What will it take to build a DIY 2km (city conditions) RF transmitter (433Mhz)?

Question

This is my first question on this site. I am not an electronics/electrical engineer so pardon my depth of knowledge.

I am trying to build an RF transmitter/receiver. These are my end goals :

• a text (bytes) transmission. (Voice not required)
• 1.5 - 2km range (Line of sight independent, in city conditions)
• as short antenna as possible
• as small circuit in size as possible
• use of free unlicensed spectrum

after reading a bit I figured out 433Mhz is one of the most used for such purposes but I am not sure if it checks all the above conditions.

Qs :

1. Am I using the right frequency or there are other options?

2. How much battery power will be required to send the signal upto 2 kms? Is it even possible to broadcast that far? I am ok if the the whole battery gets drained in 1 or 2 transmissions (don't ask why :)) but the battery has to be really small to make the whole circuit small.

3. How big will the transmitter antenna be if all the above are achievable?

A desired ideal solution would be something like remote car-door lock/unlock circuit with a greater range and no line of sight limitation.

I'll be so thankful to you guys if you can provide me these answers and any pointers for circuits or similar projects on the internet.

P.S. : I am located in India (for license reference)

Answer 1

The Friis transmission equation is usually a good start. In dB form, the loss of power between isotropic transmit and receive antennas in free space is: -

Loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)

Where f is in MHz and d is in kilometres. This equation tells you how many dB of power loss you can expect at a given distance with a given carrier frequency.

At 433MHz and 2km the loss is 32.45 dB + 52.7 dB + 6.02 dB = 91.2 dB.

But this is in free space (the perfect environment) and in a town you could easily add another 40dB to the losses taking you to about 131 dB of losses.

What does the receiver need?

The receiver power required (at ambient temeperatures) is -154 dBm + 10\$log_{10}\$(Data Rate) dBm.

This is generally accepted as a good rule of thumb for decent BERs. So if your data-rate is 100 bits per second, the receiver sensitivity needs to be -134 dBm.

Given that your transmission loss is going to be about 131 dB, you can estimate your transmit power as -3 dBm. That's 0.5 mW in real numbers to stand reasonable success of delivering 100 bits per second across 2 km of town-like environment.

If you use dipole antennas you get about 1.7 dB of gain because, unlike the theoretical isotropic antenna (that transmits power equally in all directions), the dipole only transmits perpendicular to the rise of the antenna. Clearly more power is therefore emitted in this direction so you can use the concept of antenna gain to improve the transmission.

Am I using the right frequency or there are other options?

Lower is better in terms of free space (examine the formula) but around town I wouldn't go lower than 80 MHz because penetration between and around buildings can be problematic - think of 1 MHz AM radio as you drive into a tunnel - it dies straight away whereas the standard FM band of about 100 MHz gets much further - it's a wavelength thing. The bigger wavelength doesn't fit down the hole so easily!

How big will the transmitter antenna be if all the above are achievable?

Use a quarter wave dipole - length at 300 MHz is 25cm. At 433 MHz it's about 17 cm. Lower frequencies require proportionally longer antennas.

The big problem with what you are trying to achieve is that every man and his dog will likely be using 433 MHz and across a distance of 2 km this can cause a mega-serious interference so, I'd immediately want to improve my transmission chances by boosting the output power by 30 dB (that would be about 500 mW of output power) or using directional antennas like yagis.

I think you have enough info in this answer to start figuring stuff out now.

Answer 2

Something that transmits enough power to be picked up 2 km away is going to require a license in most cases. You don't provide the all-important information on where you are, but here in the US you could possibly do this with CB, around 27 MHz. That's what old fashioned "walkie talkies" use. It used to be you could put out up to 5 W, but of course there were various rules to follow, and the signal had to be ordinary voice. I sortof remember CB has undergone some legal changes in the 40 years since I last looked at this. I don't know if it is still available, or what the restrictions might be if it is.

The ISM band at 434 MHz doesn't require a license with restrictions on power and duty cycle for unattended operation. There are other ISM bands that are license free, but again, power will be severly restricted. This is what devices like garage door openers use. They are only intended to work up to a few 10s of meters.

Another issue is that these are often actually military bands that the public can use at their own risk and with low power. There was a case in North Carolina (?) where a bunch of garage door openers stopped working because a nearby military base was using the frequency space.

Stop and actually think about what you are asking for. If everyone could build transmitters powerful enough to reach 2 km, then we'd have a mess. If all your neighbors did the same thing you are trying to do, none of you would have anything useful. This is exactly why there are regulations on the use of the RF spectrum.

Answer 3

In my opnion as an Arduino enthusiast, I would use an Xbee with an Arduino to build a working prototype. The Xbee line will communicate within a range as far as 40km on the 900Mhz band, depending on the specific model.

For guide which Xbee will work for you, see here

You can check with your authority on their management of this band, but a quick look on Wiki shows that this band is allocated to amateurs in a number of countries.

You can easily build a quick prototype using an Xbee of choice with an Arduino (UNO for example). When it is working as expected, you can shrink the size of the product by using an Arduino Mini, or make it even smaller by using a stand-alone Arduino (just Google "stand-alone Arduino").