# Can i make my own radar

In forums and blogs, the question is sometimes asked what needs to be considered when building a radar device. Often, adventurous constructions are mentioned with a satellite dish and a magnetron from a microwave, but they don't have the slightest chance of functionality.

Building your own radar, which can even be operated legally, is easier than you might think.
But in general the following applies: Steer clear of microwave magnetrons! In general, for safety reasons, one should limit oneself to semiconductor assemblies, if only because these have a much lower risk potential with their moderate operating voltage.

##### With what power does the transmitter have to be able to transmit in order to achieve a range of around 2 to 3 kilometers?

Two conditions apply to the range of a radar: on the one hand there is a time condition that must ensure that sufficient reception time is available for the necessary transit time of the electromagnetic waves; as well as an energetic condition that states that for the losses on the way there, during the reflection and on the way back, the budget of the transmission power must be large enough to still receive an evaluable echo signal at the receiver.

If you decide on pulse radar, then the pulse power according to the basic radar equation for this distance must be roughly 5 to a maximum of 10 watts and the pulse repetition frequency has values ​​of 1 to a maximum of 10 kHz, which is by far for clear measurements in this range sufficient.

If you opt for an FMCW radar, a transmission power of 10 to 30 mW is completely sufficient. Instead of the pulse repetition frequency, the burst duration (that is the duration of a measurement cycle, i.e. the duration of the linear increase in frequency) has a similar function. However, since this is usually in the millisecond range, no ambiguous range measurements are to be expected.

The fact that the FMCW radar goes further with less power than the pulse radar is due to the fact that it is not the pulse power that actually affects the range, but the energy content of the transmitted pulses. So here you can at most compare the average power of the impulse radar with the power of the FMCW radar.

There are several approved frequency bands specifically for measurement technology (and radar is measurement technology). Above all, the frequency 24.125 GHz in the K band is known, for which a large number of radar modules for handicraft purposes are already available. These frequency releases by the Federal Network Agency are generally also limited in terms of the maximum transmission power.

Projects such as the UWB radar, which only emits a few extremely short impulses that can hardly be distinguished from the natural ambient noise at a greater distance, are open. Since the Federal Network Agency cannot even measure such additional emissions of noise pulses, it cannot claim that they would interfere anywhere: That is why UWB radars with a limited pulse power have generally been approved by ordinance.

However, UWB radar is not recommended for a hobbyist with little experience in the high frequency range. There is more and faster success with the FMCW radar modules available in stores. They already contain the transmitter, the receiver and the antenna radiator and only need to be connected to low-frequency components. But be careful! Many of the very cheap modules are only suitable as CW radar, they cannot be modulated. With this you can usually not measure a distance. As a rule of thumb, it can be said that if the radar module only has three connection pins, then it is guaranteed not to be suitable as an FMCW radar.

##### Can I also use an old satellite dish as an antenna for the radar module?

Generally: yes. Since the satellite dishes are calculated for a frequency of up to 13 GHz, they are theoretically much more effective in the 24 GHz range than when operated on a satellite receiving system. You just have to make sure that the reflector is not deformed, otherwise this gain will be lost again. So trying to give the mirror a radar-typical design with a flex does not work.

When aligning, it must also be taken into account that these dishes should actually receive from a direction of about 27 °… 30 ° from above. For a horizontal radiation of the radar, this dish must be swiveled forward by this angle.

An old LNB with a neck diameter of 40 mm can easily be cannibalized and a simple radar module with only two patch antennas glued into the housing behind the horn antenna so that these patch antennas are right in the middle of the round waveguide section. This is not very effective because two patch antennas have to share the center and neither of them is right in the center. However, this construction has the advantage that the old LNB housing offers a good (original) hold in exactly the right place and is splash-proof.

However, the formal and legal side of this construction can be problematic. Even a very small satellite dish with a diameter of only 60 cm has an antenna gain of up to 36 dB, that is 4000 times as much. Taken together, all losses may not be sufficient to reliably fall below the Federal Network Agency's requirement for a maximum permissible EIRP (Effective Isotropic Radiated Power, that is transmission power multiplied by the antenna gain). But here, too, you can make do: if such a case should occur, then a small piece of foam polystyrene can be clamped into the LNB housing directly in front of the transmitting antenna group, the minimal damping of which is completely sufficient for this purpose.