 15 Nov 1998


Using Compressors and Limiters

For low power microbroadcasting a compressor/limiter can make the world of difference to the apparent strength of the received signal. The higher average modulation obtained masks the background noise that comes with the signal getting weaker, giving the appearance of a stronger signal..

This is even more noticeable when running in stereo, as the hiss that is there when the signal is weak is not so noticeable when running higher average modulation.

If you ran a 30 watt station and a 100 watt station from the same mast on clear channels and set both stations at 75 KHz deviation on the peaks of the audio, but ran the 20 watt through a processor you could find the 20 watt gives out a stronger sounding signal than the 100 watt even when the 20 watt has a lower measured signal strength. With the peaks of the 100 watt only allowing, say an average deviation of about 30 KHz (to account for the peaks not allowed to exceed 75KHz) you would have a much lower signal to noise ratio than the processed 20 watt transmitter which may have have an average modulation level of 50-65 KHz (more like 70 KHz if you are using an optimod, omnia or other high end broadcast processor). Some stations peak deviation indicators appear static when viewed from 10 foot away indicating just how much the audio (modulation) remains near to the maximum.

A peak clipper allows the average modulation level to be raised at the cost of distortion, both to the audio and the introduction of out of band harmonics. That is why more modern limiters and stereo encoders put the peak clipping process into the audio filtering section, commonly known as overshoot compensation.

The real world tests for this sort of thing are not scientific and do not need any test equipment, other than a good car stereo and antenna that you have used for a while and know well. All the equipment in the world does not mean anything compared to "I can hear it at my mates' house 30 miles away on his clock radio and in the car all the way there".

Many an engineer in the UK will judge a signal BY THE WAY IT GETS OUT - if the signal does not break up under a certain bridge or when in front of that building where others do. Usually you can tell if a signal is going the distance just by the fact that with the antenna down 5 miles away you only get a flutter when you past that legal mast on the top the hill (I have used a local example to me).

What I am trying to say is there is no golden rule to judge strength etc except the ear listening to your normal car radio that you use everyday.  While the battle for maximum volume may be a commercial power battle between stations, you cannot avoid the fact increasing the average modulation does make the signal sound stronger. I think this is probably 40 percent of the reason the commercials want the volume and the other 60 percent is down to wanting to sound louder than the same music playing competitor. For microbroadcasters, even if you do not care about sounding loud think about the fact that your signal will be improved by the addition of a limiter/compressor (as long as it is set up correctly). The only drawback is the loss of dynamic range that occurs through compression. I think this is made up for by the fact that your signal is more listenable at a greater distance.

The above does assume that you are not overmodulating, nearly all of you without limiters will probably be overmodulating (exceeding 75 KHz peak deviation). You can obtain the extra loudness by overmodulating but that is a whole new argument and for one you are breaking the broadcasting regulations (did I say that?). New problems are introduced by overmodulating including exceeding the receivers IF bandwidth, your sound will sound great under the site and locally, and sound very loud but as your signal gets a little more conservative a few miles away, you will get distortion as your modulation (audio) gets hacked up by the filters in the listeners' radio. Some cheap radios have very poor filtering and that is why you can make your station sound very loud if setting up your levels on one, but on a decent tuner it will sound awful. Always set up your levels if you do not have a limiter on a good tuner, preferably one with a narrow filter setting. If you do not sound distorted on a good tuner set to narrow you won't be far off, well under 140 KHz or so deviation.  I have heard stations that have taken up 400 KHz of bandwidth that have been unlistenable on any of my tuners, but their engineer swears blind that it sounds crystal clear on their ghetto blaster in their studio.

Posted by info from Broadcast Warehouse on 1 Aug 1998


How do I set up my limiter and stereo encoder?

Here's how to set-up the system (this assumes the user/tester has no test equipment).   If test equipment is present set point 3 to 6.75 KHz deviation and point 7 to 75 KHz deviation.

  1. Turn all audio to the transmitter off
  2. Adjust the volume control (deviation control) on the exciter until the stereo pilot light goes off on your tuner. Most tuners stereo light turns off at about 3 KHz of deviation of the transmitter by the 19 KHz pilot.
  3. Turn the level of the exciter back up very slowly until the pilot comes back on and then carry on a very little bit more past the on point, only a little mind. Bearing in mind the pilot should be back on by 4 KHz and we want 6-7 KHz. The volume level of the PLL board will probably be still way way under one quarter of the way round.
  4. Now the level of the stereo coder to exciter ratio is set, let's move to the limiter to coder ratio.
  5. Reapply audio to the limiter and set your mixer or audio source at the correct level. I will assume that you want a loud commercial sound, so we will use a slight bit of compression at all times. We can achieve this by adjusting the preset controls of the Veronica limiters until the 2nd led on the limiters is flashing indicating limiting/compression.
  6. So now the limiter is limiting, the coder to exciter is set, all is left is to adjust the limiter to stereo coder levels.
  7. Tune to some commercial stations and get an idea of the sound level (volume). Tune to your station and adjust the input pots equally on the stereo coder until your station has practically the same volume. Slightly under would be a good idea as they are using expensive processors to achieve the volume, you are not.
  8. If you have followed this step by step you should be able to push your audio source right up without overmodulation or splashing.

Posted by info from Broadcast Warehouse on 1 Nov 1998


How to Keep your Final RF Power Device Alive

  1. Using a rugged device. Some are better than others. In the MRF174 PAs I have made, I never lost one to bad VSWRs (or any other reason for that matter).
  2. Don't run at the max rated output power of the device. Back off by 25%, 33% or 50%. I run my MRF174 at 100W (125W device).
  3. Make sure the device is running efficient (65 to 70% min DC to RF efficiency). Tune for efficiency rather than max output power or gain.
  4. Lower the supply voltage. This is very effective - running a 28V device at 24 or 18V volts will greatly increase its' chances of surviving a severe mismatch.
  5. Make sure the device is adequately cooled - big heatsink with a big fan, plus a thermal cut-out on the supply if things get too hot for any reason.
  6. Also very important - use a current limited power supply, and set the current limit, so if you're drawing 5A at 28V, set the limit for 6A. This is easy to implement with a 723 based PSU.

Posted by Marconi from How to be a Community Radio Station on 27 Aug 1998


What are the European Broadcast Specifications?

These standards are set by ETSI, the European Telecommunication Standards body.  The document in question is grandly titled

Radio broadcasting systems; Very High Frequency (VHF), frequency modulated, sound broadcasting transmitters (ETS 300 384) Jan 1995

and is available here in PDF format.   Another document that may be of interest is

Radio Equipment and Systems (RES); ElectroMagnetic Compatibility (EMC) standard for VHF FM broadcasting transmitters (ETS 300 447) March 1997


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