Generally speaking, with power levels being the same, you will achieve longer ranges at lower frequencies.Originally Posted by SayCheeze
The primary downside of lower frequencies is of course, antenna length, which can be a bit of a pain on a smaller aircraft where trailing an antenna can produce mixed results depending on it's drag and stiffness, is it vertical or horizontal, well that depends on air speed and the drag of the antenna, which can be quite a variable.
Now as frequencies move up into the microwave region 2.4 GHz+, atmospheric absorption from moisture, metal objects, RF absorbing building materials, foliage reflection and absorption and rain fade all begin to become problems.
Absorption is basically turning the RF into heat and reflections produce multipath.
Having your intended radiation converted to heat reduces the amount of RF reaching your receive antenna, which as you can imagine, is undesirable.
Multipath from reflections on the other hand, can result in multiple signals of the same frequency being present at the receive antenna, the direct and desired signal and the reflections which can mix with the desired signal and depending on the phase of the reflected signal or signals can degrade the desired signal.
An extreme example would be if the reflected signal is exactly 180 degrees out of phase of the desired signal, as such it will mix with the desired signal and reduce it by the signal strength of the reflection.
Now in the real world, with model aircraft or anything else in motion this 180 degree phase shift seldom happens or remains at 180 degrees, but reflections of less than 180 can still distort the desired signal and with video cause dropouts or loss of synch and with data cause a loss of signal or a collapsing of the constellation, resulting in a lowered throughput and or excessive errors, errored seconds, severely errored seconds and it gets bad enough loss of framing.
Now reflections require the reflector to be larger than the wavelength, which is why reflections are not normally not a problem at 72 MHz, but at say 2.4 GHz, the wavelength is considerably much smaller increasing the likely hood the number of objects that can become reflectors will be present.
Now there are some workarounds such as circular polarization, but that is only truly effective if the reflection is fully reversed, but again in the real world, that is seldom the case, as reflections are not always 90 degrees and therefore not a true reversal of polarity, so multipath can still be a problem even with circular polarization.
And there are two other factors which come into play at microwave frequencies, those being diffraction which is where the signal encounters the edge of an object it cannot penetrate, and scattering which is where the signal hits an object the size of a wavelength or less and sends part of the signal off in an unintended direction.
Diffraction can best be understood by watching the morning sun rise above a building, you will see the upper edge of the sun and an inverse reflection of the sun below the top of the building.
Likewise scattering can be observed on a foggy morning where a distant light can be seen but not focused on, you know you are looking down the road at a streetlight or headlight but to your vision it is just an illuminated blob.
Wayne
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