Transponder power and its proper use are probably the most misunderstood values in the satellite communications link. The tendency is to increase power whenever there is a problem with a satellite circuit. I have heard things like, “I’ll just increase power a little bit, and nobody will notice.” That is not exactly true – as you will see the transponder High Powered Amplifier (HPA) will know. In some cases, if the transponder is running close to or at maximum useable power already, the power increase actually does the opposite of what is expected. There are certain “rules” to follow in the design and operation of a circuit. And if not followed correctly, that can lead to serious circuit problems.
Transponder power is derived from two elements:
- the satellite antenna from which the circuit will transmit from and
- the HPA assigned to the transponder on the satellite.
The combination of these two elements develops the downlink Effective Isotropic Radiated Power (or downlink EIRP) towards earth. The transponder downlink EIRP is measured in a value called Decibel Watts (or dBW). Let’s take a look at both elements separately.
The satellite antenna, made up of the single feedhorn or a multi feed array (to transmit the HPA power) and reflector (to direct the HPA power to earth), shapes the way the downlink power will cover the earth (Fig. 1). In some circumstances a multi-feed array can be used to shape the desired radiation pattern and a parabolic or “standard” antenna reflector is used. In other instances a single feedhorn is used along with a shaped reflector to make the desired downlink pattern. In still other instances, such as the steerable Intelsat Ku Spot beams, a standard feedhorn and reflector are used and the downlink radiation pattern is determined by where the downlink radiation is desired.
In the case of a shaped reflector, this is accomplished by deforming and “tuning” the reflector into the desired radiation pattern shape to deliver the desired power where it is needed. The final reflector design shape is developed through the interaction of the satellite operator (owner) and the satellite manufacturer. The design must take into account the orbital location(s) the satellite will/could operate and where exactly the coverage has to include or exclude. The satellite operator’s goal is to cover as much of earth’s marketable surface area with as much downlink power as commercially needed and required by the service to be provided. An equally important goal is to minimize the downlink power in areas that are the least marketable – or wasted power. Weighing all the variables to come up with an optimal antenna radiation pattern always requires compromise.
A multi-feed array is made up of a cluster of small feedhorns that shape the antenna radiation much the same way a tuned reflector does, only the power is shaped through how each small feedhorn is excited and how much power is sent to each one.
In all instances of footprint design another determining factor is the curvature of the earth. Once the coverage is agreed upon, the construction of the satellite antenna and/or feed array begins.
Various examples C-band and Ku-band downlink footprints show the different ways to “shape” an RF signal.
Figure 1: G-16 NA C Band (Shaped antenna) and
Figure 2: IS-705 EH C-band (Multifeed array)
High Power Amplifiers
There are various sizes of High Power Amplifiers ranging from 35 Watts to 150+ Watts. There are typically two types of transponder HPAs, Traveling Wave Tube Amplifiers (TWTAs or “tubes”) and Solid State Power Amplifiers (SSPAs also called “solid state”). Both have their advantages and disadvantages. Tubes can provide more power than SSPAs and have a higher efficiency, but SSPAs are smaller in physical volume and mass. Therefore it is easier to accommodate a larger number of SSPAs on the satellite.
Tubes and solid state amplifiers must be used correctly or problems can develop. For instance, an amplifier will correctly amplify one primary carrier all the way to saturation or full useable power. But introduce a second primary carrier (or three… or X number) and the HPA not only amplifies the primary carriers you want it to amplify, it also produces unwanted harmonic carriers based on the proximity of the primary carriers in the transponder and due to the nonlinear characteristics of the TWTA. These unwanted harmonic carriers “steal” HPA power from the primary carriers and can cause interference to the primary carriers if the HPA is operated too close to saturation. Distortion will develop, affecting the performance of the circuits, and can also cause interference to other carriers in the transponder as well.
To ensure that there is no distortion caused by multiple carriers, the power use of the HPA must be below the point at which too much distortion can occur. The point of distortion is determined by the HPA non-linearity transfer characteristics. This means testing of the HPA is necessary (or class of HPA) to determine the power level that can be used while still staying below the point at which distortion can develop. The point of maximum power without causing distortion is called “transponder back-off”. As long as you operate the transponder up to the “back-off limit,” then the carriers will be transmitted “cleanly” and without an acceptable level of distortion. TWTAs are typically backed-off more than the equivalent SSPA, since SSPAs are more linear in operation. Modern TWTAs are equipped with a linearizer circuit that improves the inherent non-linearity of the TWTA to enable operation with less back-off.
Another way to look at back-off limit is this: It is much the same way as a stereo audio amplifier. Take a car stereo that runs a maximum power output of 25 Watts and a home stereo system that can run 200 Watts or more. If you crank both up to 25 Watts, chances are the car stereo – at maximum power and amplifying multiple audio frequencies – will sound distorted and the home stereo will sound cleaner and brighter. That is because the home amplifier is nowhere near the point of using maximum power available. The home amplifier is operating in a “backed-off” state.
I hope you can see now how transponder power is designed, created and delivered for our customers to use and that transponder power management is critical within a transponder. It’s just another reason why there is a need to run a link analysis for a new or upgraded circuit—to ensure that the designed circuit will either co-exist peacefully (without interference) with already established circuits in a transponder, or it will work as the only carrier in the transponder.