## Introduction

"What sort of a range can I expect with your FM transmitter?". This is a question I used to be asked quite often. The answer is quite complex, and if you look at the table below, you will see it can also be very surprising.

The answer is that range is not very dependent on the transmitter power, but many other factors, such as:

• Indoors or outdoors?
• Type of transmitter antenna used
• Type of receiver antenna used
• Nearby objects (vehicles, buildings, water-tank, people, etc)
• Terrain, hills and valleys
• Cross-polarised antennas
• transmission bandwidth

## Ideal Conditions

Let us assume that you are using my V8 180mW FM transmitter with my VPOle antenna (opens in a new tab). The transmitter has an output of +22.5dBm. The antenna has a gain of 3dB. Your radio telescopic antenna with a gain of -3dB (a loss). In other words, antenna gains are 0dB.

Let us also assume that you are in a high hot-air baloon at both the transmitter and receiver sites: a "Free-Space Path" (FSP), or "Line Of Sight" LOS). The table below will give you the maximum distance, total pathloss, received signal power in dBm and Volts (RMS).

Table of received signal strength for 100MHz TX output = 180mW (22.5dBm)
100km112.44dB-90dBm1pW7µVHigh quality FM radio (eg. Tecsun)

One small point is that the distance is limited by the "noise floor", which is related to the signal bandwidth. Wide-band FM has a signal bandwidth of 150kHz (±75kHz), so the door is open for loads of noise. If you reduce the bandwidth (eg, AM bandwidth = 4.5kHz), then you will get only 3% of the noise, resulting in an increase in receiver sensitivity. Signal-to-noise ratio increases by 30X. This gives a much greater range, perhaps 5X the distance.

## Practical Distances

Let us now come back down-to-Earth; if the VPOle antenna were used on the roof of a house, 20m above the ground, then the distance to the horizon would be about 16km. If the receiver was on the roof of a building then it would be easy to achieve 25km, assuming there are no obstruction. If the receiver was at ground level, between buildings, then you could expect a 20dB reduction, or only about 7km. Now use a cheap US\$2.5 FM radio from E-Bay and the range could be as little as 2.5km.

Look at this in graphic form:

Graph showing received signal (negative dBm) and distance, for different power levels.

## Conclusion

So a little 200mW transmitter can give quite a decent coverage if you wanted to cover your local neighbourhood. Even my little 10mW FM V5 bug will give you 100m, but extended to 3km or more under ideal conditions with a typical domestic radio. I have not tried it, but my new Tecsun PL330 should receive the V5 for up to 10km, or more" under ideal conditions", and with a 10dB gain antenna.

The "hypothetical" scenario I gave above, with extremely high antennas, is not hypothetical for aviation. If you spew out 100mW and you are getting 5km distance with your radio receiver at ground level, then be aware that aircraft fly at typically 10km high, so any spurious output could cause serious interference over very long distances. The VHF aircraft band is 108-137MHz, which is right beside the VHF FM band.

Be aware that is is illegal in most countries to set up a radio transmitting station. In Spain, for example, all local radio stations are licensed and all are effectively pirates. If you operate sensibly in Spain, then the authorities turn a blind eye, and even help you (unofficially) to avoid interference by choice of frequency. In the United kingdom, however, you can expect heafty fines and confiscation of all your transmitting and test equipment. In some countries you could expect to be carted away, never to be seen again.

I hope that this information has given you some "food for thought". You can always e-mail me at harry.lythall@[my domain].com. You can even use oeieio@hotmail.com or hotmail@sm0vpo.com as they are both valid e-mail accounts for me 😉 although I would prefer that you visit my messageboard if you have any questions about this or any other project. I always look forward to receiving feedback, positive or negative ☺

Very best regards from Harry - SM0VPO