Why was radio contact lost with the Pioneer probes much earlier than with the Voyager probes? The Voyagers were launched only 4 years after the Pioneers, yet the Voyagers are projected to last into the 2020s, while the Pioneers stopped transmitting in 1995 and 2003 respectively. Both were powered by RTGs as well.
Interplanetary communication is mainly dependent on signal strength (for transmission) and antenna size (for reception).
The Pioneers use a 9-foot antenna and an 8-watt transmitter. The Voyagers use a 12-foot antenna and a 20-watt transmitter, allowing a substantially stronger signal to be received on Earth.
In addition to a better transmitter, the Voyagers have better power reserves: their RTGs supplied 470 W at launch, while the Pioneer RTGs supplied 160 W at launch. So the Voyager RTGs will take much longer to decay to a point where they can't power the spacecraft.
NASA seems to think RTG decay is the primary reason we can't receive Pioneer 10 any more:
We're not sure about Pioneer 11's fate:
Several factors lead to the loss of signal.
The combination of these factors leads to the cumulative failure.
Primary is that Pioneer is not producing intelligible data due to weak signal, so it was no longer cost effective to track Pioneer 10. Pioneer 11 shutdown was primarily due to power loss. ③
In looking at how the decision works - at the range they are at, the signal is very weak, and antenna accuracy (to keep Earth in the center of the antenna lobe) is a function of rotational control over attitude. Even so, the required dish size to get data to or from Pioneer 10 was a minimum 64m dish to have the required signal strength. As it goes further out, the minimum antennae to receive its signal increases. At present, only a handful of scopes can communicate with Pioneer 10 at all, and the data is so weak that it's unreliable.
The Pioneers used gas thrusters as their reaction control ④, which provides a fundamental limit to their pointing accuracy.
The drag of the interplanetary medium (IPM) and the solar wind are also issues. ② because the medium is not actually uniform, this results in slight differentials across the spacecraft, which, given the decades, are able to slow the spin and alter the angle. Keeping in mind that the IPM includes chunks of dust and even small pebbles, and the Pioneer velocities make a pebble impact comparable to a small caliber firearm hit, one at the right spot could wobble the spacecraft out of communication.
The exact cause of Pioneer 11 failure isn't actually known, but it's presumed to be power failure (as the telemetry showed decreasing power on a decay curve), possibly coupled with an impact event and/or short circuit. If it were knocked off axis, it might still be broadcasting but have earth outside the antenna's broadcast lobe. The knocked off axis hypothesis isn't likely, as the location is still on the predicted course.
The expectation of Pioneer 10 to continue to degrade in power consumption contributed in the public explanations of termination of the Pioneer 10 mission, equally as much as the expense of the 65m, 100m, and Arecibo antenna time, and the ability to spend that money on missions returning more data.
Voyager, on the other hand, has better thermocouples, younger thermocouples, more power produced, more power (18 w)⑤, and while only a 48 dB gain, it's in a better frequency noise-wise (~ 8 GHz), it's still sending useful data from instruments, and had much higher surplus power at launch.
Voyager power losses have been a shallower curve than the pioneer program; more of the initial power available is still available. Further, the original power was much higher.
The instrumentation for Voyager included several more experiments that are not of use in the extended mission; they can be turned off with no loss of data gain. This allows more of the available power to be used on the ones that are relevant.
The Voyager missions as originally conceived of also included a longer operational period - more than 10 years was expected, versus Pioneer's 2 years.