Assuming this was fixed, I calculated the calculated the expected gain that this would result in. In the diagram below I have drawn a cyan rectangle to represent the assumed aperture for my antenna(45cm^2). As a planar antenna, its hard to easily see what the aperture is, but given a bit of experimentation I was able to derive a suitable area that worked for my needs. We can calculate this aperture(area of the dish) and multiply if by the power intensity(fig2), we can work out the received power.įor my Palmtree Vivaldi antenna, I did a similar calculation. Assuming its 100% efficient(dishes are normally at least 70% efficient). It has a constant aperture, dictated by the diameter of the dish. So long as the wavelength is smaller than the area of the dish, these antennas work to capture all the energy that hits the dish. A common example of which would be a satellite dish. This is true for a type of antenna called an ‘aperture antenna’. If we can design an antenna such that its size remains the same with increasing frequency, we can compensate for this loss. Nothing is lost, we are just using a smaller bucket to catch the signal with. In fact to call it a ‘loss’ is somewhat confusing. Freespace Pathloss increase with frequency because our antennas get smaller. ![]() This brings me to what I think is the real understanding of the link between FSPL and frequency. Watts per Unit Area, it stands to reason that if the Area is smaller(because our antenna is shorter) there will be less received power.įormulas in fig1 and fig2 are different because the FSPL formula has the assumption of an isotropic antenna built in. ![]() The power intensity we calculated in fig2 is in units of W/m^2, ie. If we want to make our antenna work at a higher frequency we make it shorter. Our dipole is tuned to a specific frequency that we can change by making it shorter or longer. A standard half-wave dipole will have a gain of 2.15dBi regardless of frequency. For good reasons, we do this in a way that similar similar antennas provide the same gain when scaled for frequency. Well its because of the way we specify antenna gain. So, why does our FSPL formula differ from this. Its not complicated, and very easy to intuitively understand. This in my opinion really is the fundamental formula we should understand. There is no loss, but the power density, which would have units like Watts per Meter Squared, decreases according to the size of the sphere. The intensity of the signal decreases as the surface area of a sphere of radius d (yes, I know radius is normally r, but for some reason RF people use d to equal distance). In the denominator, we see what is clearly recognisable as the formula for the surface area of a sphere. In the formula above(fig2), we show the transmitted power in the numerator(above the line). Let us first understand that the freespace pathloss formula is derived from this basic principle. Hence Freespace Pathloss does not depend upon frequency.įig2 – Power Intensity as a function of distance from an isotropic antenna There is no Loss in a vacuum, if there were we would not be able to see the stars. Regardless, my argument here is about Freespace Pathloss, so let’s assume we are in a vacuum. We will ignore some effects like hydrogen lines and water absorption that happen in specific bands. Within the air of our atmosphere and within the frequencies we normally deal with for radio signals, air is pretty much a lossless medium. Note, that I haven’t had to discuss the colour or frequency of this light. If we placed a huge sphere around our solar system (Dyson sphere) pretty much all the light emitted from the sun would be hit the inside surface of that sphere. No, the light gets dimmer because it spreads out as it travels. Why is this so? It’s clearly not because of dust in space that is absorbing this light and turning it into heat, such an effect would cause the dust to emit black body radiation (probably IR light) and the sky would glow (at least in IR) which it doesn’t. The further a planet is from the sun, the dimmer the light that reaches it. ![]() It emits electromagnetic waves(light) that illuminate all the planets in our solar system. Our sun is a ball of light that shines in all directions.
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