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New Horizons was said to be able to communicate at ~1kbit/s at Pluto and 38kbit/s at Jupiter.

Communication with the spacecraft is via X band. The craft had a communication rate of 38 kbit/s at Jupiter; at Pluto's distance, a rate of approximately 1 kbit/s per transmitter is expected. Besides the low data rate, Pluto's distance also causes a latency of about 4.5 hours (one-way).

https://en.wikipedia.org/wiki/New_Horizons#Telecommunications_and_data_handling

However the Voyager 1 which was decades older can transmit 1.4kbit/s around 2018 at a much larger distance

Termination of Data Tape Recorder (DTR) operations (limited by ability to capture 1.4 kbit/s data using a 70 m/34 m antenna array; this is the minimum rate at which the DTR can read out data)

https://en.wikipedia.org/wiki/Voyager_1#Future_of_the_probe

The Voyager 2 is also faster at Jupiter's distance (~115.2kbit/s)

Communications are conducted over the S-band (about 13 cm wavelength) and X-band (about 3.6 cm wavelength) providing data rates as high as 115.2 kilobits per second at the distance of Jupiter, and then ever-decreasing as the distance increased, because of the inverse-square law

https://en.wikipedia.org/wiki/Voyager_2#Communications

Why is that?

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    $\begingroup$ Voyager has a 3.7 m (12 ft) dish, New Horizons a 2.1-meter (7 ft) dish. Simple as that. $\endgroup$ – Polygnome May 25 '18 at 14:09
  • $\begingroup$ yeah, forgot to check the dish size. However I don't think that's really the answer, because the antenna area is just ~3.1 times larger, whereas the distance is much much farther (about 5.83 times @2011). Moreover 3 decades later digital modulation and error correction have gotten much better, so the speed should be better IMHO $\endgroup$ – phuclv May 25 '18 at 14:18
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    $\begingroup$ They have also become better for the Voyager probes ;) $\endgroup$ – Polygnome May 25 '18 at 14:26
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The phrase "limited by ability to capture 1.4 kbit/s data using a 70 m/34 m antenna array" indicates they used 2 large dishes to receive Voyager's signal, which takes up a lot of DSN resources (only a few large antennas are available, and they must divide their time between more missions than there are antennas). New Horizons used one dish instead, so the receiving antenna gain is lower.

Voyager's transmitter has a bit more power (19 vs 12 W). And Voyager has a larger dish antenna (3.7 vs 2.1 m).

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    $\begingroup$ Yeah. Voyager was built in an era where budget wasn't the primary concern. They don't make spacecraft like these nowadays. $\endgroup$ – SF. May 25 '18 at 15:21
  • $\begingroup$ Budget does not appear to be the primary concern for JWST since it's probably the biggest cost overrun in NASA history. $\endgroup$ – Organic Marble May 25 '18 at 23:14
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    $\begingroup$ New Horizons' size was constrained by the size of the available rockets and the fact it only had one gravity assist, so it had a smaller weight budget than Voyager. Voyager did have budget constraints: a third mission was cancelled. NASA was being squeezed at the time, coming out of the Apollo program and starting the Shuttle program. $\endgroup$ – Hobbes May 26 '18 at 7:47

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