FAQ FREQUENTLY ASKED QUESTIONS

There is not an easy way to combine transmitter outputs together, it is complicated. If you simply join the outputs together the sum and difference frequencies are produced in addition to the original frequencies.

For example if you have a transmitter at 100 MHz and another at 107 MHz, then you join them together, the result will be the originals at 100MHz and 107MHz plus 107-100=7MHz, 100+107=207MHz, 100-7=93MHz, 107+7=114MHz, there will also be more, but these are the main ones. These are called Intermodulation Products or IP's. For this reason you cannot simply join transmitter outputs together as it causes interference to other users of the spectrum.

In high power installations, more than a few watts, passive combiners are used, these are basically huge narrow band filters that stop the unwanted frequencies entering the transmitters output stage and prevent the IP's.

At the 100mW level it is better to combine through resistive attenuators of about 30dB, then use ultra linear amplification of 30dB to get back to 100mW. This is not so easy, ultra linear amplifiers are more difficult to make.

The lowest cost way to operate multiple transmitters is to place some distance between the antennas. In order to keep the IP's below 60dB (close to the international agreed limit) you need about 50dB of isolation between the antennas, this is about 11 metres.

If you use 1W transmitters (these are nearly the same price as the 100mW) with a 10dB resistive attenuator at the output (this is very low cost) this will then give 100mW to the antenna. because there is an extra 10dB at the transmitter output, only 40dB of isolation is required between the antennas to keep the IP's below 60dB, this is a distance of 6 metres. This is without the a doubt the best value for money solution and probably feasible in most situations such as a gym, health club, a station, multi channel translation etc.

We have spent years trying to find maintenance engineers on the ground in Africa and the Middle East. Some engineers are really good, others you wouldn't let them change the light bulb, they were so bad. Basically it's near impossible to standardise a maintenance process. The only other option is send a team of engineers from the UK or US to work out there, but this is an very expensive option and is not cost viable for most of the community groups and NGOs we deal with.

We have been supplying transmitters to the Middle East and Africa for over 17 years and during this time we have optimised the design and service of our equipment for this situation. For example a medium size community radio station in Africa is typically 500W ERP and would use the following equipment:

- 1W FM Stereo Driver

- 200W FM Amplifier

- 20m of Antenna Cable

- 2 Way Stacked Dipole Antenna

INSTALLATION

The equipment is designed in such a way that a semi-skilled engineer, electrical contractor or electrician can install the system successfully if they read the installation instructions properly. The installation does not need a radio engineer. We have proven this to be effective over many years.

MAINTENANCE

Looking at the four pieces of equipment in the above list, the 1W FM Stereo Driver, 20m of Antenna Cable and the 2 Way Stacked Dipole Antenna are ultra reliable and will run trouble free without maintenance usually for 5 years.

Most transmitter manufacturers have the driver and amplifier as a combined unit, we have separated them into two pieces of equipment. This is because the most vulnerable part in the list is the FM Amplifier, in this case 200W. The downside to the FM Amplifier is that has cooling fans, air intakes, high internal voltages and runs hot. This mixed with the dust, sand and often salty air from the coastal areas is a challenge. The upside to the FM Amplifier is that is is easy to change only 3 connections, takes less than 5 mins, anybody can change it, even the DJ in a one man station.

We advise clients at minimum to buy a second additional FM Amplifier, even a third if the budget allows, then as part of simple maintenance program change the FM Amplifier every month to the second one and the following month back to the first and so on. The moment one FM Amplifier is taken out of service it is serviced according to our instructions. This is a brief explanation of what happens, the cover is taken off, the sand and dust is cleaned out and some of the components are visually checked for heating and corrosion. If all this is okay, the lid is replaced and the it is ready to go back into service at the scheduled monthly FM Amplifier change. If it is not okay then the clients contact us, we advise them on what they need to do to correct the problems. Depending on the exact problem and the skill of the local technician or engineer more often than not any problem can be corrected on site, if this is not the case then the FM Amplifier is returned to us by DHL, we resolve the problem and re-ship back for the scheduled monthly FM Amplifier change. The main point here is that the station stays on air operating reliably.

We have developed this process or system over the last 17 years specifically for Africa and the Middle East and we believe without a doubt it is way more most cost effective than having engineers on site.