The Yagi.
First ‘introduced’ by two Japanese gentlemen, YAGI and UDA. Formally named the Yagi-Uda array but commonly referred to as a YAGI. Most amateurs are familiar with the general principles of antennas so no need to describe them here. What you ‘need’ to know can be gleaned from any edition of the ARRL Antenna handbook or the RSGB manual on the same subject. If you feel you need to know everything! about antennas you should purchase THE ANTENNA ENGINEERING HANDBOOK which contains 1230 pages, weighs a ton! and costs (at least) two fortunes.
Important features of the Yagi-Uda array are the horizontal and vertical radiation angles together with the beamwidth. These are closely followed by radiation resistance and impedance. For satellite operating it is extremely important to fully understand the horizontal and vertical radiation angles, why, will become apparent shortly.
Lets say for the sake of argument the ‘literature’ says your yagi has a horizontal beamwidth of 30 degrees at the -3Db points. This, simply stated, means: 15 degrees each side of the main lobe the transmit power (or received energy) will have fallen by minus 3 decibels or half of the power of that at the zenith of the main lobe. The same principle applies to the vertical beamwidth. If the glossy literature says the antenna has a vertical radiation angle of 26 degrees, it means the power will ‘fall off’ by 3Db (half) 13 degrees above and below the main lobe. Hopefully, one can now see that a ‘fixed’ elevation yagi (say at 25 degrees) is still a very useful antenna for satellite operating.
If the satellite is at 30 degrees elevation your signal will be well within the ‘full power’ direction (assuming your azimuth is correct). If the satellite is at 20 degrees elevation you're still within the ‘full power’ direction. When the satellite is at 5 or 45 degrees elevation your power will have dropped by 3Db (half) or 1 S point on a calibrated signal meter. Yes, it's true, not the catastrophe you first imagined?. Exactly the same principle applies to the azimuth. If your favourite tracking program tells you the correct azimuth is 170 degrees and your antenna is pointing to 165 degrees, no difference! If your antenna happens to be pointing to 155 degrees you're at the half power point of your beam and the signal has dropped by 3Db, a WHOLE! S point.
Do not confuse fading and other idiosyncrasies of the ionosphere with your antenna not being pointed ‘exactly’ at the satellite.
The radiation resistance of your antenna depends on height above ‘electrical’ ground, spacing between elements, element length and boom diameter. The length of an EFFECTIVE yagi being measured in ‘boomlengths’ as opposed to number of elements. Important parameters being ‘front to back ratio’, (the ratio of the power transmitted in the main lobe to that transmitted in the opposite direction). Some amateurs are not aware that a lot of energy is transmitted to the sides and the rear of the antenna, depending on its design. Front to side ratio should be self explanatory. The spacing and length of elements along the boom define the radiation pattern. In some cases the front to back ratio being more important than forward gain. In satellite operations we're primarily interested in high forward gain and narrow beamwidth for high elliptical orbits but low forward gain and wide beamwidth for low earth orbit satellites. The adjustment of one parameter always changes the others.
At this point it is important to fully understand the meaning of SWR. SWR (standing wave ratio) is a term often misunderstood by some amateurs. The first thing to get fixed firmly in the mind is that a fairly high SWR ratio DOES NOT mean lost power. High SWR means the antenna's radiation resistance is not the same as the impedance of the feedline and/or the feedline's impedance differs from the load, in this case the transmitter. There are many ways to match the transmitter to the line and the line to the antenna, we needn't go into matching systems here. If the rig, line and antenna are ‘matched’ the SWR will be 1:1.
As the name suggests, SWR is a ratio, how much reflected power in relationship to forward power. It is never ‘lost’- it is reflected back and forth along the line between the transmitter and the antenna. If on the other hand the line IS matched to the antenna and the rig is matched to the line, ALL the transmitted energy will be radiated by the antenna. Conversely, if the match is poor between the rig and the line or line and the antenna the antenna will radiate poorly. Hence the saying ‘warming up the antenna’ as opposed to radiating the energy. A ‘reasonable’ SWR (depending on frequency) is NOT! the disaster some amateurs seem to think. An SWR of 1.5:1 is every bit as good as the perfect match of 1:1.
A related point to note. Assume station 1 is transmitting with 20 watts and the receiving station gives a (genuine) S meter reading of S9. If station Nr: 1 now reduces his power to 10 watts you'd need a magnifying glass! to see the difference on the same meter. THAT IS A FACT. Most experienced operators take no notice of what the S meter says but concentrate on the quality of the signal and use their ears more than their eyes.
The next term we need to look at is ERP (effective radiated power) or EIRP (effective isotropic radiated power). Most antenna manufacturers preferring to use EIRP because it sounds like you're getting more for your money. Firstly, an isotropic radiator is an ‘imaginary’ radiator in space that radiates equally in all directions with nothing to impede the energy. In the real world there is ALWAYS something that effects the radiated energy, not least of which is the earth itself. You never get ‘something for nothing’. That is to say, in a ‘beam’, energy is concentrated in a particular direction AT THE EXPENSE of energy that would otherwise have gone in a different direction. The Isotropic radiator spreading it's energy in ALL directions at the same time. We're all familiar with the stone dropped into the pool causing ripples to drift outward in ever increasing circles. This happens in the horizontal plane only, try to think 3 dimensionally with the ‘ripples’ moving in ALL directions (a doughnut shape). That's how an isotropic radiator radiates. In the real world, something ALWAYS impedes the energy so we use a dipole as a reference by which we compare all other antennas. To fully understand ‘gain’ it should be referenced to a dipole. A dipole is classed as a bi-directional antenna if it is a half wavelength in length though it can radiate in more than two directions depending on it's resonant length. For now we'll say it's a bi-directional antenna.
There are basically two types of radiators, uni and omni directional radiators.
A yagi type of antenna comprises a dipole with one or more reflectors and several directors. The directors concentrate the energy in a certain direction while the reflector ‘reflects’ energy back to the dipole (main element). Sometimes the ‘elements’ are parasitic, sometimes not, we don't need to delve too deeply into antenna mechanics here. The same ‘principles’ apply whether we're talking about a 3 or 10 element yagi, a parabolic reflector antenna or a slot antenna in the nosecone of a fighter plane.